title: Potassium hydrogenoxalate
text: # Potassium hydrogenoxalate
  
  Potassium hydrogenoxalate is a salt with formula $KHC_2O_4$ or $K^+·HO_2C-CO_2^−.$ 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)
  
  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" $K^+[C_2HO_4]^−•C_2H_2O_4$ forms and precipitates out of solution.
  
  The monohydrate $KHC_2O_4·H_2O$ 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.
  
  