Sodium polyphosphates covered by this specification are intended for use in industrial water treatment but are not intended for use in water for human consumption. They form soluble complexes with calcium and magnesium and may be used to soften water and control scale formation in condensers, heat exchangers, pipelines, and boilers. Sodium polyphosphates are interchangeable for the intended use, but possess the differences indicated below. Selection of the type and form should depend upon established patterns of usage, compatibility with equipment, and economic advantage.

Sodium tetraphosphate is primarily intended for use in water treatment for laundries.

Sodium hexametaphosphate is primarily intended for use in the treatment of boiler feedwater. This type in commonly used for the threshold treatment of industrial water. In those cases where the use of this type requires the addition of a relatively large quantity of caustic soda or soda ash, type III might be preferred.

Sodium tripolyphosphate is intended for use as a substitute for types I or II whenever it is determined to be necessary for performance or economically advantageous.