Pyrophosphoric acid as a source of hydrogen in cathodic hydrogen evolution on silver

摘要

Hydrogen evolution from pyrophosphate solution on silver was studied as a function of pH. Linear Tafel functions with slopes of 0.120 to 0.150 Vdec{sup}-1 were obtained over three orders of magnitude of current density. A diagnostic criterion forparticipation of hydrogen donors other than the hydronium ions is found in the dependence of polarisation at a constant current density on pH. It was found that all the acid species present, i.e. H{sub}4P{sub}2O{sub}7, H{sub}3P{sub}2O{sub}7{sup}-,HP{sub}2O{sub}7{sup}2- and HP{sub}2O{sup}3-, contribute to the hydrogen evolution current with rate constants/A dm mol{sup}-1 decreasing in order of 4.3×10{sup}-4, 5.0×10{sup}-7, 4.6×10{sup}-7 and 6×10{sup}-10 respectively.Each of the species makes a contribution in proportion to its abundance which is strongly pH dependent.