A chemical added to an acid to stabilize iron. The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron (Fe⁺³) or ferrous iron (Fe⁺²). If the iron is not controlled, it will precipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide (FeS), which will damage the formation.

Reducing agents change or reduce Fe⁺³ to Fe⁺² to avoid precipitation. Erythorbic acid is an effective reducing agent.