PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV, HOLDER OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR

Vol. 257　　　　　　　　　　　　　　　　　　　　　　　　　1973

ON THE CLASSIFICATION OF CURRENT–VOLTAGE METHODS

AND THEIR FEATURES

A. T. Stromberg

The literature lacks a sufficiently complete and rigorous classification of current–voltage methods and their features. In the present work an attempt is made to classify current–voltage methods in a way that would satisfy, to a sufficient degree, the requirements of completeness and purity, and would be based on the rules of logic (the rules for dividing the scope of a concept) [1].

By the term “current–voltage methods” (CVM) we mean electrochemical methods in which the measured property of the electrochemical system is the nonequilibrium potential during the passage of current or, conversely, the current at a given nonequilibrium potential.

The proposed natural classification of current–voltage methods is based on phenomena at the solution–electrode interface, and the most essential factors (features) that influence the character of the phenomena occurring at this interface are considered as the basis for the division. In determining which features are essential, we were guided by sociohistorical practice and by our personal experience in the field of current–voltage methods of analysis and investigation. We have considered 12 features (levels), which are distributed among four groups (stages).

Each subsequent feature (level) is a further detailing of current–voltage methods and their features. It is advisable to form the rational name of a CVM variant and of its features by appending to the basic root of the word (CVM) the abbreviated names of the various classification features in the order in which the levels are arranged¹).

¹) The present work does not address the question of expedient practical terminology for the various variants of CVM and their features.

The first stage includes four levels associated with the characteristics of the current and potential used in the measurements.

1st level. According to the type of current used, DC CVM (CVM DC) and AC CVM (CVM-AC) are distinguished.

2nd level. According to the manner in which current or potential is varied, DC CVM are divided into variants: potentiostatic (PS)\(^2\); with a potential varying linearly with time (LP); with a potential varying nonlinearly (NLP); galvanostatic (GS); with a current varying linearly (LC); with a current varying nonlinearly (NLC).

According to the form of the alternating current, AC CVM should be divided into CVM with sinusoidal alternating current (SAC); CVM with square-wave current (SWC); CVM with triangular-wave current (TWC), and other forms of alternating current. These variants of CVM are sometimes used without additional devices in electrochemical measurements, but because of the large capacitive current their use at low concentrations of the substance in solution is difficult.

3rd level. According to the features of signal recording, for the six CVM-DC variants considered at the second level, the following cases are distinguished at the 3rd level: recording with a pen recorder (PR); oscillographic recording (OR).

For sinusoidal alternating current, the following signal-recording features are distinguished at the 3rd level: recording of the complete signal (RCS); recording of the active component—vector polarography (VAP); recording of the second harmonic—second-harmonic AC polarography (SH); recording of the low-frequency component of a modulated high-frequency current—high-frequency polarography (HFP) [2].

For square-wave current, the following are distinguished: recording at the end of the half-period—square-wave polarography (SWP); recording

\(^2\) In the literature the terms chronogalvanometric (for the potentiostatic method) and chronopotentiometric (for the galvanostatic method) are also used. These names emphasize the nature of the variation of the measured quantity, rather than of the potential or current applied to the cell. In the present article the second method of naming the CVM variants is used.

of selected pulses at the end of the half-period—pulse polarography (IMP). For triangular-wave alternating current, division at the 3rd level is still absent.

4th level. According to the characteristics of recording the transformed signal, for each of the 19 variants of CVM considered at the 3rd level, one should distinguish at the 4th level: recording of the untransformed (primary) signal (RNS); recording of the time derivative of the signal—differential methods (DIF); recording of the difference between signals at two electrodes—difference methods (RZN).

In all, after the first stage (after four levels), 57 variants of CVM are obtained, differing in the characteristics of the current used and in the method of recording.

A rational notation for a CVM variant after the 1st stage,

CVM – PER – SNV – IMP – DIF

denotes pulse polarography with differential polarogram. The notation CVM – PST – IMP – OSC – RZN denotes direct-current polarography with linearly varying potential, with oscillographic recording using a difference circuit.

The basic classification of CVM according to the nature of the current and apparatus used (the nature of the external current, the characteristics of signal recording) ends at the first stage. However, it is expedient to continue the classification of the features of the CVM variants identified at the first stage in the next three stages and eight levels.

The second stage includes three levels (5, 6, 7), which characterize the features of the electrochemical system under study, i.e., the nature of the substance considered at the electrode and the solvent. Each of the 57 CVM variants (CVM-I) obtained at the 1st stage is distinguished at the three levels of the 2nd stage by the following features:^[1)]

5th level. According to the direction of the substance flux (mass transfer), a direct method (CVM-I-PRM) and a reverse method (CVM-I-OBT) are distinguished. By the term “direct” it has become customary to denote the flux of substance from solution to the electrode; “reverse”—from the electrode into solution. The electrode process may in each of these cases be anodic or cathodic.

1) By the term CVM-I is meant any of the 57 CVM variants according to the classification at the 1st stage.

depending on the nature of the substance (cation; anion).

6th level. According to the aggregate state of the substance at the electrode, two cases are distinguished: when the substance under investigation at the electrode is in the form of a liquid solution of the substance in the electrode (for example, in the form of an amalgam or a liquid alloy, LLS); or in the solid state (SES).

