Etidronic acid (INN), or 1-hydroxyethane 1,1-diphosphonic acid (HEDP), is a bisphosphonate used in detergents, water treatment, cosmetics, and pharmaceuticals. Etidronate is a salt of etidronic acid, abbreviated MnHEDP (M: a cation, n: the maximum number of M is 4).
Etidronic acid is a sequestering agent and can be added to bind or, to some extent, supplement the effects of substances such as calcium, iron, or other metal ions that may be discharged as a component of gray wastewater and contaminate groundwater resources. As a phosphonate, it has corrosion inhibiting properties on untreated steel. Etidronic acid also delays the acidification and oxidation of fatty acids.