Potassium hydrogenoxalate

Potassium hydrogenoxalate is a salt with formula KHC2O4 or K+·HO2C-CO2−. It is one of the most common salts of the hydrogenoxalate anion, and can be obtained by reacting potassium hydroxide with oxalic acid in 1:1 mole ratio.
The salt is also known as: potassium hydrogen oxalate, potassium bioxalate, acid potassium oxalate, or monobasic potassium oxalate. In older literature, it was also called: Salt of sorrel, sorrel salt, sel d'oseille, sal acetosella; or, "inaccurately, salt of lemon" (due to the similar acidic "“lemony”" taste of the edible "common sorrel" or "garden sorrel"; see "sorrel" below).
Potassium hydrogenoxalate occurs in some plants, notably sorrel. It is a commercial product used in photography, marble grinding, and removing ink stains.
Properties.
The anhydrous product is a white, odorless, crystalline solid, hygroscopic and soluble in water (2.5 g/100 g at room temperature). The solutions are basic. Below 50 °C the much less soluble "potassium tetraoxalate" • forms and precipitates out of solution.
The monohydrate KHC2O4·H2O starts losing the water at 100 °C.
The anhydrous salt was found to have remarkable elastic anisotropy, due to its crystal structure that consists of relatively rigid columns of hydrogen-bonded hydrogenoxalate anions, joined into sheets by ionic K–O bonds.
Toxicity.
Potassium hydrogenoxalate is strongly irritating to eyes, mucoses and gastrointestinal tract. It may cause cardiac failure and death.