Metachors and dissociation of salts of oxo acids of group VI heavy metals in aqueous solutions

Abstract

The backround of the previously introduced concept of the metachor is outlined. This concept is applied to aqueous solutions of Se, Te, Mo, and W oxo acids and their sodium and potassium salts for which the metachors, calculated from the experimental surface tension values, are used to gain insight into their dissociation state in solution. The normal salts with mononuclear and binuclear anions (Me2XO3, Me2XO4, and Me2X2O7 types) in aqueous solutions at concentrations of 0.1-0.5 mol dm-3 are completely dissociated in the 2nd step and their metachors are about 10 cm3 mol-1, whereas KHSeO4 and Na2W3O10 with its bulky trinuclear anion are dissociated in the 1st step only and their metachors at the above concentrations lie in the 4.5 to 5.5 cm3 mol-1 range (at concentrations as low as 0.025 mol dm-3 for the latter). The metachor of the free acid H2SeO4 is as low as 2.5-1.5 cm3 mol-1, corresponding to an appreciable association in the 1st dissociation step. The metachor only reflects the "chemical" association due to short-range forces, whereas it is indifferent to physical outer-sphere ion pairing.