Cathodic stripping reactions

Reactions (2.205) and (2.206) are called second-order cathodic stripping reactions [134]. If the reacting ligand has a tendency to adsorb on the electrode snrface, the following mechanisms are encountered [136,137] ... 

Reaction (2.208) is a first-order cathodic stripping reaction with adsorption of the ligand [136], whereas reaction (2.210) is of second order [137]. Considering a mercurous salt formation, reaction (2.210) is written in the following form ... 

Here, cp = (E —E ) is a dimensionless potential and rs = 1 cm is an auxiliary constant. Recall that in units of cm s is heterogeneous standard rate constant typical for all electrode processes of dissolved redox couples (Sect. 2.2 to 2.4), whereas the standard rate constant ur in units of s is typical for surface electrode processes (Sect. 2.5). This results from the inherent nature of reaction (2.204) in which the reactant HgL(g) is present only immobilized on the electrode surface, whereas the product is dissolved in the solution. For these reasons the cathodic stripping reaction (2.204) is considered as an intermediate form between the electrode reaction of a dissolved redox couple and the genuine surface electrode reaction [135]. The same holds true for the cathodic stripping reaction of a second order (2.205). Using the standard rate constant in units of cms , the kinetic equation for reaction (2.205) has the following form ... 

Substituting the solutions for surface concentrations into the corresponding kinetic equations, one obtains integral equations for each cathodic stripping reaction. Numerical solution for the quasireversible electrode mechanism (2.204) is ... 

Gulaboski R, Mirceski V, Komorsky-Loviic S, Lovric M (2004) Square-wave voltammetry of cathodic stripping reactions, diagnostic criteria, redox kinetic measurements, and analytical applications. Electroanalysis 16 832-842. 

Most of the voltammetric features of a reversible cathodic stripping reaction of a second order (2.205) are similar to reaction (2.204) [134]. The main differences arise due to the influence of the concentration of the ligand on the position of the voltammetric response. The peak potential depends linearly on log(cL) with a slope of = —2.3, which is an inherent characteristic of a second-order reaction. Nevertheless, the dimensionless net peak current is virtually independent on c. Hence, the real net peak current is a linear function of the ligand concentration, which permits application of this mechanism for analytical purposes. A representative theoretical response of a quasireversible reaction (2.205) is shown in Fig. 2.86b. In addition to k and y, the dimensionless response is a function of the concentration c ... 

Some electrodes are made of substances that participate in the redox reactions that transfer electrons. These are active electrodes. Other electrodes serve only to supply or accept electrons but are not part of the redox chemistry these are passive electrodes. In Figure 19-7. both metal strips are active electrodes. During the redox reaction, zinc metal dissolves from the anode while copper metal precipitates at the cathode. The reactions that take place at these active electrodes are conversions between the metals contained in the electrodes and their aqueous cations. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

For each cathodic stripping mechanism, the dimensionless net peak current is proportional to the amount of the deposited salt, which is formed in the course of the deposition step. The amount of the salt is affected by the accumulation time, concentration of the reacting ligand, and accumulation potential. The amount of the deposited salt depends sigmoidally on the deposition potential, with a half-wave potential being sensitive to the accumulation time. If the accumulation potential is significantly more positive than the peak potential, the surface concentration of the insoluble salt is independent on the deposition potential. The formation of the salt is controlled by the diffusion of the ligand, thus the net peak current is proportional to the square root of the accumulation time. If reaction (2.204) is electrochemically reversible, the real net peak current depends linearly on the frequency, which is a common feature of all electrode mechanism of an immobilized reactant (Sect. 2.6.1). The net peak potential for a reversible reaction (2.204) is a hnear function of the log(/) with a slope equal to typical theoretical response... 

Most of the voltammetric features of a reversible cathodic stripping reachon of a second order (2.205) ate similar to reaction (2.204) [134]. The main differences arise due to the influence of the concentration of the ligand on the position of the voltammetric response. The peak potenhal depends linearly on log(c ) with a slope of... 

The second-order reaction with adsorption of the ligand (2.210) signifies the most complex cathodic stripping mechanism, which combines the voltammetric features of the reactions (2.205) and (2.208) [137]. For the electrochemically reversible case, the effect of the ligand concentration and its adsorption strength is identical as for reaction (2.205) and (2.208), respectively. A representative theoretical voltammo-gram of a quasireversible electrode reaction is shown in Fig. 2.86d. The dimensionless response is controlled by the electrode kinetic parameter m, the adsorption... 

SWV has been applied to study electrode reactions of miscellaneous species capable to form insoluble salts with the mercury electrode such as iodide [141,142], dimethoate pesticide [143], sulphide [133,144], arsenic [145,146], cysteine [134, 147,148], glutathione [149], ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) [150], 6-propyl-2-thiouracil (PTU) [136], 5-fluorouracil (FU) [151], 5-azauracil (AU) [138], 2-thiouracil (TU) [138], xanthine and xanthosine [152], and seleninm (IV) [153]. Verification of the theory has been performed by experiments at a mercury electrode with sulphide ions [133] and TU [138] for the simple first-order reaction, cystine [134] and AU [138] for the second-order reaction, FU for the first-order reaction with adsorption of the ligand [151], and PTU for the second-order reaction with adsorption of the ligand [137]. Figure 2.90 shows typical cathodic stripping voltammograms of TU and PTU on a mercuiy electrode. The order of the... 

Ion-exchange reactions were used for the accumulation of europium(III) [158] and iron(III) [159] ions on the surface of GCE coated with Nafion , and chromium(VI) ions on the surface of GCE covered by a pyridine-functionalized sol-gel film [160], which were combined with the stripping SWV Furthermore, a cathodic stripping SWV was used for the determination of sulfide [161,162], thiols [163-166], selenium(lV) [167-170], halides [171-173] and arsenic [174] accumulated on the snrface of mercury electrode. 

Xanthine and xanthosine were investigated on HMDE, applying out-of-phase ac and dc voltammetries [74]. It has been shown that both compounds are strongly adsorbed and interact chemically. In the cathodic stripping process, one could determine both compounds at trace level. Naidu et al. [146] have performed polaro-graphic studies to show that the product of anodic reaction (prewave) of potassium isobutyl xanthate is strongly adsorbed at the mercury electrode. 

The electrode at which oxidation takes place is called the anode (the zinc strip in this example), and the electrode at which reduction takes place is called the cathode (the copper strip). The anode and cathode half-reactions must add to give the overall cell reaction ... 

The figures span the distance from the center of an anodic strip (0.5 cm wide) to the center of an adjacent cathodic strip 1.5 cm wide (i.e., the center-to-center distance for the strips is 1.0 cm). It is assumed that the anodic and cathodic reactions are confined to the respective areas, as stated above. Current flows in the solution as positive ions from the anodic area where the reaction, M —> Mm+ + me, occurs to the cathodic... 

Palecek. E. (1980). Reaction of nucleic acid bases with the mercury electrode determination of purine derivatives at submicromolar concentrations by means of cathodic stripping voltammetry. AwaZ Bzoc/zem 108, 129-138. 

The galvanic stripping reactions involved in separating iron from a zinc sulphate electrolyte using solvent extraction can be assumed to comprise the following half cell reactions. The major anodic and cathodic steps are ... 

---