Oxalic acid is a potent but toxic cleaner that removes rust and stains caused by iron,... SF Gate Logo Hearst Newspapers Logo. ... Proper application may differ depending on how you'll be using it.

To use oxalic acid, mix one-half cup of the powder in a gallon of warm water and mop it on the stain. Let it work for five to 10 minutes, keeping the stain wet, then scrub with a stiff-bristled ...

Oxalic acid, or oxalate, is a compound in many plant foods. In large quantities, it can inhibit the absorption of some nutrients. Learn more.