7th level. Depending on the nature of the solvent, the following are distinguished: aqueous solutions (AS); nonaqueous solutions (NAS); mixtures of molten salts (MMS). In all, there are 12 features at the three levels of the second stage.

The notation OVM—LLS—NAS after the name of one of the 57 variants of first-stage voltammetry (VAM-I) means that an inverse method is being considered (dissolution of the substance from the electrode), when the obtained substance participating in the electrode reaction is on the electrode in liquid form (liquid metal, alloy, amalgam), and a nonaqueous solution is used.

The third stage includes three levels (8, 9, 10), which characterize essential features of the electrochemical cell affecting the solution–electrode boundary. Each of the 57 variants of first-stage VAM, taking into account the 12 features of the second stage (a total of 684 variants and features of VAM-I, II), may additionally differ in the following features at the three levels of the third stage.

8th level. Depending on the aggregate state of the electrode, the following are distinguished: liquid electrode (LE) and solid electrode (SE).

9th level. According to the shape of the liquid electrode, the following are distinguished: spherical electrode (usually a mercury drop) (SPE); film electrode (usually a mercury film on a silver substrate) (FE). According to the features of the solid electrode, the following are distinguished: electrodes made of graphite or inert metal—graphite, platinum, etc. (GP); paste electrode (PE).

10th level. Depending on the character of the relative motion of the electrode and the solution, one should distinguish: electrodes without convective delivery of the substance to the electrode, i.e., a stationary electrode without stirring of the solution (ST); with convective delivery of the substance to the electrode or its removal from the electrode surface (CONV). Convection of the solution near the electrode surface may be produced both by motion of the electrode (vibration, rotation, in particular, a rotating disk electrode) and by motion of the solution (stirring of the solution with a stirrer, gas bubbles, circulation of the solution through a capillary with an electrode in a circulating electrolyzer, etc.). These features are not included in the classification under consideration.

differ, except for one case, because of its great practical importance. Among the various spherical liquid electrodes with convective stirring, we shall single out the dropping mercury electrode (DME), which is widely used in classical polarography.

The notation: SLE—CSR—DME—after one of the 684 variants of CVM-I, II means that an additional feature of this variant of CVM after the II-th stage is the use of a liquid mercury electrode in the form of a sphere as the dropping electrode. Thus, after the third stage, 9 additional features are obtained, and in all, after three stages at the 10th level, there are about 7000 variants and features of current–voltage methods.¹)

The fourth stage comprises two levels that characterize the mechanism of the electrode process. Each of the 7000 variants and features of CVM-I, II, III after the three stages considered above may differ further by the mechanism of the electrode process at the solution–electrode boundary.

II-th level. Depending on the nature of the principal limiting stages of the electrode process, one should distinguish: a reversible process, when the limiting stage is the delivery of substance to the surface or the removal of substance from the electrode surface (REV); a quasi-reversible process, when, along with delivery or removal of substance, the limiting stage is a two-sided electrochemical reaction (QVR); an irreversible process, when, along with delivery or removal of substance, the limiting stage is a one-sided electrochemical reaction (IRR).

¹) One more additional level (sublevel) may be included in the fourth stage, characterizing the dimensions of the electrode and the solution from the standpoint of diffusion in an unstirred medium, namely, finite and infinite dimensions of the medium. In this case, for a liquid electrode in an unstirred solution four additional variants are obtained; for a solid electrode in an unstirred solution and for a liquid electrode in a stirred solution, two additional variants are obtained.

12th level. Depending on the nature of the additional limiting stages that complicate the main electrode process (see level II), the following are distinguished: an uncomplicated process (UEP); a preceding chemical reaction (PCR) (in direct current-voltage methods this may be, for example, dissociation of complexes in solution, and in inverse current-voltage methods, dissociation of intermetallic compounds in mercury); a subsequent chemical reaction (SCR); adsorption of surface-active substances (SAS), and other additional stages. In all, there are 12 features of the electrode process at the two levels of the 4th stage.

According to the classification considered, for each of the 57 variants of CVM-I in the first stage there may be about 1300 features in the subsequent three stages. Altogether, for the 57 variants of CVM-I there are more than 70 thousand features. This means that the dependence of the useful signal on various factors can be expressed in the form of 70 thousand different regularities (mathematical equations).

The natural classification considered above makes it possible to clearly distinguish the different variants of current-voltage methods, taking their features into account. At the same time, it shows the commonality of the various variants of CVM. On the basis of this classification, it is possible to plan scientific research on the development of current-voltage methods, and also to plan the order of presentation of various questions of voltammetry in various kinds of textbooks and monographs.

The scientific, electrochemical classification presented here may also be supplemented by an auxiliary classification (stage) according to the features of the analytical application of CVM. This question will not be considered in the present article.

Conclusion

A sufficiently complete and clear natural (scientific) classification of current-voltage methods and their features is proposed, based on 12 characteristics (levels) distributed over four groups (stages).

References

1. B.S.E., 21, 362, 1953; 628, 1952.
2. B. S. Bruk. Polarographic Methods. Energiya Publishing House, Moscow–Leningrad, 1965.