Turbidimetric study of fluorite nucleation in solution

Precipitation of fluorite (calcium fluoride) from solution at 30 and 37 degrees C has been studied by measurement of turbidity at different wavelengths, analyzing the data with the aid of the Mie theory of light scattering from small particles. In this way both number density and average particle size were obtained as functions of time. The values were confirmed by sedimentation analysis. In the later part of the process the results were strongly influenced by agglomeration and sedimentation, but the initial part of the curve of number density vs time was useful in determination of nucleation kinetics. The trend of nucleation frequency vs supersaturation agreed well with the classical theory of Becker, Döring, Volmer, Zel'dovich, and Frenkel, and surface energies equal to 102 mJ/m(2) at 30 degrees C and 89 mJ/m(2) at 37 degrees C were found. On the other hand, the absolute rate of nucleation was many orders of magnitude lower than that predicted by the theory, which is ascribed to an overestimate of the rate of growth of postcritical nuclei.