Role of Phosphate Species and Speciation Kinetics in DetergencySolutions

The present investigation addresses the mechanisms of inorganic phases precipitation from detergency solutions of low polyphosphate formulation. In washing machine systems the main precipitation agent is Na4P2O7·10H2O that constitutes up to 10 wt % commercial Na5P3O10·5H2O. The solid incrustation on the heating coil is mainly formed by amorphous calcium pyrophosphate, subsequently transforming first into amorphous calcium orthophosphate and then into low crystallinity hydroxylapatite. The tripolyphosphate eventually present in the precipitates hydrolyzes as well at the working temperature present at the surface of the washing machine coil (100 °C). The pyrophosphate presence strongly inhibits the crystallization of hydroxylapatite in the washing system; thus, thermal treatment of amorphous calcium−pyrophosphate is a viable mechanism to produce hydroxylapatite with controlled crystallinity. A range of analytical techniques including X-ray diffraction, IR spectroscopy, and liquid- and solid-state NMR were employed to quantify the speciation kinetics in solution, to characterize the nature of the precipitates, and to monitor the phase transformations in the solid.