Kinetic methods of analysis offer very convenient procedures for trace determination, especially when the analyte acts as a catalyst and the signal intensity may be controlled by substrate concentration. Usually, the reaction rate is monitored in an indirect way, most frequently by the spectrophotometric method.

Alternatively, the voltammetric methods show important advantages when the analyte-catalyst is involved in some way in the electrode process. This makes the transduction process more reliable and also enables more flexibility by performing in situ chemical and electrochemical manipulations of the analyte. In this respect, stripping voltammetry is particularly advantageous [1]. Maybe the best known applications of electrocatalysis in voltammetry are that involving a transition metal which may exist in two different oxidation states. Such a metal ion can act as a mediator in the electron transfer to a molecular substrate that is reduced in the direct way with a very high overvoltage. The resulting catalytic current is a function of the metal ion concentration whereas the regeneration of the analyte by a redox reaction with the excess substrate brings about an important signal enhancement. This contribution presents the application of some other electrocatalytic schemes in stripping voltammetry, namely (A) catalytic reduction of Ni²⁺ or Co²⁺ ; (B) catalytic hydrogen evolution. Both versions enable the determination of  organic or inorganic compounds that are not electroactive but display catalytic activity in one of the above electrode processes.
 

A. Catalytic reduction of Ni²⁺ or Co²⁺

A different catalytic scheme, that does not requires special redox properties of the analyte, was developed by this group starting from 1994 in conjunction with stripping voltammetry [2]. This new procedure relies on the properties of some organic compounds to catalyze the reduction of the Ni²⁺ ion on the mercury electrode. The actual electron acceptor in this process is a nickel-analyte complex compound that is reduced with a lower overvoltage compared with the [Ni(H₂O)₆]²⁺ form. The electrode process may be represented by the Scheme 1 where L stands for the ligand-analyte (the charge was omitted for simplicity).

Fig. 1 presents the curves recorded by differential pulse cathodic stripping voltammetry on the hanging mercury drop electrode for an electrode process of this type, involving mercaptopurine-riboside (MPR) as an analyte [3]. The deposition at a sufficiently positive potential, in the absence of nickel ion, results in the oxidation of mercury to Hg²⁺ followed by the formation of a sparingly soluble compound with the analyte. This is accumulated on the electrode surface and the reduction of Hg²⁺ in this compound during the cathodic scan gives the peak A on the curve 1. The peak B on the curve B results from the catalytic nickel reduction according to Scheme 1, whereas the peak C is due to the direct reduction of [Ni(H₂O)₆]²⁺.

In order to avoid the deviation of the calibration graph from the rectilinear shape, Ni²⁺ is added in large excess so as Ni²⁺ concentration gradient is negligible during the occurrence of the catalytic process. Consequently, the [NiL] complex forms in the solution phase.Both the pre-concentration process and the stripping step are dependent on the stability of this complex, as follows.

1. If the complex is very stable (log(beta) about 10 or higher) two different cases can be distinguished:

1.a) If the deposition potential is negative enough (e.g. –0.4 V vs. SCE) to keep very low the activity of Hg²⁺, the accumulation will occur only by the adsorption of the [NiL] complex. During the stripping step, the adsorbed [NiL] form enter the catalytic process shown in Scheme 1. Such conditions are very convenient for the determination of a complex forming compound (e.g. cysteine, [2, 4] or penicillamine [4, 5]) in the presence of a related compound with no complexing properties (e.g. N-acetylcysteine in excess). The last one may be pre-concentrated only as a mercury salt, but this process does not occur at the above-mentioned potential.

1.b) At a sufficiently positive potential, the high Hg²⁺ activity makes the pre-concentration process to occur with the formation of mercury thiolate (HgL) as the accumulated product. However, in the presence of Ni²⁺ the re-dissolution process (formulated schematically) occurs as follows:

2. Compounds that form relatively slight nickel complexes (e.g. glutathione [6]) are mostly accumulated as a mercury salt. The catalytic process during the cathodic scan occurs after the analyte was released by mercury ion reduction in this salt. This last reaction gives a cathodic peak (like A in Fig. 1) even in the presence of Ni²⁺ but the interference of a non-catalytic thiol (e.g. N-acetylcysteine) is still not important when measuring the catalytic Ni²⁺ current.

