Effect of chain length on storage stability and hydrolytic reversion of sodium polyphosphate glasses

Abstract

At low concentrations in hard water, the polyphosphates revert to orthophosphate at rates which are not predictable from data obtained at higher concentrations. A reversal in the effect of pH on reversion rate at lower concentrations appears to be due to complex formation between the polyphosphate and the hard water metal ions. A minimum in the rate of reversion of polyphosphate glasses to orthophosphate occurs at an average chain length between about 13 and 40 under the low concentration hard water conditions. Phosphate glasses having an average chain length below 30 show good storage stability but longer-chain glasses undergo a decrease in chain length under normal "dry" storage conditions, with the rate of degradation increasing with increasing chain length.