Solid-state hydrolysis of calcium tripolyphosphate scales

Precipitation of calcium tripolyphosphate (CaTPP) from strongly supersatura ted solution is accompanied by tripolyphosphate hydrolysis if the bulk Ca/T PP mole ratio is in excess of about 2. Along with the tendency for hydrolys is, a change in the crystal habit from crystalline to amorphous is observed beyond this bulk ratio. The composition of the material changes as well fr om mainly Ca2NaP3O10 at a 2:1 Ca/TPP bulk ratio to Ca-5(P3O10)(2) at an 8:1 bulk ratio, though the water of hydration contents are similar. The crysta llinity change is reflected in the IR and solid-state P-31 NMR spectra, but , beyond these expected differences, the amorphous samples also appear to h ave a larger fraction of more mobile crystal water. Through a series of seq uential experiments, it was identified that hydrolysis of these materials m ainly occurs via a mechanism consisting of precipitation of CaTPP followed by hydrolysis of the precipitate. Bulk water does not play a critical role in the hydrolysis nor does surface adsorbed water, to the extent that it ca n clearly be differentiated from the crystal or bound water. Kinetic studie s of the hydrolysis of the 8:1 material showed that its solid-state reactio n rate (7.1 x 10(-3) min(-1) at 70 degrees C) is 2 orders of magnitude high er than that of NaTPP. The hydrolysis mechanism is found to lay midway betw een first order and diffusion controlled, leading us to suspect that loosel y bound water within the crystal lattice constitutes the diffusing species. Although the facile hydrolysis at the stronger supersaturation can possibl y be explained in terms of the higher Ca/TPP ratio in the precipitate leadi ng to destabilization of the P-O-P bond, the observed trends seem to rather suggest the diffusion of crystal water within the strongly distorted cryst al lattice as the dominant factor.