Squaric acid

Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula C₄H₂O₄.

The conjugate base of squaric acid is the hydrogensquarate anion C₄HO₄; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C₄O₄. This is one of the oxocarbon anions, which consist only of carbon and oxygen.