VOLTAMMETRY FOR REDUCTION OF HYDROGEN-IONS FROM MIXTURES OF MONO-PROTIC AND POLYPROTIC ACIDS AT PLATINUM MICROELECTRODES

The steady-state voltammetric behavior for reduction of several polypr otic acids and mixtures of strong and weak mono-and polyprotic acids w as studied at platinum microelectrodes. The results demonstrated that over the potential range accessible to reduction of acids in water (up to similar to-1 V vs Ag/AgCl) via a preceding chemical reaction (CE m echanism), the reduction of weak polyprotic acids and mixtures of acid s 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 solution s. The overall reduction mechanism for most of the mixtures examined w as interpreted on the basis of a series of CE processes associated to the hydrogen evolution. This interpretation was supported by digital s imulation procedures. A theoretical relationship for predicting the st eady-state limiting current for any mixture of acidic species, whose d issociation steps are fast, was also derived. This equation proved val id for all those acids with equilibrium constants larger than similar to 10(-6) On the basis of this theoretical relationship, a simple diag nostic criterion to assess whether or not the reduction process of a m ixture of acids is under a kinetic control was also established.