The stability of hydrated aluminium phosphate, AlPO4 x 1.5H2O.

The dissolution mechanism and stability of a synthetic hydrated aluminium phosphate, AlPO4 x 1.5H20 (AlPO4-H3) was investigated at 22 degrees C over the pH range 2.4 to 8.8. AlPO4 x 1.5H2O was found to be more soluble than the better known dihydrate (variscite, AIPO4 x 2H2O) hence proving to be metastable. This material was determined to undergo incongruent dissolution at pH around 3.0 and higher producing initially amorphous Al(OH)3 which gradually (within 30-day dissolution period) converted to the stable gibbsite phase. Upon evaluation of the experimental data with PHREEQC it was possible to calculate the solubility product ( logKsp ) and the standard Gibbs free energy of formation (deltaGf,0) for AlPO4 x 1.5H2O to be respectively-- 20.46+/-0.40 and -1980.5+/-.0 kJ mol(-1) at 22 degrees C.