“Electrochemical methods of analysis and their modern hardware design: a review of WEB sites of companies selling chemical analytical equipment”

Introduction

Chapter 1. Classification of electrochemical methods

1.1 Voltammetry

1.2 Conductometry

1.3 Potentiometry

1.4 Amperometry

1.5 Coulometry

1.6 Other electrochemical phenomena and methods

1.7 Applied electrochemistry

Chapter 2. Electrochemical methods analysis and their role in protection environment

Chapter 3. Devices based on electrochemical methods of analysis

Chapter 4. Review of WEB sites of companies selling chemical analytical equipment

Literature

INTRODUCTION

Electrochemical methods of analysis (electroanalysis), which are based on electrochemical processes, occupy a worthy place among methods for monitoring the state of the environment, as they are capable of determining a huge number of both inorganic and organic environmentally hazardous substances. They are characterized by high sensitivity and selectivity, rapid response to changes in the composition of the analyzed object, ease of automation and the possibility of remote control. And finally, they do not require expensive analytical equipment and can be used in laboratory, industrial and field conditions. Direct relation Three electroanalytical methods are relevant to the problem under consideration: voltammetry, coulometry and potentiometry.

CHAPTER 1. CLASSIFICATION OF ELECTROCHEMICAL METHODS

Electrochemical methods of analysis (EMA) are based on the study of processes occurring on the surface of the electrode or in the near-electrode space. The analytical signal is an electrical parameter (potential, current, resistance, etc.), functionally related to the concentration of the solution component being determined and amenable to correct measurement.

The EMA classification proposed by IUPAC has undergone certain changes over the past decades; clarifications (explanations) and additions have been made to it.

Significant attention is paid to electrochemical cells and analytical signal sensors (electrode systems, various electrochemical sensors); it is these primary electrochemical converters that determine the analytical capabilities of any method. Currently, the most advanced and fastest processing of the signal from the sensor, calculation of statistical characteristics of both the original signal and the results of the entire analysis as a whole are not a problem. This is why it is important to obtain a reliable initial signal in order to calibrate it in concentration units.

According to the general classification proposed

IUPAC, EMA are divided into methods in which the excited electrical signal is constant or equal to zero and into methods in which the excited signal varies over time. These methods are classified as follows:

voltammetric - voltammetry,I ≠ 0; E = f(t);

potentiometric – potentiometry, (I = 0);

amperometric – amperometry (I ≠ 0; E=const);

chronopotentiometric,E = f(t); I =const;

impedance, or conductometric- measurements using the application of low amplitude alternating voltage; other, combined(for example, spectroelectrochemical).

1.1 VOLTAMPEROMETRY

VOLTAMPEROMETRY- a set of electrochemical methods of research and analysis based on studying the dependence of the current in an electrolytic cell on the potential of an indicator microelectrode immersed in the analyzed solution, on which the electrochemically active (electroactive) substance under study reacts. In addition to the indicator electrode, an auxiliary electrode with a much larger surface is placed in the cell so that when current passes, its potential practically does not change (non-polarizing electrode). The potential difference between the indicator and auxiliary electrodes E is described by the equation E = U - IR, where U is the polarizing voltage, R is the resistance of the solution. An indifferent electrolyte (background) is introduced into the analyzed solution in high concentration in order, firstly, to reduce the value of R and, secondly, to eliminate the migration current caused by the action electric field for electroactive substances (obsolete - depolarizers). At low concentrations of these substances, the ohmic voltage drop IR in solution is very small. To fully compensate for the ohmic voltage drop, potentiostating and three-electrode cells are used, which additionally contain a reference electrode. In these conditions

Stationary and rotating ones are used as indicator microelectrodes - made of metal (mercury, silver, gold, platinum), carbon materials (for example, graphite), as well as dripping electrodes (made of mercury, amalgam, gallium). The latter are capillaries from which liquid metal flows drop by drop. Voltammetry using dropping electrodes, the potential of which changes slowly and linearly, is called. polarography (method proposed by J. Heyrovsky in 1922). Electrodes of the second type are usually used as reference electrodes, for example. calomel or silver chloride (see Reference electrodes). Dependence curves I =f(E) or I =f(U) (voltammograms) are recorded with special devices - polarographs of various designs.

Voltammograms obtained using a rotating or dripping electrode with a monotonic change (linear sweep) of voltage have the form shown schematically in the figure. The current increasing section is called wave. Waves m.b. anodic, if the electroactive substance is oxidized, or cathodic, if it is reduced. When the solution contains oxidized (Ox) and reduced (Red) forms of a substance that react quickly (reversibly) on the microelectrode, a continuous cathode-anode wave is observed on the voltammogram, crossing the x-axis at a potential corresponding to the redox potential of the Ox/Red system in this environment. If the electrochemical reaction on the microelectrode is slow (irreversible), an anodic wave of oxidation of the reduced form of the substance and a cathodic wave of reduction of the oxidized form (at a more negative potential) are observed on the voltammogram. The formation of the limiting current area on the voltammogram is associated either with a limited rate of mass transfer of the electroactive substance to the electrode surface by convective diffusion (limiting diffusion current, I d), or with a limited rate of formation of the electroactive substance from the analyte component in solution. This current is called the limiting kinetic current, and its strength is proportional to the concentration of this component.

Waveform for reversible electro chemical reactions described by the equation:

where R is the gas constant, T-absolute temperature, E 1/2 - half-wave potential, i.e. potential corresponding to half the wave height (I d /2;). The E 1/2 value is characteristic of a given electroactive substance and is used for its identification. When electrochemical reactions are preceded by the adsorption of the analyte on the electrode surface, peaks rather than waves are observed in the voltammograms, which is associated with the extreme dependence of adsorption on the electrode potential. In voltammograms recorded during a linear change (sweep) of potential with a stationary electrode or on one drop of a dripping electrode (obsolete - oscillographic polarogram), peaks are also observed, the descending branch of which is determined by the depletion of the near-electrode layer of the solution in the electroactive substance. The height of the peak is proportional to the concentration of the electroactive substance. In polarography, the limiting diffusion current (in μA), averaged over the lifetime of a drop, is described by the Ilkovich equation:

where n is the number of electrons participating in the electrochemical reaction, C is the concentration of the electroactive substance (mM), D is the diffusion coefficient (cm 2 / s), the lifetime of a mercury drop (s), m is the flow rate of mercury (mg/s) .

With a rotating disk electrode, the limiting diffusion current is calculated using the equation:

where S is the surface area of the electrode (cm 2), is the circular frequency of rotation of the electrode (rad/s), v-kinematic viscosity solution (cm 2 /s), F-Faraday number (C/mol).

Cyclic voltammetry (voltammetry with a relatively fast triangular potential scan) allows you to study the kinetics and mechanism of electrode processes by observing on the screen of an oscilloscope tube with an afterglow simultaneously voltammograms with anodic and cathodic potential scans, reflecting, in particular, the electrochemical reactions of electrolysis products.

The lower limit of the determined concentrations of Cn in V. methods with linear potential sweep is 10 -5 -10 -6 M. To reduce it to 10-7 -10 -8 M, improved instrumental options are used - alternating current and differential pulse voltammetry.

In the first of these options, a small amplitude alternating component of a sinusoidal, rectangular (square wave voltammetry), trapezoidal or triangular shape with a frequency usually in the range of 20-225 Hz is superimposed on the constant component of the polarization voltage. In the second option, voltage pulses of the same magnitude (2-100 mV) with a duration of 4-80 ms are applied to the constant component of the polarization voltage with a frequency equal to the frequency of the mercury dripping electrode, or with a frequency of 0.3-1.0 Hz when using stationary electrodes. In both options, the dependence of the alternating current component on U or E is recorded with phase or time selection. In this case, voltammograms have the form of the first derivative of a conventional voltammetric wave. The peak height on them is proportional to the concentration of the electroactive substance, and the peak potential serves to identify this substance from reference data.

