The Precipitation of Strontium and Lead Sulphates from Aqueous Solution. Kinetics of the Crystal Growth

The precipitation of strontium and lead sulphates from well‐stirred supersaturated aqueous solutions, of initial solute concentrations C 0 = 0.001 to 0.020 M and C 0 = 0.0002 to 0.003 M respectively, was studied at 20° and 40°C by chemical analysis and optical microscopy. Nucleation occurred during induction periods and continuous regular growth then took place onto the nuclei formed during these periods. Crystallisation was complete after 4 to 48 hr. The crystal growth was rate‐controlled by the rate of deposition of metal salt ions onto the growing crystal surfaces. This rate ( dC / dt ), at any growth time, then depended on both the overall surface area ( A t ) and on the residual excess solute concentration (Δ C t ) in solution according to the relation\documentclass{article}\pagestyle{empty}\begin{document}$$\frac{{dC}}{{dt}}= - k_c A_t \,\, \Delta C_t^2\,\, {\rm M}\sec ^{ - 1} $$ \end{document} while the growth rate ( d α/ dt ), expressed in terms of degree of crystallisation, was\documentclass{article}\pagestyle{empty}\begin{document}$$\frac{{d\alpha }}{{dt}} = k_\alpha\,\, \alpha _t^{2/3} \left[ {\Delta C_0 \left( {1 - \alpha _t } \right)} \right]^2 \sec ^{ - 1}$$ \end{document} . The rate constants ( k α ) for the crystal growth of strontium and lead sulphates at 20°C were 22 and 4200 sec −1 M −2 respectively — that is, greatest for the salt with least cation hydration –; these constants increased 4 to 6 times for 20°C temperature rise. The rate‐determining process for the metal salt deposition was probably the ion dehydration.