Potassium Oxalate, K₂C₂O₄,H₂O

Oxalic acid forms with potassium not only the two salts which, according to the dibasic nature of the acid, are to be expected, but also another salt which can be regarded as a compound of oxalic acid with acid potassium oxalate. Of the salts of oxalic acid, those with potassium are the best known, because they occur in the juices of various plants, from which they were early prepared, and have led to the knowledge of oxalic acid.

Normal potassium oxalate, K₂C₂O₄,H₂O, is a white salt soluble in water, and is used in photography.

Acid potassium oxalate

Acid potassium oxalate, KHC₂O₄ + 1H₂O, is called salt of sorrel, because it was first obtained by evaporation and crystallisation from the juice of the wood-sorrel. It is less soluble than the normal salt, and is used for removing iron and ink stains, since it converts iron salts into soluble (complex) compounds.

Potassium tetroxalate

Potassium tetroxalate is the name given to the salt KHC₂O₄. H₂C₂O₄ + 2H₂O, which is easily obtained by mixing one of the previous salts with the necessary excess (or rather more) of oxalic acid in warm, concentrated solution. It then quickly crystallises out, since it is rather sparingly soluble. This salt is used in volumetric analysis in place of free oxalic acid, because it does not effloresce so readily as the latter, and allows therefore of definite amounts being accurately weighed out.

If two equal amounts of acid potassium oxalate are weighed out, and one converted, by heating, into potassium carbonate and then dissolved along with the second portion, a liquid having a perfectly neutral reaction is obtained, after the carbonic acid is boiled off. This is a proof that in the acid oxalate exactly half as much potassium is contained as is necessary for the preparation of the normal salt. In this simple manner, the law of multiple proportions was proved by Wollaston as early as the year 1808.