Nucleation behavior in the precipitation of a sparingly soluble salt - AgBr

Nucleation behavior during the early stages of the precipitation of a sparingly soluble salt (AgBr) was studied experimentally. Direct electron microscopy and light-scattering photometry were used to determine the average diameter and the number of the AgBr nuclei. For a given reactant addition rate R, the number of AgBr nuclei increased at first, reached a maximum, and then started to decrease. The slope of the ln-ln plot of the maximum number versus R was {most positive} 1.5, indicating a bulk-diffusion growth mechanism for the AgBr nuclei. The average diameter of the nuclei was 0.008-0.015 μm, and changed with precipitation time. The supersaturation ratio S, calculated from the average nuclei diameter and the Gibbs-Thomson equation, showed the same behavior as that from a theoretical prediction. However, the calculated S values were significantly smaller than a critical value reported for the homogeneous nucleation of AgBr nuclei. These smaller S values can be explained qualitatively by a nuclei survival concept.