Measurement of the Metastable Zone Width of Phosphoric Acid Hemihydrate in the Presence of Impurity Ions

The effect of five different impurities, the cations Al 3+, Fe 2+, and Mg 2+ and the anions SO 4 2- and Cl -, on the metastable zone width of phosphoric acid hemihydrate has been investigated using a laser method with a linear cooling rate. The metastable zone width was measured as a function of impurity concentration and cooling rate in the seeded system. The influences of impurity ions on the metastable zone width of phosphoric acid hemihydrate take place by different mechanisms. Mg 2+ has enlargement effects on the metastable zone width, whereas Al 3+ and Fe 2+ have both enlargement or suppression effects under different impurity concentrations. Cl - and SO 4 2- have relative weak effects on the metastable zone width.