Kinetics of Cupferron Electroreduction on the Dropping Mercury Electrode

Cupferron electroreduction was studied polarographically in buffer media with pH 4.3-7.5 and 8-10. Irreversible reduction with slow transfer of the first electron was found, with ?=0.45 in alkaline and ?=0.33 in acidic solutions. In acidic media E1/2 of cupferron wave depends on pH linearly with a slope 0.125 V. This implies reversible protonation step preceding the electron transfer. The wave shape at the lowest pH studied clearly indicates adsorption influence on kinetics of the process, i.e. protonated cupferron reduces from adsorbed state. Mairanovskii model of surface waves was successfully applied, with ? values corrected to 0.38. There is no prior protonation in alkaline solutions, but some kinetics features (such as unusually high slope of E1/2, logt1 dependence) also imply some influence of adsorption. Analysis of published data revealed that contribution of slow tautomeric equilibrium proposed in earlier works is very doubtful. Double protonation of cupferron anion at low pH should be considered instead