URIC ACID, as explained fully in the article Nutrition (vol. xvii. p. 683), 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 (q.v.), 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 soda 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 the preparation of uric acid (C 5 H 4 N 4 O 3 ), Guano (q.v. ) 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. For its purification it is dissolved in hot dilute caustic potash or soda ley, the solution filtered, and the filtrate saturated with carbonic acid. An almost insoluble acid urate of alkali separates out, which is collected on a filter, washed, and decomposed by adding it in instalments to hot dilute hydrochloric acid. Uric acid separates out as a white precipitate, which is filtered off, washed, and dried, to be re-purified by a repetition of the alkali process or otherwise. Pure uric acid forms a snow-white microcrystalline 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. When heated it suffers complete decomposition....
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. To identify the precipitate, it is dissolved in a few drops of nitric acid on a watch-glass, and the solution cautiously evaporated to dryness. The residue, if exposed to ammonia gas as it comes off from the stopper of a liquor ammoniae bottle, assumes the intense purple colour of “murexide.”
Urates.— Uric acid dissolves in excess of caustic potash ley as dipotassic salt, C 5 H 2 K 2 N 4 O 3 , soluble in 44 parts of cold water, with the formation of a strongly alkaline solution. On the saturation of the solution with carbonic acid one-half of the potash is eliminated as bicarbonate, and a neutral salt, C 5 H 3 KN 4 O 3 , comes down as a precipitate, soluble in 800 parts of cold water. Soda and the alkaline earths behave similarly. But more interesting than its salts are the numerous derivatives obtainable from it, chiefly by the action of oxidizing agents. Most of these were discovered and investigated by Liebig and Wohler in a classical research published in 1838. From 1861 to 1863 Adolf Bayer supplemented their work by important new discoveries. He showed that all those metamorphoses of uric acid fall in naturally with the assumption that the molecule of uric acid consists of two urea rests—(NH)(CO)(NH)— united each by its two nitrogen affinities with a tricarbon group, (C 3 O), by carbon affinities of the latter. To give an example of the practical meaning of the theory,—if uric acid is oxidized cautiously with nitric acid, one of the urea rests by uniting with the 2H of an H 2 O is split off as urea, while the rest unites with the oxygen of the H 2 O and other O + H 2 O into alloxan,— the ureid of mesoxalic acid, HO - (CO . C(OH) 2 . CO) - OH. On the application of more energetic oxidizing agents, that one urea rest still survives, but the tricarbon nucleus burns down into C 2 O 2 , the radical of oxalic acid, with formation of parabanic acid,
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