Cathodic stripping reactions second order

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 ... 

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 ... 

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... 

---