The crystal growth and dissolution of barium sulfate in the presence of additives

Abstract

The crystallization and dissolution of barium sulfate in the presence of N,N,N′,N′ triethylenediaminetetra(methylene phosphonic acid) (TENTMP), sodium tetrametaphosphate, and sodium tripolyphosphate have been studied conductimetrically at 25°C. Quadratic equations, second order with respect to relative supersaturation and subsaturation, have been satisfactorily used to interpret the kinetic data for the growth and dissolution reactions, respectively. Delay periods in the growth of barium sulfate crystals in the presence of sodium tripolyphosphate strongly suggest that this additive is incorporated into the crystals during growth. In the corresponding dissolution reactions, the presence of polyphosphates results in an initial surge indicating that the adsorption of the polyphosphates on the surface of the barium sulfate seed crystals is not an instantaneous process. The marked inhibition of dissolution in the presence of 4.4 × 10 −8 M sodium tripolyphosphate adds strong support to the suggested surface control for this reaction. For all the additives, the relative effectiveness as inhibitors both for crystal growth and dissolution is sodium tripolyphosphate > sodium tetrametaphosphate > TENTMP.