The influence of polyphosphate ions on the precipitation of calcium oxalate

We studied the growth of calcium oxalate monohydrate (COM) crystals in stirred suspensions by following the changes in ionic conductivity of supersaturated solutions at 25 +/- 0.02C. At a pH of 6.9-7.2, in the presence of polyphosphate ion, crystallization proceeded by an initial rapid step followed by a markedly reduced growth rate as compared with that in pure supersaturated solution. The initial surge was reduced as the polyphosphate concentration reached 1.6 X 10(-6) M, which can be interpreted as evidence of either surface nucleation or the incomplete adsorption of inhibitor at the active growth sites. In spontaneous precipitation experiments made in the presence of polyphosphate ions, the formation of calcium oxalate dihydrate and trihydrate is favored together with appreciable coprecipitation of additive ions.