The peaks of various electroactive substances are usually better resolved than the corresponding voltammetric waves, and the peak height in the case of an irreversible electrochemical reaction is 5-20 times less than the peak height in the case of a reversible reaction, which also determines the increased resolution of these voltammetric options. For example, irreversibly reduced oxygen practically does not interfere with the determination of electroactive substances by alternating current voltammetry. The peaks in alternating current voltammograms reflect not only the electrochemical reactions of electroactive substances, but also the processes of adsorption and desorption of non-electroactive substances on the electrode surface (non-Faraday admittance peaks, obsolete - tensammetric peaks).

For all variants of voltammetry, a method of reducing Cn is used, based on preliminary electrochemical, adsorption or chemical accumulation of the determined component of the solution on the surface or in the volume of a stationary microelectrode, followed by registration of a voltammogram reflecting the electrochemical reaction of the accumulation product. This type of voltammetry is called inversion voltammetry (the old name for inversion voltammetry with accumulation on a stationary mercury microelectrode - amalgam polarography with accumulation). In stripping voltammetry with preliminary accumulation, Cn reaches 10 -9 -10 -11 M. The minimum values of Cn are obtained using thin-film mercury indicator electrodes, incl. mercury-graphite, consisting of tiny droplets of mercury, electrolytically separated onto a substrate of specially treated graphite.

For phase and elemental analysis solids stripping voltammetry is used with electroactive carbon electrodes (so-called mineral-carbon paste electrodes). They are prepared from a mixture of coal powder, the powdered substance being studied and an inert binder, for example. Vaseline oil. A variant of this method has been developed, which makes it possible to analyze and determine the thickness of metal coatings. In this case, a special device (clamping cell) is used, which makes it possible to record a voltammogram using a drop of background electrolyte applied to the surface under study.

Application

Voltammetry is used: for the quantitative analysis of inorganic and organic substances in a very wide range of contents - from 10 -10% to tens of%; to study the kinetics and mechanism of electrode processes, including the stage of electron transfer, preceding and subsequent chemical reactions, adsorption of initial products and products of electrochemical reactions, etc.; to study the structure of the electrical double layer, the equilibrium of complexation in solution, the formation and dissociation of intermetallic compounds in mercury and on the surface of solid electrodes; to select amperometric titration conditions, etc.

1.2 Conductometry

Conductometry - based on measuring the electrical conductivity of a solution and is used to determine the concentration of salts, acids, bases, etc. In conductometric determinations, electrodes made of identical materials are usually used, and the conditions for their conduct are selected in such a way as to minimize the contribution of potential jumps at both electrode/electrolyte interfaces (for example, high-frequency alternating current is used). In this case, the main contribution to the measured cell potential is made by the ohmic voltage drop IR, where R is the solution resistance. The electrical conductivity of a one-component solution can be related to its concentration, and measuring the electrical conductivity of electrolytes of complex composition allows one to estimate the total ion content in the solution and is used, for example, in monitoring the quality of distilled or deionized water. In another type of conductometry - conductometric titration - a known reagent is added in portions to the analyzed solution and the change in electrical conductivity is monitored. The equivalence point at which is marked sudden change electrical conductivity, is determined from a graph of the dependence of this value on the volume of added reagent.

1.3 Potentiometry

Potentiometry - used to determine various physical and chemical parameters based on data on the potential of a galvanic cell. The electrode potential in the absence of current in the electrochemical circuit, measured relative to the reference electrode, is related to the concentration of the solution by the Nernst equation. In potentiometric measurements, ion-selective electrodes are widely used that are sensitive primarily to one ion in solution: a glass electrode for measuring pH and electrodes for the selective determination of sodium, ammonium, fluorine, calcium, magnesium, etc. ions. The surface layer of the ion-selective electrode can include enzymes, and the result is a system that is sensitive to the appropriate substrate. Note that the potential of an ion-selective electrode is determined not by the transfer of electrons, as in the case of substances with electronic conductivity, but mainly by the transfer or exchange of ions. However, the Nernst equation, which relates the electrode potential to the logarithm of the concentration (or activity) of a substance in solution, is also applicable to such an electrode. In potentiometric titration, the reagent is added to the solution being analyzed in portions and the change in potential is monitored. The S-shaped curves characteristic of this type of titration allow one to determine the equivalence point and find thermodynamic parameters such as the equilibrium constant and standard potential.

1.4 Amperometry

The method is based on measuring the limiting diffusion current passing through a solution at a fixed voltage between the indicator electrode and the reference electrode. In amperometric titration, the equivalence point is determined by the break in the current curve - the volume of the added working solution. Chronoamperometric methods are based on measuring the dependence of current on time and are mainly used to determine diffusion coefficients and rate constants. Miniature electrochemical cells that serve as sensors at the output of liquid chromatograph columns operate on the principle of amperometry (as well as voltammetry). Galvanostatic methods are similar to amperometric ones, but they measure the potential when a certain amount of current passes through the cell. Thus, in chronopotentiometry, the change in potential over time is controlled. These methods are used mainly to study the kinetics of electrode reactions.

1.5 Coulometry.

In coulometry, at a controlled potential, complete electrolysis of a solution is carried out by intensively mixing it in an electrolyzer with a relatively large working electrode (bottom mercury or platinum mesh). The total amount of electricity (Q, C) required for electrolysis is related to the amount of the forming substance (A, g) by Faraday’s law:

where M – mol. mass (g/mol), F – Faraday number. Coulometric titration involves using a constant current to electrolytically generate a reagent that reacts with the substance being determined. The progress of the titration is controlled potentiometrically or amperometrically. Coulometric methods are convenient because they are absolute in nature (i.e., they allow you to calculate the amount of the analyte without resorting to calibration curves) and are insensitive to changes in electrolysis conditions and electrolyzer parameters (electrode surface area or stirring intensity). In coulogravimetry, the amount of substance that has undergone electrolysis is determined by weighing the electrode before and after electrolysis.

There are other electroanalytical methods. In alternating current polarography, a low amplitude sinusoidal voltage is applied to a linearly varying potential over a wide frequency range and either the amplitude and phase shift of the resulting alternating current or the impedance is determined. From these data, information is obtained about the nature of substances in solution and about the mechanism and kinetics of electrode reactions. Thin-layer methods use electrochemical cells with an electrolyte layer 10–100 µm thick. In such cells, electrolysis proceeds faster than in conventional electrolyzers. To study electrode processes, spectrochemical methods with spectrophotometric registration are used. To analyze substances formed on the surface of the electrode, their absorption of light in the visible, UV and IR regions is measured. Changes in the properties of the electrode surface and the medium are monitored using electrical reflection and ellipsometry methods, which are based on measuring the reflection of radiation from the electrode surface. These include methods of specular reflection and Raman scattering of light (Raman spectroscopy), second harmonic spectroscopy (Fourier spectroscopy).

1.6 Other electrochemical phenomena and methods

With the relative movement of the electrolyte and charged particles or surfaces, electrokinetic effects occur. An important example This type is electrophoresis, in which the separation of charged particles (for example, protein molecules or colloidal particles) moving in an electric field occurs. Electrophoretic methods are widely used to separate proteins or deoxyribonucleic acids (DNA) in gels. Electrical phenomena play big role in the functioning of living organisms: they are responsible for the generation and propagation of nerve impulses, the occurrence of transmembrane potentials, etc. Various electrochemical methods are used to study biological systems and their components. It is also of interest to study the effect of light on electrochemical processes. Thus, the subject of photoelectrochemical research is the generation electrical energy and the initiation of chemical reactions under the influence of light, which is very important for increasing the efficiency of converting solar energy into electrical energy. Semiconductor electrodes made of titanium dioxide, cadmium sulfide, gallium arsenide and silicon are commonly used here. Another interesting phenomenon is electrochemiluminescence, i.e. generation of light in an electrochemical cell. It is observed when high-energy products are formed on the electrodes. Often the process is carried out in a cyclic manner to obtain both oxidized and reduced forms of a given compound. Their interaction with each other leads to the formation of excited molecules, which pass to the ground state with the emission of light.

