The precipitation of barium chromate powders from aqueous solution at different pHs: Nucleation, final crystal numbers and sizes

Barium chromate precipitations were studied from equivalent aqueous solutions of initial overall metal salt concentrations from 0.00013 to 0.010 M at ambient temperature and at p Hs from 8 to 3. At p Hs from 8 to 5, precipitation mainly occurred through homogeneous nucleation: the reciprocal induction periods (and nucleation rates) and the crystal numbers generally decreased with reduction of p H but the values at any effective mean metal salt concentration increased appreciably with increasing acidity. Presumably, both M ++ A and M ++ HA − species were taking part in the nucleation process. – At p Hs below 5, heterogeneous nucleation predominated in most precipitations: the crystal numbers and nucleus numbers at any effective metal salt concentration increased with reduction of p H in these systems. Presumably, more active sites for heterogeneous nucleation were developed.