Voltammetry for Reduction of Hydrogen Ions from Mixtures of Mono- and Polyprotic Acids at Platinum Microelectrodes

The steady-state voltammetric behavior for reduction of several polyprotic acids and mixtures of strong and weak mono- and polyprotic acids was studied at platinum microelectrodes. The results demonstrated that over the potential range accessible to reduction of acids in water (up to ∼−1 V vs Ag/AgCl) via a preceding chemical reaction (CE mechanism), the reduction of weak polyprotic acids and mixtures of acids can produce either a single well-defined wave or two waves separated to a different extent, depending on the dissociation constant of each acidic form, on the analytical concentration of each acid, and on the mutual ratio of the acids present at equilibrium in the bulk solutions. The overall reduction mechanism for most of the mixtures examined was interpreted on the basis of a series of CE processes associated to the hydrogen evolution. This interpretation was supported by digital simulation procedures. A theoretical relationship for predicting the steady-state limiting current for any mixture of acidic species, whose dissociation steps are fast, was also derived. This equation proved valid for all those acids with equilibrium constants larger than ∼10 - 6 . On the basis of this theoretical relationship, a simple diagnostic criterion to assess whether or not the reduction process of a mixture of acids is under a kinetic control was also established.