1.7 Applied electrochemistry

Electrochemistry has many practical applications. With the help of primary galvanic cells (disposable elements) connected to batteries, chemical energy is converted into electrical energy. Secondary current sources - batteries - store electrical energy. Fuel cells are primary power sources that generate electricity through a continuous supply of reactants (such as hydrogen and oxygen). These principles underlie portable power sources and batteries used on space stations, electric vehicles and electronic devices.

Large-scale production of many substances is based on electrochemical synthesis. The electrolysis of brine in the chlor-alkali process produces chlorine and alkali, which are then used to produce organic compounds and polymers, as well as in the pulp and paper industry. The products of electrolysis are compounds such as sodium chlorate, persulfate, sodium permanganate; Industrially important metals are obtained by electroextraction: aluminum, magnesium, lithium, sodium and titanium. It is better to use molten salts as electrolytes, since in this case, unlike aqueous solutions, the reduction of metals is not complicated by the release of hydrogen. Fluorine is produced by electrolysis in molten salt. Electrochemical processes serve as the basis for the synthesis of some organic compounds; for example, adiponitrile (an intermediate in the synthesis of nylon) is obtained by hydrodimerization of acrylonitrile.

Electroplating of silver, gold, chromium, brass, bronze and other metals and alloys is widely practiced on various objects in order to protect steel products from corrosion, for decorative purposes, for the manufacture of electrical connectors and printed circuit boards in the electronics industry. Electrochemical methods are used for high-precision dimensional processing of workpieces made of metals and alloys, especially those that cannot be processed by conventional mechanical methods, as well as for the manufacture of parts with complex profiles. When the surface of metals such as aluminum and titanium is anodized, protective oxide films are formed. Such films are created on the surface of workpieces made of aluminum, tantalum and niobium in the manufacture of electrolytic capacitors, and sometimes for decorative purposes.

In addition, studies of corrosion processes and the selection of materials that slow down these processes are often based on electrochemical methods. Corrosion of metal structures can be prevented using cathodic protection, for which an external source is connected to the structure being protected and the anode and the structure is maintained at a potential such that its oxidation is excluded. The possibilities of practical application of other electrochemical processes are being explored. So, electrolysis can be used to purify water. A very promising direction is the conversion of solar energy using photochemical methods. Electrochemical monitors are being developed, the operating principle of which is based on electrochemiluminescence.

Electrochemical methods of analysis (electroanalysis), which are based on electrochemical processes, occupy a worthy place among methods for monitoring the state of the environment, as they are capable of determining a huge number of both inorganic and organic environmentally hazardous substances. They are characterized by high sensitivity and selectivity, rapid response to changes in the composition of the analyzed object, ease of automation and the possibility of remote control. Finally, they do not require expensive analytical equipment and can be used in laboratory, industrial and field conditions. Three electroanalytical methods are directly related to the problem under consideration: voltammetry, coulometry and potentiometry.

Brief historical background. The beginning of the development of electroanalysis is associated with the emergence of the classical electrogravimetric method (around 1864, W. Gibbs). The discovery of the laws of electrolysis by M. Faraday in 1834 formed the basis of the coulometry method, but the use of this method began in the 30s of the twentieth century. A real turning point in the development of electroanalysis occurred after the discovery of the polarography method in 1922 by J. Heyrovsky. Polarography can be defined as electrolysis with a dropping mercury electrode. This method remains one of the main methods of analytical chemistry. In the late 50s and early 60s, the problem of environmental protection stimulated the rapid development of analytical chemistry, and in particular electroanalytical chemistry, including polarography. As a result, improved polarographic methods were developed: alternating current (Mr. Barker, B. Breuer) and pulsed polarography (Mr. Barksr, A. Gardnsr), which significantly surpassed in their characteristics the classic version of polarography proposed by J. Heyrovsky. When using solid electrodes made of various materials instead of mercury (used in polarography), the corresponding methods began to be called voltammetric. At the end of the 50s, the work of V. Kemuli and Z. Kublik laid the foundation for the method of stripping voltammetry. Along with the methods of coulometry and voltammetry, methods based on measuring electrode potentials and electromotive forces galvanic cells, - methods of potentiometry and ionometry (see).

Voltammetry. This is a group of methods based on studying the dependence of the current in an electrolytic cell on the potential applied to an indicator microelectrode immersed in the analyzed solution. These methods are based on the principles of electrolysis; the analytes present in the solution are oxidized or reduced at the indicator electrode. In addition to the indicator electrode, a reference electrode with a much larger surface is placed in the cell so that when current passes, its potential practically does not change. The most commonly used indicator microelectrodes are stationary and rotating electrodes made of platinum or graphite, as well as a mercury dripping electrode, which is a long narrow capillary, at the end of which small mercury drops with a diameter of 1-2 mm are periodically formed and separated (Fig. 1). High quality and quantitative compositions solution can be established from voltammograms.

Rice. 4. Electrochemical cell with a dropping mercury electrode: 1 - solution to be analyzed, 2 - dropping mercury electrode, 3 - reservoir with mercury, 4 - reference electrode

Voltammetric techniques, especially sensitive variants such as differential pulse polarography and stripping voltammetry, are routinely used in all fields chemical analysis and are most useful in solving environmental problems. These methods are applicable for the determination of both organic and inorganic substances, for example, for the determination of most chemical elements. Using the stripping voltammetry method, the problem of determining traces of heavy metals in waters and biological materials is most often solved. For example, voltammetric methods for the simultaneous determination of Cu, Cd and Pb, as well as Zn and Pb or Ti in drinking water are included in the standard Germany. An important advantage of voltammetry is the ability to identify the forms of metal ions in waters. This makes it possible to assess the quality of water, since different chemical forms of existence of metals have to varying degrees toxicity. From organic substances, it is possible to determine compounds that have groups capable of reduction (aldehydes, ketones, nitro-, nitroso compounds, unsaturated compounds, halogen-containing compounds, azo compounds) or oxidation ( aromatic hydrocarbons, amines, phenols, aliphatic acids, alcohols, sulfur-containing compounds). The possibilities for determining organic substances by stripping voltammetry are significantly expanded when using chemically modified electrodes. By modifying the electrode surface with polymer and inorganic films, including reagents with specific functional groups, including biomolecules, it is possible to create conditions for the component being determined where the analytical signal will be practically specific. The use of modified electrodes provides selective determination of compounds with similar redox properties (for example, pesticides and their metabolites) or electrochemically inactive on conventional electrodes. Voltammetry is used to analyze solutions, but it can also be used to analyze gases. Many simple voltammetric analyzers have been designed for use in the field.

Coulometry. An analysis method based on measuring the amount of electricity (Q) passed through an electrolyzer during electrochemical oxidation or reduction of a substance at the working electrode. According to Faraday's law, the mass of an electrochemically converted substance (P) is related to Q by the ratio:

P = QM/ Fn,

where M is the molecular or atomic mass of the substance, n is the number of electrons involved in the electrochemical transformation of one molecule (atom) of the substance, p is Faraday’s constant.

A distinction is made between direct coulometry and coulometric titration. In the first case, an electrochemically active substance is determined, which is deposited (or transferred to a new oxidation state) on the electrode at a given electrolysis potential, while the amount of electricity expended is proportional to the amount of the reacted substance. In the second case, an electrochemically active auxiliary reagent is introduced into the analyzed solution, from which a titrant (coulometric titrant) is electrolytically generated, and it quantitatively chemically interacts with the substance being determined. The content of the component being determined is assessed by the amount of electricity passed through the solution during the generation of the titrant until the completion of the chemical reaction, which is determined, for example, using color indicators. It is important that when carrying out coulometric analysis, there are no foreign substances in the test solution that can enter into electrochemical or chemical reactions under the same conditions, that is, no side electrochemical and chemical processes occur.

