New Electrocatalytic Reactions at a Mercury Electrode in the Presence of Homocysteine or Cysteine and Cobalt or Nickel Ions

Abstract

Homocysteine (Hcy) and cysteine (Cys) mercury thiolate layers were prepared by anodic polarization of a mercury electrode in amino acid containing solutions and then investigated in the cathodic regime in the presence of Ni 2þ or Co 2þ ions. The sulfhydryl function in the mercury thiolate undergoes a slow disintegration resulting in surfaceattached mercury sulfide. During the cathodic scan, Hg 2þ substitution by Ni 2þ or Co 2þ yields minute amounts of the relevant metal sulfide. Such a species catalyzes hydrogen evolution at À 1.3 V vs. Ag j AgCl j KCl(3 M). Hcy experiences a faster decomposition and, consequently, displays a stronger catalytic effect. Each compound catalyzes the reduction of Ni 2þ or Co 2þ , but only Cys (bound in metal complexes) induces typical catalytic hydrogen evolution processes such as the Brdička reaction (with Co 2þ ; pH around 9), or the catalytic hydrogen prewave (CHP) (with Ni 2þ ; pH near 7). On the other hand, Hcy catalyzes the hydrogen evolution in the presence of Co 2þ at À 1.5 V in the same way than sulfur derivatives with no amine function do. Metal sulfide formation does not interfere with CHP and Brdička processes. Correlations between the physical state of the metal sulfide (adsorbed molecule or aggregate form) and its catalytic properties are discussed and possible analytical applications suggested.

Homocysteine Voltammetry at a Mercury Electrode in the Presence of Nickel Ions

“…Therefore, the Ni 2þ ion acts as masking agent for the thiol and prevents the formation of mercury cysteinate. As the nickel ion in this complex is reduced at a more negative potential [14], masking by the nickel ion is effectual within the whole potential region of the peak A. Adsorption of the nickel complex on the positively-charged mercury surface is driven by its negative charge (Figure 3). Taking into account the disposition of the carboxylate anions, it could be supposed that the adsorbed complex adopts a parallel orientation with respect to the electrode surface.…”

“…We proved recently [14] that, in the presence of cobalt or nickel ion, significant differences between Hcy and Cys behavior can be noticed as far as the catalytic hydrogen …”

Electrode Modified with Cobalt Cyclohexylbutyrate for the Determination of Low Molecular Weight Thiol Group Bearing Compounds Using Catalytic Stripping Voltammetry

“…In the above mentioned matrices, the methods belonging to the second group which work in cathodic region of potential may be more appropriate. These methods comprise the reaction between thiol group and a transition metal complex compound (Hg 2þ [16 -18], Co 2þ (Brdicka reaction) and Ni 2þ [19,20] are the most typical central metal ions) followed by subsequent electroreduction of the resulting thiol-metal complex. Complexes of these metals with thiols are easier to reduce than the original metal compounds themselves.…”