3.16. POLYPHOSPHATES IN WATER
Phosphorus occurs as many oxoanions, anionic forms in combination with
oxygen. Some of these are strong complexing agents. Since about 1930, salts of
polymeric phosphorus oxoanions have been used for water treatment, for water
softening, and as detergent builders. When used for water treatment, polyphosphates
“sequester” calcium ion in a soluble or suspended form. The effect is to reduce the
equilibrium concentration of calcium ion and prevent the precipitation of calcium carbonate
in installations such as water pipes and boilers. Furthermore, when water is
softened properly with polyphosphates, calcium does not form precipitates with soaps
or interact detrimentally with detergents.
The simplest form of phosphate is orthophosphate, PO43-