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Uric Acid

1911 Encyclopedia Britannica

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C 5 H 4 N 4 0 3, in organic chemistry, an acid which is one of the penultimate products of the tissue waste in the human body. While the bulk of the nitrogen of the albuminoids passes off through the bladder as urea, a small portion of it stops at the uric acid stage. Human urine contains only a fraction of a per cent. of the acid, chiefly as sodium salt; abundance of uric acid is met with in the excrement of serpents and birds, with whom it is the principal nitrogenous product of tissue waste. For its preparation guano is boiled repeatedly with a solution of borax in 120 parts of water. The filtered solution is acidified with hydrochloric acid, when impure uric acid separates out as a brown precipitate, which is washed with cold water; it is then dissolved in hot dilute caustic potash or soda, the solution filtered, and the filtrate saturated with carbon dioxide. An almost insoluble urate is precipitated, which is filtered, washed and decomposed by hot dilute hydrochloric acid. Uric acid separates as a white precipitate, which is filtered off, washed and dried, to be repurified by a repetition of the alkali process or otherwise. Pure uric acid forms a snow-white micro-crystalline powder, devoid of smell or taste, soluble in 1800 parts of boiling and in 14,000 parts of cold water, but insoluble in alcohol and in ether. For its detection in urine, the urine is mixed with excess of hydrochloric acid, and allowed to stand, when the uric acid separates out, generally coloured reddish by impurities. The precipitate is dissolved in a few drops of nitric acid and the solution cautiously evaporated to dryness. The residue when exposed to ammonia gas assumes the intense purple colour of murexide.

The acid, which was discovered by C. Scheele in 1776 in urinary calculi, was afterwards investigated by Liebig and Wohler. The determination of its constitution, and its relation to other vegetable and animal products, followed from the researches of A. von Baeyer and E. Fischer (see Purin).