Voltammetry for Reduction of Hydrogen Ions from Mixtures of Mono- and Polyprotic Acids at Platinum Microelectrodes Salvatore Daniele,* ,† Irma Lavagnini, ‡ M. Antonietta Baldo, † and Franco Magno ‡ Department of Physical Chemistry, University of Venice, Calle Larga S. Marta, 2137, 30123 Venice, Italy, and Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita’ di Padova, Via Marzolo,1, 35131 Padova, Italy 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 polypro- tic 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 relation- ship 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. Recently, the electrode reaction involving discharge of hydro- gen ion arising from strong and weak acids at platinum micro- electrodes has been the object of several investigations. 1-9 Measurements have been carried out in solutions without and with varying concentrations of supporting electrolyte, and the effects of ionic strength on mass transport properties have been examined. 2-6 Other studies were aimed at verifying the kinetics of the electrode process involved in the reduction of weak monoprotic acids for which a CE mechanism applies. 1,7-9 It was verified that, using platinum microdisk electrodes having radius over the range 10-12.5 μm, the chemical reaction preceding the electron transfer is fast for those acids whose dissociation constant, K a , is larger than ∼1 × 10 -6 . 7,8 In solutions of strong acids, which are completely dissociated, it was found that the reduction wave height depends linearly on concentration over a wide range. 2,3,6 On the other hand, in solutions of monoprotic weak acids the dependence of the steady- state limiting currents on the acid concentration is not strictly linear, though in restricted concentration ranges an apparent linearity can be observed. 7,8 For weak monoprotic acids, whose dissociation step is fast, the steady-state limiting current, I l , can be predicted by 2,7,8,10 where D HA and [HA] b are the diffusion coefficient and the bulk equilibrium concentration of the undissociated acid, D H + and [H + ] b are the diffusion coefficient and bulk equilibrium concentration of proton, the electron number n is equal to 1, and the other symbols have their usual meanings. This relationship has been derived by resorting to the apparent diffusion coefficient of the acid, which is given by the mole fraction weighted average of H + and HA present in the bulk solution at equilibrium, in analogy with similar situations where two or more species, involved in a homogeneous equilibrium, are characterized by different diffusion coefficient values. 11-13 In fact, in water, the diffusion coefficient of proton and undissociated acid differ generally by ∼1 order of magnitude. 7,14 The wave position of weak monoprotic acids was found to depend on both the equilibrium dissociation constant and the † University of Venice. ‡ University of Padova. (1) Fleischmann, M.; Lasserre, F.; Robinson, J.; Swan, D. J. Electroanal. Chem. Interfacial Electrochem.1984 , 177, 97. (2) Ciszkowska, M.; Stojek, Z.; Morris, S. E.; Osteryoung, J. G. Anal. Chem. 1992 , 64, 2372. (3) Troise, F. M. H; Denault, G. J. Electroanal. Chem. Interfacial Electrochem. 1993, 354, 3311. (4) Stojek, Z.; Ciszkowska, M.; Osteryoung, J. G. Anal. Chem. 1994 , 66, 1507. (5) Ciszkowska, M.; Stojek, Z.; Osteryoung, J. G. J. Electroanal. Chem. Interfacial Electrochem. 1995 , 398, 49. (6) Perdicakis, M.; Piatnicki, C.; Sadik, M.; Pasturaud, R.; Benzakour, B.; Bessiere, J. Anal. Chim. Acta. 1993 , 273, 81. (7) Daniele, S; Lavagnini, I; Baldo, M. A.; Magno, F. J. Electroanal. Chem. Interfacial Electrochem. 1996 , 404, 105. (8) Daniele, S; Baldo, M. A.; Simonetto, F. Anal. Chim. Acta 1996 , 331, 117. (9) Ciszkowska, M; Jaworski, A.; Osteryoung, J. G. J. Electroanal. Chem. Interfacial Electrochem. 1997 , 423, 95. (10) Oldham, K. B. Anal. Chem. 1996 , 68, 4173. (11) Carofiglio, T.; Magno, F.; Lavagnini, I. J. Electroanal. Chem. Interfacial Electrochem. 1994, 373, 11. (12) Rusling, J. F.; Shi, C. N.; Kumosinski T. F. Anal. Chem. 1988 , 60, 1260. (13) Evans, D. H. J. Electroanal. Chem. Interfacial Electrochem. 1989 , 258, 451. (14) Heyrovsky, J.; Kuta, J. Principles of Polarography, Academic Press: New York, 1966. I l ) 4nFr( D H + [H + ] b + D HA [ HA] b ) (1) Anal. Chem. 1998, 70, 285-294 S0003-2700(97)00666-5 CCC: $15.00 © 1998 American Chemical Society Analytical Chemistry, Vol. 70, No. 2, January 15, 1998 285 Published on Web 01/15/1998