Polarography of oxyanions: I. The reduction of iodate and periodate from unbuffered solutions

Abstract

Summary The polarographic wave of IO 3 − in neutral unbuffered solutions splits into two parts in the presence of small amounts of strong or weak, mono-or polybasic acids. The first wave, developing at more positive potentials, is due to the reduction of part of the IO 3 − with the participation of free-protons or undissociated acid molecules, while the second wave represents the reduction of the remaining part of the IO 3 − from the neutral solution next to the electrode surface. The limiting diffusion current of the first wave depends on the type of acid used, and varies linearly with its concentration in solution. Comparison is made between the experimentally measured current of the wave, and that calculated on the basis of the theory of H + -ion impoverishment in the reaction layer. In the case of formic, acetic and propionic acids, large differences between the two figures are noted. These are mainly due to variations in the dissociation constants of the acids, on the one hand, and to their adsorption on the electrode surface, on the other. Basic proton-donors, e.g. HCO 3 − , HPO 4 2− and NH 4 + , shift the IO 3 − wave as a whole towards less negative potentials. this occurs despite the increase in the pH value of the solution. These ions are probably adsorbed on the Hg surface, and act as a contact proton source for the reducible IO 3 − ion. Very small acid additions to neutral unbuffered solutions of IO 4 − cause general exaltation of its double-wave polarogram. Large concentrations effect the splitting of the IO 3 − /I − wave, and finally its complete displacemtnt towards positive potentials. A decrease in the height of the IO 4 − polarogram is noted when the solution is made basic by addition of NaOH. In all solutions the limiting currents are diffusion-controlled. The results are explained on the basis of variation of the diffusion coefficient of the periodate species, due to association with water molecules. The measurement of the specific conductivity of KIO 4 -HCl mixtures and the pH-titration of KIO 4 with NaOH support this conclusion. A scheme is presented of the most probable acid-base and association-dissociation equilibria existing in aqueous periodate solutions.