J. Koryta

A contribution to the voltammetric study of cystine and cysteine at Pt electrodes in 0.5 M H2SO4

Abstract Both cysteine and cystine adsorb at the Pt electrode according the Frumkin—Temkin isotherm with the heterogeneity factor f = 51 for cysteine and 21 for cystine. Both the adsorbed cysteine and cystine give in a solution without any dissolved cystine or cysteine almost identical first cyclic voltammetric curves. Each substance dissolved in the electrolyte gives two oxidation peaks which differ when the oxidation is carried out at a “reduced” or an “oxidized” Pt electrode. On the basis of the dependence of the height and potential of the peaks on polarization rate and concentration (in the case of oxidation of dissolved substances) and of coulometric measurements the following conclusions have been made concerning the kinetics and mechanism: (i) Neither cysteine nor cystine change their oxidation state on adsorption at the electrode. (ii) The final oxidation product of both adsorbed cysteine and cystine may be the cysteic acid. (iii) For cysteine there are two adsorbed species, one strongly adsorbed, the other one weakly adsorbed. (iv) The oxidation of dissolved cysteine takes place via the weakly adsorbed species, the surface concentration of which is influenced by the coverage of the strongly adsorbed species. This process is described by an electrode reaction rate equation. (v) In the overall oxidation of cysteine one electron is transferred while the detailed mechanism requires an oxidation by splitting-off two electrons with a subsequent ion—substrate dimerization reaction.

Complex forming and extraction properties of oligo benzo-fused crown ethers

The complex forming properties with alkali metal and ammonium ions of a series of oligo benzo-condensed 18-crown-6 ethers1–8 having a different gradation of lipophilicity and of molecular rigidity are investigated by voltammetry at the interface of two immiscible electrolyte solutions (ITIES) and by a liquid-liquid extraction technique. The experimental results obtained in the two phase system H2O/nitrobenzene are discussed in relation to the structure of the crown and the cation type. The stability constants for the 1 : 1 complexes of Na+, K+, Rb+, Cs+ and NH4+ in nitrobenzene have been determined and compared with the extraction constants for the 1: 1 complexes of Na+ and K+ and for the 1 : 1 and 1 : 2 complexes of Cs+, showing the effect of oligo benzo condensation for the 18-crown-6 system.