Zones of Sparingly Soluble Inorganic Salts with Membrane Properties in Gels

Abstract

Polyacrylamide gels containing potassium oxalate (sodium oxalate) are brought into contact with aqueous solutions of Pb(NO3)2, CaCl2, SrCl2 and BaCl2 respectively. The electrolyte concentrations in the gel phase and in the solutions are varied systematically. There exists a system specific characteristic concentration c̃s below which advancing precipitation zones of PbOx, CaOx, SrOx, and BaOx (Ox: oxalate) within the gel are observed. Above c̃s the precipitation zones do not grow after advancing for about 2 mm maximally. Such precipitation zones have membrane properties (e. g. Impermeability to ions forming the precipitate, osmotic property). c̃s corresponds to a supersaturation ratio of the order 103. The experiments indicate that the precipitation zones with membrane properties are generated by homogeneous nucleation. – The kinetics of formation of oxalate precipitates in cellophane skins is studied by measurements of the membrane potential as function of time. In these systems precipitate is formed by interdiffusion of two solutions containing the appropriate ions. A system specific delay time Δtdel is observed after which the precipitation zones acquire membrane properties. It is found that this time is related to the rate of formation of precipitate which is a function of the solubility product and the square of electrolyte concentration. The precipitation zone acquires membrane properties as soon as the local concentration of precipitate has reached a value of the order 10−3 mol cm−3. The existence of the characteristic concentration c̃s and of the delay time Δtdel are interpreted in terms of an absorption charge model proposed by Hirsch‐Ayalon.