The precipitation of sparingly-soluble alkaline-earth metal phosphates in gelatine gels by diffusion of anion (i). the precipitation of calcium hydrogen phosphate dihydrate and basic calcium phosphates

The precipitation of different calcium phosphates in gelatine gels was studied by diffusion of disodium hydrogen phosphate solutions into ten percent gels containing calcium cation at liquid gel pHs from 6 to 11. At pHs 6.0 to 7.5, calcium hydrogen phosphate dihydrate (CaHPO₄ · 2 H₂O) was deposited as a continuous precipitate and then as rings of precipitate (the Liesegang phenomenon): their spacing coefficients varied with the reciprocal initial anion concentrations according to the Packter-Matalon relation. At liquid gel pHs 8.0, 8.5, tretacalcium phosphate (Ca₄H(PO₄)₃) was deposited as a continuous precipitate. At liquid gel pHs from 8.5 to 11, hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, Ca/P ratios = 1.50 to 1.67) was deposited as a continuous precipitate. The mechanisms of precipitation of the different calcium phosphates (in gelatine gels) is briefly discussed.