Barreswil found that a strongly alkaline solution of copper sulphate and potassium sodium tartrate (Rochelle salt) remained unchanged on boiling, but yielded an immediate precipitate of red cuprous oxide when a solution of glucose was added.
The tartrate and acid tartrate are also diuretic in their action and, as well as the sulphate, are valuable hydragogue saline purgatives.
Edmund Davy first made acetylene in 1836 from a compound produced during the manufacture of potassium from potassium tartrate and charcoal, which under certain conditions yielded a black compound decomposed by water with considerable violence and the evolution of acetylene.
Another mode of separating the two acids is to convert them into calcium salts, which are then treated with a perfectly neutral solution of cupric chloride, soluble cupric citrate and calcium chloride being formed, while cupric tartrate remains undissolved.
Citric acid is also distinguished from tartaric acid by the fact that an ammonia solution of silver tartrate produces a brilliant silver mirror when boiled, whereas silver citrate is reduced only after prolonged ebullition.
Tartaric acid, which is sometimes present in large quantities as an adulterant in commercial citric acid, may be detected in the presence of the latter, by the production of a precipitate of acid potassium tartrate when potassium acetate is added to a cold solution.
During the alcoholic fermentation of grape juice it is deposited in the form of an impure acid potassium tartrate which is known as argol, and when purified as cream of tartar.
For the preparation of the acid the crude argol is boiled with hydrochloric acid and afterwards precipitated as calcium tartrate by boiling with milk of lime, the calcium salt being afterwards decomposed by sulphuric acid.
This consists chiefly of cream of tartar (bitartrate of potash), tartrate of lime, yeast cells and of albuminous and colouring matters.