Spectacular Breeding of Crystals on Silica Gel

Abstract

During experiments with silicic acid sols blended with thiosulfate solutions, an interesting phenomenon was observed. Namely, after gelation of the systems and evaporation of excessive water at ambient temperature, the gels cracked and on their surface grasslike offsprings appeared, which continued to grow. A closer inspection of these species showed them to be sodium thiosulfate, slightly less hydrated than the parent salt (pentahydrate). A literature search for explanation of this phenomenon led the authors to a class of minerals known as antholytes, whose representative is a rare mineral chalcanthite, CuSO4•5H2O. Further experiments with CuSO4•5H2O, Cr2(SO4)3•18H2O, Al2(SO4)3•18H2O, NiSO4•7H2O, Na2SO4•10H2O, and (NH4)2SO4 gave similar results. With colorful salts, the ”antholytes“ differed in hydration degree and morphology of crystals as compared to those of the parent salts. These experiments are particularly recommendable for undergraduate students for demonstration of several reactions and phenomena such as preparation of silicic acids by ion-exchange from sodium metasilicate, sol–gel transition, aging of silica gels, preparation of antholytes, and demonstration of the labile nature of monosilicic and thiosulfuric acids.