Coulometry is used to determine both trace (at the level of 109-10 R mol/l) and very large quantities of substances with high accuracy. Coulometrically, it is possible to determine many inorganic (almost all metals, including heavy metals, halogens, S, NO3, NO2) and organic matter(aromatic amines, nitro- and nitroso compounds, phenols, azo dyes). Automatic coulometric analyzers for determining very low levels (up to 104%) of gaseous pollutants (S02"Oz, H2S, NO, N02) in the atmosphere have successfully proven themselves in field conditions.

Potentiometry. An analysis method based on the dependence of the equilibrium electrode potential E on the activity of the a components of the electrochemical reaction: aA + bB + ne = mM + pP.

In potentiometric measurements, a galvanic element is formed from an indicator electrode, the potential of which depends on the activity of one of the components of the solution, and a reference electrode, and the electromotive force of this element is measured.

There are direct potentiometry and potentiometric titration. Direct potentiometry is used to directly determine the activity of ions from the potential value (E) of the corresponding indicator electrode. In the potentiometric titration method, the change in E during the reaction of the analyte with a suitable titrant is recorded.

When solving problems of environmental protection, the most important method is direct potentiometry using membrane ion-selective electrodes (ISE) - ionometry. Unlike many other methods of analysis, which allow one to estimate only the total concentration of substances, ionometry allows one to estimate the activity of free ions and therefore plays a large role in studying the distribution of ions between their different chemical forms. Automated monitoring methods are especially important for monitoring environmental objects, and the use of ISE is very convenient for this purpose.

One of the main indicators in characterizing the state of the environment is the pH value of the environment, which is usually determined using glass electrodes. Glass electrodes coated with a semi-permeable membrane with a film of the appropriate electrolyte are used in the analysis of water and atmosphere to control pollutants (NH3, SO 2 NO, NO 2, CO 2, H 2 S). ISE is usually used to monitor the content of anions, for which there are traditionally much fewer determination methods than for cations. To date, ISEs have been developed and are widely used for the determination of F, CI, Br, I, C1O4, CN, S2, NO] and NO2, allowing the determination of the listed ions in the concentration range from 10 -6 to 10 -1 mol/l .

One of the important areas of application of ionometry is hydrochemical studies and determination of the concentration of anions and cations in different types waters (surface, sea, rain). Another area of application of ISE is food analysis. An example is the determination of NO – 3 and NO 2 – in vegetables, meat and dairy products, and baby food products. A miniature needle-shaped ISE has been created to determine NO - 3 directly in the pulp of fruits and vegetables.

Ionometry is also widely used to determine various biologically active compounds and drugs. At present, we can already say that there are carriers that are selective to almost any type of organic compounds, which means that it is possible to create an unlimited number of corresponding ISEs. A promising direction is the use of enzyme electrodes, the membrane of which contains immobilized enzymes. These electrodes have the high specificity inherent in enzymatic reactions. With their help, for example, it will be possible to determine cholinesterase-inhibiting insecticides (organophosphorus compounds, carbamates) at concentrations of -1 ng/ml. The future of the method is associated with the creation of compact, specific sensors, which are modern electronic devices in combination with ion-selective membranes, which will make it possible to avoid the separation of sample components and will significantly speed up analyzes in the field.

Wastewater analysis

Electroanalytical methods, which are usually used in water analysis to determine inorganic components, are often inferior in sensitivity to gas and liquid chromatography and atomic adsorption spectrometry. However, cheaper equipment is used here, sometimes even in the field. The main electroanalytical methods used in water analysis are voltammetry, potentiometry and conductometry. The most effective voltammetric methods are differential pulse polarography (DIP) and stripping electrochemical analysis (IEA). The combination of these two methods makes it possible to carry out determinations with very high sensitivity - approximately 10 -9 mol/l, while the instrumentation is simple, which makes it possible to carry out analyzes in the field. Fully automated monitoring stations operate on the principle of using the IEA method or a combination of IEA with DIP. The DIP and IEA methods, in a direct version, as well as in combination with each other, are used to analyze water contamination with heavy metal ions and various organic substances. At the same time, sample preparation methods are often much simpler than in spectrometry or gas chromatography. The advantage of the IEA method is (in contrast to other methods, for example, atomic adsorption spectrometry) the ability to “distinguish” free ions from their bound chemical forms, which is important both for assessing the physicochemical properties of the analyzed substances and from the point of view of biological control ( for example, when assessing the toxicity of water). Analysis time is sometimes reduced to several seconds by increasing the polarizing voltage sweep speed.

Potentiometry using various ion-selective electrodes is used in water analysis to determine a large number of inorganic cations and anions. The concentrations that can be determined in this way are 10 0 -10 -7 mol/l. Monitoring using ion-selective electrodes is simple, fast and allows for continuous measurements. Currently, ion-selective electrodes have been created that are sensitive to certain organic substances (for example, alkaloids), surfactants and detergents. Water analysis uses compact probe-type analyzers using modern ion-selective electrodes. In this case, a circuit that processes the response and a display are mounted in the probe handle.

Conductometry used in the operation of detergent analyzers in wastewater, in determining the concentrations of synthetic fertilizers in irrigation systems, in assessing quality drinking water. In addition to direct conductometry, indirect methods can be used to determine some types of pollutants, in which the substances being determined are reacted with specially selected reagents before measurement and the recorded change in electrical conductivity is caused only by the presence of the corresponding reaction products. In addition to the classical variants of conductometry, its high-frequency version (oscilloscopy) is also used, in which the indicator electrode system is implemented in continuous conductometric analyzers.

Chapter 3. Devices based on electrochemical methods of analysis

The voltammetric method of analysis today is considered one of the most promising among electrochemical methods, due to its wide capabilities and good performance characteristics.

Modern stripping voltammetry, which has replaced classical polarography, is a highly sensitive and rapid method for determining a wide range of inorganic and organic substances with redox properties.

This is one of the most universal methods for determining trace amounts of substances, which is successfully used for the analysis of natural geo- and biological, as well as medical, pharmaceutical and other objects.

Voltammetric analyzers make it possible to simultaneously determine several components (up to 4 - 5) in one sample with a fairly high sensitivity of 10 -8 - 10 -2 M (and stripping voltammetry - up to 10-10 - 10 -9 M).

Adsorption stripping voltammetry, based on preliminary adsorption concentration of the element being determined on the surface of the electrode and subsequent recording of the voltammogram of the resulting product, is considered the most promising in analytical chemistry today. In this way, it is possible to concentrate many organic substances, as well as metal ions in the form of complexes with organic ligands (especially nitrogen- and sulfur-containing ones). With a sequential accumulation time of 60 s and using a differential pulse mode for recording a voltammogram, it is possible to achieve detection limits at the level of 10 -10 - 10 -11 mol/l (10 -8 - 10 -9 g/l or 0.01 - 0.001 μg/dm 3 ).

Voltammetric complex for metal analysis "IVA - 400MK" (NPKF "Akvilon", Moscow) designed for the analysis of 30 elements (Cu, Zn, Pb, Cd, As, Co, Ni, Cr, and other metals), sensitivity 0.1 - 10 -3 μg/dm 3.

Voltammetric analyzer with UV irradiation of samples - TA-1M (Tomsk), which, in addition to metal ions, allows you to determine a number of organic compounds. The device is characterized by the following features:

· simultaneous analysis in three electrochemical cells,

· small sample (0.1 - 1.0 g),

· low cost of sample preparation and analysis.

