The precipitation of barium sulphate and chromate powders from aqueous solution with slow development of supersaturation. Crystal numbers and final crystal sizes

The precipitations of barium sulphate and chromate were studied by slow addition of anion to metal cation solution at 20°C, to give final equivalent metal salt solutions; the final solute concentrations (C_fin) were varied from 0.002 to 0.30 mol l⁻¹ while the rates (R) of addition of anion were varied from 10⁻⁴ to 10⁻² ion l⁻¹. At first, mainly heterogeneous nuclei formed continuously during induction periods; then, as the metal salt concentration in solution increased, homogeneous nucleation soon predominated. This second nucleation process probably attained its maximum rate when the metal salt concentration in solution reached its maximum value (C_max) and then probably terminated quite rapidly. Some further nuclei also formed during the growth process when crystal growth was prolonged. The final nucleus numbers (N), and thence the crystal numbers for slow precipitations from very dilute solutions were then rather higher than the number N ∞ (het) of heterogeneous nuclei in solution: nucleus numbers then increased with increasing mixing rate according to the relation, (where β = 0.7–0.9) (where β = 0.7–0.9). The final average crystal lengths of any precipitate were then 2 to 40 times the sizes noted for rapid precipitation from equivalent solutions of the same concentration: generally, final lengths varied with mixing rate according to the relation, .