Abstract
AbstractSodium stannate hydrolyses completely in 87% w/w concentrated hydrogen peroxide solution to form a stable lyophobic sol of stannic acid. Precipitation of this sol results by a reduction in the concentration of the potential determining ion (OH− or O2 H−) or by the electrostatic adsorption of small quantities of polyvalent cations, e.g., Al3+. In the presence of colloidal stannic acid the hydrolysis equilibrium of this ion is automatically displaced towards the formation of a stable AlOH3+ ion which, by adsorption, neutralises negative charges on the colloid. Adsorption of aluminium ions is the critical factor governing the stability of stannic acid sols in concentrated hydrogen peroxide solution stored in aluminium tanks. Protection of the sol is afforded by orthophosphate and pyrophosphate ions which are adsorbed and increase its tolerance for aluminium. Pyrophosphate, in addition, provides an alternative sink for aluminium ions which are precipitated as aluminium pyrophosphate, AlHP2O7.
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