In St. Pereburg NFT “Volta” produces the ABC-1 voltammetric complex with a rotating disk glassy carbon electrode, which allows for the analysis of toxic elements in water, food products and various materials. The detection limit without sample concentration is: 0.1 mg/l for Pb, 0.5 mg/l for Cd, 1.0 μg/l for Cu. The sample volume is 20 ml, the time to obtain a current-voltage curve is no more than 3 minutes.

"AZHE - 12" (Vladikavkaz) is intended for express analysis of the ionic composition of waste and circulating waters. The analyzer uses a traditional mercury electrode. Controlled components - Cu, Zn, Pb, Cd, In, Bi, Tl, Sb, As, Co, Ni, Cr, CN -, Cl -, S 2-. The analyzer allows you to carry out measurements without sample preparation.

"Ecotest-VA" ("Ekoniks", Moscow) - portable voltammetric analyzer. It is made on a modern microprocessor element base and is equipped with a whole complex of electrodes - graphite, glassy carbon, microelectrodes made of precious metals and a mercury dripping electrode.

Instruments of this series are intended for the determination of metals Cu, Zn, Pb, Cd, As, Bi, Mn, Co, Ni, Cr, as well as acetaldehyde, furfural, caprolactam and other substances in samples of drinking, natural, waste water, soil, and after appropriate sample preparation - in food and feed.

The capabilities of many analytical methods for water analysis can be significantly expanded when using flow-injection concentrating attachments operating in automatic mode - for example, the BPI-M and BPI-N types - in the sample preparation process.

BPI-M - designed for automated sample preparation, it includes microcolumns with highly efficient sorbents. The unit's productivity is 30-60 analyzes per day with full automation of the process. The use of the block allows you to increase sensitivity by 20 times per minute of concentration. The unit works best in combination with atomic absorption detection, as well as X-ray fluorescence, atomic absorption and electrochemical methods.

BPI-N- designed for concentrating metal ions on selective sorbents simultaneously in four microcolumns with DETATE sorbent or on 4 thin-layer sorption DETATE filters. It can be used with X-ray fluorescence, atomic absorption, atomic emission, and electrochemical methods.

Analyzers based on voltammetry

Instruments based on the principle of inverse voltammetry are used in Lately special demand. They combine selectivity and high sensitivity with ease of analysis.

With regard to the determination of elemental composition (for example, for heavy metals), these devices successfully compete with atomic absorption spectrophotometers, since they are not inferior to them in sensitivity, but are much more compact and cheaper (about 5 - 10 times). They do not require additional consumables, and also provide the opportunity for simultaneous rapid determination of several elements.

Polarograph ABC - 1.1 (NTF "Volta" St. Petersburg).

The detection limits for metals without sample concentration are (mg/l): Cd, Pb, Bi - 0.0001, Hg - 0.00015, Cu - 0.0005, Zn, Ni - 0.01. Cost $1700.

Analyzers based on the conductometric principle are designed to quantitatively determine the total salt content in water. "EKA-2M" (St. Petersburg) measures salt content in a wide range of values from 0.05 to 1000 µS/cm ($900). “ANION”, “MARK”, KSL (from 330 to 900 $), COD - analyzers ($750).

Gas analyzers for harmful substances

An automatic gas analyzer is a device in which air sampling, determination of the amount of the controlled component, issuance and recording of analysis results are carried out automatically according to a given program without operator participation. To control the air environment, gas analyzers are used, the operation of which is based on various principles.

Thermoconductometric gas analyzers.

The operating principle is based on the dependence of the thermal conductivity of the gas mixture on its composition. The sensitive element of this type of analyzers is thin platinum filaments. Depending on the composition of the gas, the temperature of the sensitive element changes, and a current arises, the strength of which is proportional to the concentration of the controlled component.

Coulometric gas analyzers.

The operating principle is based on measuring the limiting electric current that occurs during the electrolysis of a solution that contains the substance being determined, which is an electrochemical depolarizer. The mixture to be analyzed, containing, for example, sulfur dioxide, is fed into an electrochemical cell. It reacts with iodine to form hydrogen sulfide, which is then electrooxidized at the measuring electrode. Electric current is a measure of the concentration of the component being determined.

CHAPTER 4. OVERVIEWWEB– SITES OF COMPANIES SELLING CHEMICAL ANALYTICAL EQUIPMENT

"AGILENT.RU"

Modern testing, measuring and monitoring equipment for the development, manufacture and implementation of new electronic devices and technologies...

http://www.agilent.ru

"ACADEMLINE", JSC, Moscow

Supplies a wide range of measuring chemical and analytical equipment...

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"AKTAKOM"

The registered trademark AKTAKOM combines a wide range of world-class instrumentation and control equipment. All the best from foreign and domestic manufacturers...

http://www.aktakom.ru

"ANALITPRIBOR"

Offers gas analyzers

http://www.analytpribor.ru

"WATSON", JSC, Mytishchi, Moscow region.

Instruments and measuring instruments;

http://www.watson.ru/

"DIPOL", NPF, St. Petersburg

http://www.dipaul.ru/

"EuroLab SPb", LLC, St. Petersburg

Spectral analysis devices, chromatographs.

http://www.eurolab.ru

"IZME.RU"

http://www.izme.ru/

"INSOVT", JSC

Development and production of gas analyzers

http://www.insovt.ru

"Institute of Information Technologies", Minsk, Belarus

Specializes in the development and production of measuring instruments for fiber optics...

"KIPARIS", LLC, St. Petersburg

http://www.kiparis.spb.ru/

"CONTINENT", Gomel

http://www.continent.h1.ru

"Instrumentation and equipment", Volgograd

http://www.oscilloscop.ru

"Kontur", ITC, LLC, Novosibirsk

http://www.kip.ru/

"KraiSibStroy", LLC, Krasnoyarsk

http://www.kipkr.ru/

"Christmas+", JSC, St. Petersburg

http://www.christmas-plus.ru

"KURS", LLC, St. Petersburg

http://www.kypc.spb.ru

"LUMEX", St. Petersburg

http://www.lumex.ru/

"METTEC"

http://www.mettek.ru

"METTLER TOLEDO"

http://www.mt.com

"MONITORING", STC, St. Petersburg

http://www.monitoring.vniim.ru

"Scientific Instruments", JSC, St. Petersburg

http://www.sinstr.ru

"NevaLab", JSC, St. Petersburg

http://www.nevalab.ru

"OVEN", PO, Moscow

http://www.owen.ru/

"OCTAVA+", Moscow

http://www.octava.ru/

"OPTEK", CJSC, St. Petersburg

Develops and produces gas analyzers and analytical systems for various purposes for use in ecology, industry and scientific research...

http://www.optec.ru

"POLITECHFORM", Moscow

http://www.ptfm.ru

"Praktik-NTs", JSC, Moscow, Zelenograd

http://www.pnc.ru/

"INSTRUMENTS AND ANALYTICAL TECHNOLOGY"

Instruments for chemical analysis.

http://www.zhdanov.ru/

"Sartogosm", JSC, St. Petersburg

http://www.sartogosm.ru

"Special", JSC, Moscow

http://www.special.ru

"TKA"

http://www.tka.spb.ru/

"TST", CJSC, St. Petersburg

http://www.tst-spb.ru

"ECOPRIBOR", NPO, Moscow

Offers gas analyzers and gas analysis systems...

http://ecopribor.ru

"ECOTECH", SME, Ukraine

http://ecotech.dn.ua

"ECOTECHINVEST", NPF, Moscow

http://ecotechinvest.webzone.ru

"Axis", JSC, Moscow, Zelenograd

http://www.eksis.ru/

"ELIX"

http://www.eliks.ru/

"EMI", LLC, St. Petersburg

Production of optical gas analyzers, petroleum product analyzers.

http://www.igm.spb.ru

"ENERGOTEST", JSC, Moscow

http://www.energotest.ru, http://www.eneffect.ru

HIMMED

Analytical Instruments and Chromatography

e-mail:[email protected]

LITERATURE

1. Geyrovsky J., Kuta J., Fundamentals of polarography, trans. from Czech., M., 1965;

2. Gal yus 3., Theoretical basis electrochemical analysis, trans. from Polish, M., 1974;

3. Kaplan B. Ya., Pulse polarography, M., 1978;

4. Brainina X. Z., Neiman E. Ya., Solid-phase reactions in electroanalytical chemistry, M., 1982;

5. Kaplan B. Ya., Pats R. G., Salikhdzhanova R. M.-F., AC voltammetry, M., 1985.

6. Plambeck J. Electrochemical methods of analysis. / Per. from English M.: Mir, 1985. 496 p.

7. Brief chemical encyclopedia. M.: Soviet Encyclopedia, 1964. Volume 1. A–E. 758 p.

8. Classification and nomenclature of electrochemical methods // Journal. analyte chemistry. 1978. T. 33, no. 8. pp. 1647–1665.

