Influence of some phosphates and polyphosphate on the precipitation of calcium sulfate dihydrate in sodium chloride solutions

The inhibiting effect of phosphates and polyphosphates on the precipitation rate of calcium sulfate dihydrate (gypsum) in sodium chloride solutions has been studied using constant composition technique in which the super-saturation and ionic strength were maintained constant at 25°C. The presence of phosphoric acid H3PO4, orthophosphate NaH2PO4, metaphosphate Na2HPO4, pyrophosphate PYP and hexametaphosphate HMP, even at relatively low concentration 10−6 M, markedly reduced the rates of precipitation. The action of additives can be interpreted in terms of adsorption, following the Langmuir isotherm, of additives at the active crystal growth sites. Dependence of the degree of inhibition with the change in driving force has been observed. The order of the degree of inhibition on the rate of precipitation of gypsum for various additives studied is as follows: H3PO4 > NaH2PO4 > Na2HPO4> HMP > PYP.