A salt of phosphoric acid. In the water industry, polyphosphates are used as sequestering agents to control iron and hardness and as coating agents to control corrosion by formation of a thin passivating film on metal surfaces.
The complex phosphates also are a group of sequestering agents widely used in detergent formulations (except where phosphates are banned by law) because of their superiority in chemical water softening, sequestering, and other builder functions.
Waters containing concentrations of iron, manganese, calcium, or magnesium sometimes can be treated with a sequestering agent such as polyphosphate and kept from depositing these mineral precipitates or scales for a period of time. However, polyphosphate sequestering is not permanent, and therefore may not be as effective as actually removing the iron, manganese, and hardness minerals, as is done with iron filters and ion exchange water softening, for example. The sequestering value of polyphosphates is destroyed when they revert (hydrate) to orthophosphate. Polyphosphate reversion or hydration to orthophosphate occurs naturally in water with time. Intentions would be for this reversion not to happen and not to drop the sequestered water hardness, iron, and manganese out until after it reaches the waste water. But, the polyphosphate reversion process can be accelerated by various uncontrolled conditions, such as low pH, high temperature, and the presence of the oxides of certain heavy metals, including iron, calcium, copper, and zinc in water. It is important in phosphate feed water treatment operations to: 1)maintain a stable pH within the phosphate product’s performance rage. 2) determine the polyphosphate composition or blend that is most compatible with the specific water quality objectives and conditions, and 3) apply the appropriate dosage of phosphate to accommodate the system demand. Because of the difficulty in maintaining phosphate stabilities in the presence of varying pH, time, temperature, and metal oxides in most natural water supplies, the actual removal of iron, manganese, and water hardness is generally a more assuredly effective water treatment method.