9. Recommended Terms, Symbols and Definitions for Electroanalytical Chemistry // Pure & Appl. Chem. 1979. Vol. 51. P. 1159–1174.

10. On the use of the concept of “chemical equivalent” and related quantities: Zhurn. analyte chemistry. 1989. T. 44, no. 4. pp. 762–764; Journal analyte chemistry. 1982. T. 37, no. 5. P. 946; Journal analyte chemistry. 1982. T. 37, no. 5. P. 947.

11. Neiman E.Ya. Terminology of modern analytical chemistry and its formation // Journal. analyte chemistry. 1991. T. 46, no. 2. pp. 393–405.

12. Presentation of the results of chemical analysis (IUPAC Recommendations 1994) // Journal. analyte chemistry. 1998. T. 53. No. 9. pp. 999–1008.

13. Compendium of Analytical Nomenclature (Definitive Rules 1997). 3rd ed., IUPAC, Blackwell Science, 1998. 8.1–8.51 (Electrochemical Analysis).

Send your good work in the knowledge base is simple. Use the form below

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Posted on http://www.allbest.ru

Ministry of Education and Science of the Russian Federation

Federal State Budgetary Educational Institution

higher education

"Irkutsk National Research Technical University"

Department of Metallurgy of Non-Ferrous Metals

(name of the department)

"Electrochemical research methods"

Abstract on the discipline

“Physico-chemical methods for studying metallurgical processes”

Completed by a student from group MCM-16-1

Zakharenkov R.I.

Checked by the teacher of the MCM department

Kuzmina M.Yu.

Irkutsk 2017

INTRODUCTION

Electrochemistry is a branch of physical chemistry that considers systems containing ions (solutions or melts of electrolytes) and processes occurring at the boundary of two phases with the participation of charged particles.

The first ideas about the relationship between chemical and electrical phenomena were known in the 18th century, since a huge number of physical and chemical experiments were carried out with electric and lightning discharges, with charges located in Leyden jars, but all of them were random in nature due to the lack of constant powerful source of electrical energy. The origin of electrochemistry is associated with the names of L. Galvani and A. Volta. While studying the physiological functions of the frog, Galvani accidentally created an electrochemical circuit. It consisted of two different metals and a prepared frog's leg. The paw was both an electrolyte and an indicator of electric current, but the conclusion was given incorrectly, i.e., according to Galvani, this electric current that arose in the circuit was of animal origin, i.e., it was associated with the functional characteristics of the frog’s body (theory “ animal electricity").

The correct interpretation of Galvani's experiments was given by A. Volta. He created the first battery of galvanic cells - a voltaic column. The battery cells consisted of copper and zinc disks, and the electrolyte was a sponge material soaked in salt water or acid. It was this connection that made it possible to obtain electric current. Soon, through the works of the great scientists A. Volta, J. Daniel, B. S. Jacobi, P. R. Bagration, G. Plante and others, powerful galvanic cells and batteries, easy to use, appeared. Then A. Volta developed a series of metal voltages. If two different metals are brought into contact and then separated, then using physical means, such as an electroscope, it can be seen that one metal has acquired a positive charge and the other a negative one. This series of metals, in which each preceding metal is charged positively, but after contact with any subsequent one, i.e. the Volta series, turned out to be similar to the voltage series.

Next, in early XIX century, electrolysis was developed, and M. Faraday established the quantitative laws of electrolysis. Scientists made a great contribution to the development of electrochemistry: S. A. Arrhenius, V. F. Ostwald, R. A. Colley, P. Debye, W. Nernst, G. Helmholtz, etc. Now electrochemistry is divided into theoretical and applied. Through the use of electrochemical methods, it is connected with other branches of physical chemistry, as well as with analytical chemistry and other sciences.

electrochemical potentiometry conductometry coulometry

1 . ELECTROCHEMICAL METHODS OF RESEARCH

The need to use a variety of methods to study electrochemical processes is due to the wide range of variations in the rate of electron transfer in electrode reactions. Each of the methods has a certain limit on the determined value of the exchange current density, above which the electrochemical parameters of the electrode reaction cannot be determined. In relation to each specific object, it is necessary to choose the method that provides the maximum amount of reliable information. When conducting electrochemical studies, it is necessary to know the chemical composition of the starting substances and reaction products. To determine the composition of the electrolyte, various physical and chemical methods are used: spectrophotometric, potentiometric, analytical and others. When conducting electrochemical studies, the following conditions must be observed.

1. Maximum purity of the reagents used; the composition of the electrodes must be strictly known, as well as the condition of their surfaces. Care should be taken to ensure that the surface of the electrodes does not undergo changes during the measurement process.

2. The design of the electrochemical cell and the electrodes located in it must ensure uniform distribution of current over the entire surface of the working electrode.

3. Measurements should be carried out at strictly controlled temperatures.

4. Maintain constant pressure and composition of the gas phase above the electrolyte. As a rule, studies are carried out in an inert gas environment (N 2, Ar, Ne, He H 2), since oxygen in the gas phase can have a significant effect on the mechanism of the process.

5. It is necessary to ensure such experimental conditions under which the potential drop in the diffuse part of the electrical double layer would be minimal or precisely known. To reduce this potential, as a rule, a background electrolyte is used, the concentration of which should be no less than 20 times higher than that of the main substance. However, you must first make sure that the background electrolyte does not distort the polarization curve of the reaction being studied.

6. Accurate measurement of working electrode potential. To do this, it is necessary to eliminate the diffusion potential between the electrolyte under study and the electrolyte of the reference electrode. This potential takes on its maximum value when approaching the limiting current and can significantly distort the measurement results. To eliminate the diffusion potential between the electrolyte under study and the electrolyte of the reference electrode, it is desirable: a) select a reference electrode that has the same electrolyte composition as the one being studied. For example, when researching in chloride solutions, it is convenient to use silver chloride, calomel, and chlorine electrodes; in acidic sulfate solutions - mercury-sulfate electrodes, etc.; b) use a reference electrode with an electrolyte at the boundary of which with the electrolyte under study the diffusion potential can be calculated using known formulas.

When measuring in solutions with a constant ionic strength, and at high background concentrations - with a constant ionic concentration, you can, in principle, use any reference electrode. The diffusion potential in this case can be very large, but also constant - it can be calculated or determined experimentally.

In all cases of studying the kinetics of electrochemical processes, it is necessary to measure the current density. Usually they start by finding out using analytical chemistry and coulometry methods to determine whether only one reaction under study occurs at the electrode or whether it is complicated by side reactions. In the case of side reactions, it is necessary to find out what proportion of the current is due only to the implementation of the reaction being studied (construct the so-called partial polarization characteristic for the reaction being studied).

The mechanism of the electrode reaction can be most simply interpreted only in the case when the starting substance is converted into one product with 100% current efficiency. Checking the reaction for compliance with Faraday's law or performing coulometric measurements allows you to simultaneously determine the number of electrons participating in the total electrode reaction. Knowledge of the composition starting material and the reaction product, as well as the total number of electrons transferred, makes it possible to write down the equation of the total electrode reaction.

