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Chelating agent

Also known as
chelant, sequestrant, chelator

A chelating agent, or chelant, is a molecule that binds a metal ion at two or more points, wrapping it in a ring-shaped complex called a chelate. Chelants keep metal ions such as calcium, magnesium and iron in solution and prevent them from reacting, precipitating or forming scale.

What is a chelating agent?

The word comes from the Greek for claw. A chelating agent has several atoms, typically oxygen or nitrogen, that can each bond to the same metal ion, gripping it from multiple sides. The resulting complex, a chelate, is much more stable than one formed by a molecule that binds at a single point. While bound, the metal ion is effectively taken out of play: it cannot precipitate with other ions, deactivate surfactants or catalyze unwanted reactions. This is also called sequestering.

Common chelating agents

  • EDTA (ethylenediaminetetraacetic acid), a widely used, strong chelant that biodegrades poorly.
  • GLDA and MGDA, chelants marketed as readily biodegradable alternatives to EDTA.
  • Citrate and gluconate, milder chelants from natural sources.
  • Phosphonates, used in scale control and cleaning.

Uses

  • Cleaning products. Chelants bind calcium and magnesium from hard water, protecting surfactant performance. They are one type of detergent builder.
  • Scale prevention and removal in water systems and oilfield operations, alongside scale inhibitors.
  • Agriculture. Chelated micronutrients, such as iron, zinc and manganese, remain available to plants in soils where they would otherwise become insoluble.
  • Personal care and food, to protect product stability and appearance.

Frequently asked questions

A chelant binds metal ions in roughly fixed proportions, so it is typically used at concentrations related to the amount of metal. A threshold scale inhibitor works at very low doses by disrupting crystal growth rather than binding all the ions.

Some are and some are not. EDTA biodegrades poorly, which has driven interest in more readily biodegradable alternatives.

  • Water hardness

    Water hardness is the amount of dissolved calcium and magnesium in water, usually expressed as milligrams per liter of calcium carbonate. Hard water reduces the performance of soaps, surfactants and some pesticides, and causes scale in pipes and equipment.

  • Scale inhibitor

    A scale inhibitor is a chemical that prevents or slows the formation of mineral scale from water, usually at very low concentrations, by interfering with how scale crystals form and grow. Phosphonates and polymers such as polyacrylates are common types.

  • Detergent builder

    A detergent builder is an ingredient that improves a detergent's cleaning, mainly by softening the water: it removes or ties up calcium and magnesium ions that would otherwise interfere with the surfactants. Builders also often supply alkalinity and help keep removed soil suspended.

Sources

  1. chelation, IUPAC Compendium of Chemical Terminology (Gold Book)
  2. Chelating agents in cleaning products, American Cleaning Institute