Definition - What does Polyphosphate mean?

Polyphosphates are water purification chemicals that are employed to correct problems caused by inorganic groundwater contaminants (iron, manganese, calcium, etc.) and also to preserve water quality in distribution systems.

In the treatment of potable (drinking) water polyphosphates are used to:

  • Prevent "red" (from iron) and "black" (from manganese) water
  • Reduce soluble lead and copper in potable water
  • Clean or dissolve precipitated mineral scale already existent in water distribution lines
  • Prevent and/or retard scale formation (from minerals depositing) and corrosion (from low pH and/or dissimilar metals) in the water distribution system

Corrosionpedia explains Polyphosphate

Polyphosphate is a type of salt or ester of polymeric oxyanion formed from tetrahedral PO4 (phosphate) structural units connected together by sharing oxygen atoms. Polyphosphates can have linear or cyclic ring structures.

In water treatment general orthophosphate and polyphosphate are used. They may be used either alone or mixed together to stabilize water quality and minimize corrosion in drinking water systems. Polyphosphate-type chemicals react with soluble metals (Fe, Mn, Ca, Mg, etc.) by sequestering (binding) the metals to maintain their solubility in water. The sequestering process minimizes the risk of:

  • Discoloration
  • Staining
  • Scaling
  • Chlorine demand
  • Foul taste/odor
  • Other water quality issues

Polyphosphates were one of the first chemicals used as corrosion inhibitors and later as chromate substitutes. Polyphosphate is recognized as a cathodic inhibitor, which forms a durable polarizing film on the cathodic surfaces of most metals by an electrodeposition mechanism. Polyphosphate has the added advantage of being a scale inhibitor at threshold levels as low as 1-5 ppm.

The best corrosion results are not usually attained when all of the inorganic phosphate (polyphosphate and orthophosphate) is in polyphosphate form. Specifically, studies indicate a ratio of 65% polyphosphate to 35% orthophosphate provides the best corrosion inhibition on an inorganic phosphate treatment program.

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