The next step in studying the mechanism of the electrode reaction is to find out which stage is limiting.

If the limiting stage is the discharge-ionization stage, and all others proceed reversibly, then the main kinetic parameters of the process can be determined graphically or analytically by applying the equations of the slow discharge theory to the polarization characteristics.

1.1 Electrochemical methods of analysis

Electrochemical methodsanalysis- this is a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the medium under study or at the interface and associated with changes in structure, chemical composition or analyte concentration.

There are direct and indirect electrochemical methods. Direct methods use the dependence of the current strength (potential, etc.) on the concentration of the component being determined. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of titration of the analyte with a suitable titrant, i.e. The dependence of the measured parameter on the titrant volume is used.

For any kind of electrochemical measurements, an electrochemical circuit or electrochemical cell is required, of which the analyzed solution is an integral part.

Electrochemical methods are classified depending on the type of phenomena measured during the analysis process. There are two groups of electrochemical methods:

1. Methods without imposing extraneous potential, based on measuring the potential difference that occurs in an electrochemical cell consisting of an electrode and a vessel with the test solution. This group of methods is called potentiometric. Potentiometric methods use the dependence of the equilibrium potential of the electrodes on the concentration of ions participating in the electrochemical reaction on the electrodes.

2. Methods with the imposition of extraneous potential, based on measurement:

a) Electrical conductivity of solutions? conductometry;

b) The amount of electricity passing through the solution? coulometry;

c) Dependence of the current on the applied potential? volt-amperometry;

d) The time required for the electrochemical reaction to occur - chronoelectrochemical methods(chronovoltammetry, chronoconductometry).

In the methods of this group, an extraneous potential is applied to the electrodes of the electrochemical cell.

The main element of instruments for electrochemical analysis is the electrochemical cell. In methods without imposing extraneous potential, it is galvanic cell, in which an electric current occurs due to chemical redox reactions. In a cell such as a galvanic cell, two electrodes are in contact with the analyzed solution - an indicator electrode, the potential of which depends on the concentration of the substance, and an electrode with a constant potential - a reference electrode, against which the potential of the indicator electrode is measured. The potential difference is measured using special devices - potentiometers.

In methods with the imposition of extraneous potential, electrochemical cell, so named because at the electrodes of the cell, under the influence of an applied potential, electrolysis occurs - the oxidation or reduction of a substance. In conductometric analysis, a conductometric cell is used in which the electrical conductivity of a solution is measured. According to the method of application, electrochemical methods can be classified into direct ones, in which the concentration of substances is measured according to the readings of the device, and electrochemical titration, where the indication of the equivalence point is recorded using electrochemical measurements. In accordance with this classification, potentiometry and potentiometric titration, conductometry and conductometric titration, etc. are distinguished.

Instruments for electrochemical determinations, in addition to the electrochemical cell, stirrer, and load resistance, include devices for measuring potential difference, current, solution resistance, and amount of electricity. These measurements can be carried out with pointer instruments (voltmeter or microammeter), oscilloscopes, and automatic recording potentiometers. If the electrical signal from the cell is very weak, then it is amplified using radio amplifiers. In devices of methods with the imposition of extraneous potential, an important part is the device for supplying the cell with the appropriate potential of stabilized direct or alternating current (depending on the type of method). The power supply unit for electrochemical analysis devices usually includes a rectifier and a voltage stabilizer, which ensures constant operation of the device.

1.2 Potentiometry

Potentiometry is based on measuring the difference in electrical potentials that arises between dissimilar electrodes immersed in a solution with the substance being determined. Electric potential arises at the electrodes when a redox (electrochemical) reaction occurs on them. Redox reactions occur between an oxidizing agent and a reducing agent with the formation of redox pairs, the potential E of which is determined by the Nernst equation by the concentrations of the pair components [ok] and [rec]:

Where E°- standard electrode potential, V;

n- the number of electrons participating in the process.

Potentiometric measurements are carried out by lowering two electrodes into the solution - an indicator electrode, which reacts to the concentration of the ions being determined, and a standard or reference electrode, against which the potential of the indicator is measured. Several types of indicator and standard electrodes are used.

Electrodes of the first kind reversible with respect to the metal ions of which the electrode consists. When such an electrode is lowered into a solution containing metal cations, an electrode pair is formed: M n + /M.

Electrodes of the second kind sensitive to anions and are metal M coated with a layer of its insoluble salt MA with an anion A- to which the electrode is sensitive. When such an electrode comes into contact with a solution containing the specified anion A-, potential E arises, the value of which depends on the product of salt solubility

ETC M.A. and anion concentration [ A-] in solution.

Electrodes of the second type are silver chloride and calomel. Saturated silver chloride and calomel electrodes maintain a constant potential and are used as reference electrodes against which the potential of the indicator electrode is measured.

Inert electrodes- a plate or wire made of difficult-to-oxidize metals - platinum, gold, palladium. They are used to measure E in solutions containing a redox couple (for example, Fe 3+ /Fe 2+).

Membrane electrodes different types have a membrane on which membrane potential E arises. The value of E depends on the difference in concentrations of the same ion on different sides of the membrane. The simplest and most commonly used membrane electrode is the glass electrode.

Mixing insoluble salts type AgBr, AgCl, AgI and others with some plastics (rubbers, polyethylene, polystyrene) led to the creation ion selective electrodes on Br-, Cl-, I-, selectively adsorbing the indicated ions from solution due to the Paneth - Faience - Hahn rule. Since the concentration of detectable ions outside the electrode differs from that inside the electrode, the equilibria on the membrane surfaces are different, which leads to the appearance of a membrane potential.

To carry out potentiometric determinations, an electrochemical cell is assembled from an indicator reference electrode, which is immersed in the solution being analyzed and connected to a potentiometer. The electrodes used in potentiometry have a high internal resistance (500-1000 MOhm), so there are types of potentiometers that are complex electronic high-resistance voltmeters. To measure the EMF of the electrode system in potentiometers, a compensation circuit is used to reduce the current in the cell circuit.

Most often, potentiometers are used for direct measurements of pH, indicators of the concentrations of other ions pNa, pK, pNH?, pCl and mV. Measurements are carried out using appropriate ion-selective electrodes.

To measure pH, a glass electrode and a reference electrode - silver chloride - are used. Before carrying out analyses, it is necessary to check the calibration of pH meters using standard buffer solutions, the fixation of which is attached to the device.

pH meters in addition to direct determinations pH, pNa, pK, pNH?, pCl and others allow potentiometric titration of the ion being determined.

1.3 Potentiometric titration

Potentiometric titration is carried out in cases where chemical indicators cannot be used or when a suitable indicator is not available.

In potentiometric titration, potentiometer electrodes placed in the titrated solution are used as indicators. In this case, electrodes are used that are sensitive to titrated ions. During the titration process, the ion concentration changes, which is recorded on the measuring scale of the potentiometer. Having recorded the potentiometer readings in pH or mV units, plot their dependence on the titrant volume (titration curve), determine the equivalence point and the volume of titrant consumed for titration. Based on the data obtained, a potentiometric titration curve is constructed.

The potentiometric titration curve has a form similar to the titration curve in titrimetric analysis. The titration curve is used to determine the equivalence point, which is located in the middle of the titration jump. To do this, tangents are drawn to sections of the titration curve and the equivalence point is determined in the middle of the tangent of the titration jump. Highest value changes ? pH/?V acquires at the point of equivalence.

The equivalence point can be determined even more accurately by Gran’s method, which is used to construct the dependence ? V/?E from the titrant volume. Using the Gran method, potentiometric titration can be carried out without bringing it to the equivalence point.

Potentiometric titration is used in all cases of titrimetric analysis.

