Measuring induction period for calcium sulfate dihydrate precipitation

Homogeneous nucleation of CaSO4. 2H(2)O (gypsum) based on an optical diagno stic technique was studied within a supersaturation range of 1-4 at 25-90 d egrees C. The experiments were carried out using an experimental apparatus consisting of a batch crystallizer with the related measurement devices. Si gnals of scattered and transmitted light coming from a He Ne laser source w ere analyzed to measure the induction period (t(ind)), that is, the time de lay necessary for homogenous nucleation to take place. As expected from the ory, it was found that t(ind) decreases when either temperature or supersat uration increase; from the dependence of t(ind) on supersaturation, it was possible to distinguish between the mechanism of homogenous and heterogenou s nucleation. From the experimental data relative to homogeneous nucleation , the interfacial tension (gamma(s)) between CaSO4. 2H(2)O and the surround ing aqueous solution and the activation energy (E-att) for CaSO4. 2H(2)O cr ystallization were evaluated. In particular, the dependence of t(ind) on te mperature made it possible to evaluate E-att at 30 kJ/mol and the dependenc e of t(ind) on supersaturation offered the value of gamma(s) to be about 37 mJ/m(2), which does snot vary with temperature in the interval explored.