Oxalic Acid is an organic acid that has the formula HO2C−CO2H. It occurs naturally in many foods like leafy greens, nuts, and legumes. It is a white crystalline solid that forms a colorless solution in water.
Oxalic Acid's main use is for cleaning or bleaching. It is used to get rid of rust and other tough stains off of metal, concrete, and even wood. Typically a solution is made with water and sprayed on the surface (letting it set for particularly bad stains), scrubbed, and rinsed clean. When Oxalic Acid comes in contact with rust, a double displacement chemical reaction occurs. This turns the iron oxide (rust) into iron oxalate, which is water soluble and dissolves and washes away. After treating with Oxalic Acid, rinse with plenty of water to avoid excess Oxalic Acid staying on metal for too long. It can lead to leaching of metal ions into solutions.
Oxalic Acid is also known for it's wood-bleaching properties, in fact, home improvement stores frequently sell it as "wood bleach". Once an old finish to a wooden surface like a deck is stripped off, then Oxalic Acid can be applied to lighten the color and soften the tone of the wood. A bright, clean surface is left for a new finish to be applied. This process can be repeated years down the road when the wood is looking dull, gray, and/or faded again. Also removing rust stains (from nails in particular) from wood is something bleach cleaners don't touch, but Oxalic Acid works like a charm!
Polishing stone is another fantastic benefit of the properties of Oxalic Acid. It is used in many floor cleaners for marble floors to restore shine.
An interesting use of Oxalic Acid is in beekeeping. Varroa Mites are tiny reddish-brown external parasites of honey bees. They feed and reproduce on mostly the larvae and pupae of the colony, causing malformation and weakening of the hive as well as transmitting viruses. Oxalic Acid has been shown to be effective against the Varroa Mites while leaving the bees unharmed.
**If using Oxalic Acid in beehive applications, be aware that the end user must be EPA certified.