Acid-base titration uses a glass electrode and a reference electrode. Since the glass electrode is sensitive to changes in the pH of the medium, when they are titrated, changes in the pH of the medium are recorded on the potentiometer. Acid-base potentiometric titration is successfully used in the titration of weak acids and bases (pK?8). When titrating mixtures of acids, it is necessary that their pK differ by more than 4 units, otherwise part of the weaker acid is titrated together with the strong one, and the titration jump is not clearly expressed.

This allows the use of potentiometry to construct experimental titration curves, select indicators for titration and determine acidity and basicity constants.

In precipitation potentiometric titration, an electrode made of a metal that forms an electrode pair with the ions being determined is used as an indicator.

When complexometric titration is used: a) a metal electrode reversible to the ion of the metal being determined; b) a platinum electrode in the presence of a redox couple in the solution. When one of the components of the redox couple is bound by the titrant, its concentration changes, which causes changes in the potential of the indicator platinum electrode. Back titration of an excess EDTA solution added to a metal salt with a solution of an iron (III) salt is also used.

For redox titration, a reference electrode and a platinum indicator electrode, sensitive to redox couples, are used.

Potentiometric titration is one of the most used methods of instrumental analysis due to its simplicity, accessibility, selectivity and wide capabilities.

1.4 Conductometry. Conductometric titration

Conductometry is based on measuring the electrical conductivity of a solution. If two electrodes are placed in a solution of a substance and a potential difference is applied to the electrodes, an electric current will flow through the solution. Like every conductor of electricity, solutions are characterized by resistance R and its inverse quantity - electrical conductivity L:

Where R- resistance, Ohm;

Specific resistance, Ohm. cm;

S - surface area, cm 2.

Where L - electrical conductivity, Ohm-1;

R- resistance, Ohm.

Conductometric analysis is carried out using conductometers - devices that measure the resistance of solutions. By resistance value R determine the electrical conductivity of solutions that is its inverse value L.

The concentration of solutions is determined by direct conductometry and conductometric titration. Direct conductometry used to determine the concentration of a solution using a calibration graph. To create a calibration graph, the electrical conductivity of a series of solutions with a known concentration is measured and a calibration graph of the electrical conductivity versus concentration is constructed. Then the electrical conductivity of the analyzed solution is measured and its concentration is determined from the graph.

More often used conductometric titration. In this case, the analyzed solution is placed in a cell with electrodes, the cell is placed on a magnetic stirrer and titrated with the appropriate titrant. Titrant is added in equal portions. After adding each portion of titrant, measure the electrical conductivity of the solution and plot the relationship between electrical conductivity and titrant volume. When a titrant is added, a change in the electrical conductivity of the solution occurs. the titration curve inflects.

From n the mobility of ions depends on the electric conductivity of the solution: the higher the mobility b ions, the greater the electrical conductivity of the solution.

Conductometric titration has several advantages. It can be carried out in turbid and colored environments, in the absence of chemical indicators. The method has increased sensitivity and allows you to analyze dilute solutions of substances (up to 10-4 mol/dmі). Mixtures of substances are analyzed by conductometric titration, because the differences in the mobility of the various ions are significant and can be titrated differentially in the presence of each other.

Conductometric analysis can be easily automated if the titrant solution is supplied from a burette at a constant speed, and the change in the electrical conductivity of the solution is recorded on a recorder.

This type of conductometry is called chrono- conductometric analysis.

In acid-base titration, strong acids, weak acids, salts of weak bases and strong acids can be determined by conductometry.

IN precipitative conductometrictitration the electrical conductivity of the titrated solutions first decreases or remains at a certain constant level due to the binding of the titrated electrolyte into a precipitate, after i.e. when an excess of titrant appears, it increases again.

IN complexmetric conductometric titration changes in the electrical conductivity of the solution occur due to the binding of metal cations into a complex with EDTA.

Redox conductometrictitro- tion based on a change in the concentration of reacting ions and the appearance of new ions in the solution, which changes the electrical conductivity of the solution.

IN last years developed high frequency conductometry, in which the electrodes do not contact the solution, which is important when analyzing aggressive media and solutions in closed vessels.

Two options have been developed - straighthigh-frequency conductometry and high-frequency titration.

Direct high-frequency conductometry is used to determine the moisture content of substances, grain, wood, the concentration of solutions in closed vessels - ampoules, and when analyzing aggressive liquids.

High-frequency titration is carried out on special titrators - TV-6, TV-6L.

High-frequency conductometric titrations are carried out as acid-base, redox or precipitation titrations in cases where a suitable indicator is not available or when analyzing mixtures of substances.

1.5 Coulometry. Coulometric titration

In coulometry, substances are determined by measuring the amount of electricity spent on their quantitative electrochemical transformation. Coulometric analysis is carried out in an electrolytic cell into which a solution of the substance being determined is placed. When an appropriate potential is applied to the electrodes of the cell, electrochemical reduction or oxidation of the substance occurs. According to the laws of electrolysis, discovered by Faraday, the amount of substance reacted at the electrode is proportional to the amount of electricity passing through the solution:

Where g- mass of released substance, g;

n- the number of electrons transferred in the electrode process;

F- Faraday number (F = 96485 C/mol);

I- current strength, A;

t- time, s;

M- molar mass of the released substance, g/mol.

Coulometric analysis makes it possible to determine substances that do not deposit on electrodes or escape into the atmosphere during an electrochemical reaction.

There are direct coulometry and coulometric titration.. The high accuracy and sensitivity of methods for measuring electric current provides coulometric analysis with a unique accuracy of 0.1-0.001%, and sensitivity up to 1 10 -8? 1 10 -10 g. Therefore, coulometric analysis is used to determine microimpurities and destruction products of substances, which is important when monitoring their quality.

To indicate i.e. When coulometric titration, chemical and instrumental methods can be used - adding indicators, detecting colored compounds photometrically or spectrophotometrically.

Unlike other methods of analysis, coulometry can be fully automated, which minimizes random determination errors. This feature was used to create automatic coulometric titrators - sensitive instruments used for particularly accurate analyzes when other methods are not sensitive enough. When analyzing substances that are poorly soluble in water, coulometry can be carried out on acetylene black electrodes, which are a good adsorbent and remove such substances from the reaction medium with sufficient completeness. Coulometric titration is a promising method of instrumental analysis. It can find wide application for solving a number of special analytical problems - analysis of impurities, small quantities medicines, determination of toxic substances, trace elements and other compounds in biological material and the environment.

CONCLUSION

The work provides a review of the main electrochemical research methods, describing in detail their principle, application, advantages and disadvantages.

Electrochemical methods of analysis are a group of methods of quantitative chemical analysis based on the use of electrolysis.

Varieties of the method are electrogravimetric analysis (electroanalysis), internal electrolysis, contact exchange of metals (cementation), polarographic analysis, coulometry, etc. In particular, electrogravimetric analysis is based on weighing the substance released on one of the electrodes. The method allows not only to carry out quantitative determinations of copper, nickel, lead, etc., but also to separate mixtures of substances.

In addition, electrochemical methods of analysis include methods based on measuring electrical conductivity (conductometry) or electrode potential (potentiometry). Some electrochemical methods are used to find the end point of a titration (amperometric titration, conductometric titration, potentiometric titration, coulometric titration).

LIST OF REFERENCES USED

1. Fundamentals of modern electrochemical analysis. Budnikov G.K., Maistrenko V.N., Vyaselev M.R., M., Mir, 2003.

2. J. Plambeck, ed. S. G. Mayranovsky Electrochemical methods of analysis. Fundamentals of theory and application: trans. from English / Vidannya: Mir, 1985.

3. Damaskin B.B., Petriy O.A., Tsirlina G.A. Electrochemistry - M.: chemistry, 2001. 624 p.

4. STO 005-2015. Quality Management System. Educational and methodological activities. Decor course projects(works) and final qualifying works of technical specialties.

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