Disulfide forms of cysteine [2, 4] or glutathione [7] undergoes a conversion to the thiol form during the deposition step and can be determined in the same way as the thiol counterpart. Other sulfur containing compounds, as, for example, 2-mercaptobenzothiazole [8], and trimercapto-S-triazine [9] shows an analogous behavior. Moreover, Co²⁺ catalytic reduction proved as being more convenient in the case of 2-mercaptobenzothiazole [10].
 

B. Catalytic hydrogen evolution
 

An alternative to the catalytic Ni²⁺ or Co²⁺ reduction is the catalytic hydrogen evolution in the presence of Co²⁺ as a complex form with the analyte. Although well known from long time, this electrochemical reaction was only recently introduced in stripping voltammetry as a method for the determination of an electrochemically inactive analyte. Sulfide ion behavior was first investigated [11] in this way. A very characteristic signal was recorded by linear scan cathodic stripping voltammetry (LS-CSV) after performing the accumulation in the presence of the Co²⁺ ion. Although mercury sulfide forms on the electrode surface during the accumulation step, the actual catalyst in hydrogen evolution is a Co²⁺- sulfide compound that forms in the bulk of the solution and is co-adsorbed jointly with HgS. It is difficult to assert which is the structure of the catalyst, but the effect of the deposition potential demonstrates that it is a positively charged particle.

The same kind of electrode process proved useful for the determination of some sulfur-containing organic compounds, as it was first demonstrated with thiohydantoin derivatives of amino acids [12] and further with thiourea itself and other thiourea derivatives [13]. The principles of this method are illustrated by Fig. 2 with reference to thiourea that forms the reactive structural unit in many sulfur-containing derivatives. During the deposition step at 0.0 V, the carbon-sulfur bond splits and mercury sulfide forms by the reaction of resulted sulfide ion with Hg²⁺ produced by the anodic reaction of the mercury electrode. In the absence of Co²⁺, the subsequent cathodic scan gives only the peak A due to mercury ion reduction in HgS. If Co²⁺ is present during the deposition step, the peak C in Fig. 2 appears. This peak shows all the characteristics of the catalytic hydrogen peak recorded in the Co²⁺ - sulfide ion system [11]. Consequently, it may be assumed that the same kind of cobalt compound plays the role of the catalyst in both cases.

As it was already demonstrated [11, 12], catalytic hydrogen evolution in the presence of Co²⁺ represents a convenient option for the determination of either sulfide ion or various organic sulfur compounds down to the nanomolar concentration level. This method can be equally used after the appropriate derivatisation of some non-sulfur containing compounds. Amino acids conversion to thiohydantoins by the Edman reaction is a typical example in this respect.

Both above electrocatalytic methods avoid the detection by direct reduction of a metal ion (Hg²⁺or Cu⁺) in an adsorbed sparingly soluble compound. If such a reaction still occurs, its role is to release the analyte previously accumulated and make it available for the detection by the catalytic process. In this way, some distortions and interference that are common to film dissolution processes are avoided. Clearly, the catalytic effect connected with the generation of the analytical signal brings about an improvement in sensitivity. Additionally, selectivity is enhanced, as the catalytic effect is more sensitive to structural details when compared with the direct reduction process.

See ref [14] for a review of Electrocatalysis applications in Analytical Chemistry

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References (with links to reprints available at  NTNU Library, Trondheim)

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12. F. G. Banica, N. Spataru, Talanta, 48, 491 (1999).
13. N. Spataru, F. G. Banica, Analyst, 126, 1907 (2001)  (Abstract)  .
14. F. G. Banica, A. Ion, "Kinetics: Electrocatalysis-based Determination", Encyclopedia of Analytical Chemistry: Instrumentation and Applications, J. Wiley, New York, 2000, pp. 11115–11144. (Abstract) ||  Encyclopedia home page