Dissolution of (50-x)Na2O-xCaO-50P2O5 metaphosphate glasses in different saline solutions : mechanism and kinetic control

The dissolution of (50-x)Na 2 O-xCaO 50P 2 O metaphosphate glasses has been investigated at pH 3 in solutions containing Na + , Ca 2+ , Cl - , NO 3 - and (PO 3 - ) n ions. Whatever the conditions, a hydrated layer develops over a surface covered with etch pits. Na + and Cl - have no specific effect on the dissolution kinetics and mechanism. In Ca 2+ -containing solutions, XPS, electrophoretic mobility and 31 P NMR show that Na + is exchanged for Ca 2+ in the hydrated layer, resulting in a decreased dissolution rate. Polyphosphate anions, (PO 3 - ) n , act as complexing agent for Ca 2+ , and counterbalance its inhibiting effect on the dissolution rate.