Precipitation of calcium oxalates from high-ionic-strength solutions. Part 6.—Kinetics of precipitation from solutions supersaturated in calcium oxalates and phosphates

Abstract

The kinetics of precipitation of calcium oxalate trihydrate (COT) and calcium hydrogenphosphate dihydrate (DCPD) from high-ionic-strength solutions (I= 0.26 mol dm^–3, made up with NaCl) supersaturated in both solid phases [S_i(COT)=P_i/K_sp= 6.86 – 47.20; S_i(DCPD)= 2.10 – 3.57] have been investigated. Under the experimental conditions employed [e.g. initial total reactant concentrations c(Ca)=c(PO₄)=(2.6 – 2.8)× 10^–2 mol dm^–3, c(C₂O₄)=(1.5 – 10)× 10^–4 mol dm^–3; pH_i 5; temperature 298 K; magnetic stirring alone or followed by mechanical stirring] COT precipitated first and effectively initiated the precipitation of DCPD. Under the conditions at which the induction periods t_i(COT) and t_i(DCPD) differed by more than 1 h, previously defined quantitative criteria were employed to determine the influence of excess phosphate and calcium ions on the kinetics of crystal growth and aggregation of COT. It is shown that phosphate ions inhibit crystal growth, but have no influence on aggregation. A comparison with previous results obtained at equimolar total calcium and oxalate concentrations [e.g. c(Ca)=c(C₂O₄)] shows that in the presence of excess calcium ions the efficiency of the inhibition of crystal growth by aggregation is enhanced, while the collision rate coefficient and rate-controlling mechanism of aggregation are not significantly affected.