The precipitation of sparingly-soluble metal salts from aqueous solution: Heterogeneous nucleus numbers at low to intermediate supersaturation

The precipitation of series of alkaline-earth metal and transition hydroxides, sulphates, chromates and molybdates, hydrogen phosphate carbonates, oxalates and ‘oxinates’ were studied in aqueous solution of low to intermediate supersaturation. Heterogeneous nucleation probably occurred onto micro-crystalline particles of some siliceous mineral (of the trigonal, hexagonal or cubic system), dispersed in the solution. The heterogeneous nucleus numbers for these precipitations then depended on the rates of the heterogeneous nucleation onto these substrates and the rates of the mononuclear growth of nuclei to crystallites (during the induction periods). Generally, N_het values in polypropylene and glass beakers, at low supersaturation, varied from 10⁴ to 10¹³ dm⁻³: the N_het values then increased slightly with concentration and supersaturation according to the relation N_het = K_N^β, where K_N is a function of the metal salt surface energy and an ‘epitaxy’ factor; β = 0.4–0.5. In turn, at any supersaturation, log N_het = log N + Fσ, where N and F were constants for any precipitation: N_het values then increased from 10⁴ to 10⁸ times for increase in σ from 50 to 150 mJ m⁻². At any supersaturation and surface energy, N_het values increased in the order monoclinic < orthohombic < tetragonal < trigonal crystals.