Is Phosphonate a Chelating or Sequestering agent? Why Phsphonates are threshold scale inhibitor?

“Chelating agent, sequestering agent and phosphonates are all antiscalants and corrosion inhibitors. But their mode of action is different.”

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Phosphonates are not chelating agents like EDTA and NTA which require one mole of chelating agent to react with one mole of calcium ions, magnesium ions, iron (II) or iron (III) ions not removed by pretreatment methods such as ion exchange or demineralization in boilers. Lower dosages of phosphonates are required because they inhibit the active growth sites of insoluble calcium salts rather than reacting stoichiometrically with calcium ions or magnesium ions or iron (II) or iron (III) ions. The colloidal particles formed by phosphonates with these growth sites may only be stable for a few days in some cooling water or boiler water applications. Maintaining phosphonate levels and controlling blowdown rates can reduce deposition on heat transfer surfaces. Heat transfer rates should be monitored closely because these colloidal particles are not stable indefinitely.

Commented on October 21, 2018 15:35 by Paul Labine, Adjunct Professor of Chemistry at Palm Beach State College