Kinetic analysis of the influence of additives on spontaneous precipitation from electrolytic solutions

Abstract

Kinetic data describing the precipitation of calcium oxalates (calcium oxalate trihydrate (COT) and mixtures of COT and the monohydrate (COM)) in the presence and absence of sodium and potassium chloride, phosphate ions, and/or glutamic acid (Glu) have been employed to obtain information on the rate controlling precipitation processes, e.g., nucleation, crystal growth and aggregation. Precipitation was induced from unseeded high ionic strength solutions at pH 6.0 or 6.5 and the kinetics followed by Coulter counter or calcium selectrode. In selected samples the composition of the precipitates was determined by thermogravimetric analysis and x-ray diffraction. It was found that phosphate ions retard crystal growth but do not influence nucleation or aggregation of the crystals. The neutral electrolytes induced aggregation at concentrations comparable to their critical coagulation concentration. Glu affected the composition of the precipitate by inducing the formation of COM. Its effect on the intensity of precipitation was concentration dependent. At low concentrations (c(Glu)=3×10−6−5×10−5 mol dm−3) reduction of the rate of crystal growth and simultaneous enhancement of aggregation was observed, while at higher concentrations (c(Glu)≥10−4 mol dm−3), precipitation of calcium oxalate was enhanced. The effects of the aminoacid were attributed to its interaction at the stage of heterogeneous nucleation. A comprehensive discussion on the effect of additives on precipitation processes is included.Key wordsCalcium oxalateadditivesnucleationprecipitationcrystal growthaggregation