.COMPOSITION AND CHARACTERS OF URINE 1249 



stand for two days. The uric acid is thrown down in small dark-red or brown 

 crystals. They can be collected on a filter, dissolved in alkali, decolorised by 

 boiling with animal charcoal, and the pure acid thrown down as before hydro- 

 chloric acid. 



A more convenient method of preparation from human urine is based on 

 the fact that ammonium urate is insoluble in concentrated solutions of ammo- 

 nium chloride (Hopkins). The urine is saturated with crystals of ammonium 

 chloride and a few drops of strong ammonia added. A gelatinous precipitate of 

 ammonium urate is produced. This is collected on a filter, washed off with a 

 minimum amount of hot water into a beaker, and a few drops of hydrochloric 

 acid added. The mixture is boiled and then allowed to cool, when the pure acid 

 crystallises out. 



TESTS FOR URIC ACID 



(1) MUREXIDE TEST. If a small quantity of uric acid be treated with 

 a little strong nitric acid and the whole evaporated to dryness on the water- 

 bath, an orange-red residue is obtained, which on treatment with ammonia 

 yields a fine purple colour. If a drop of sodium hydrate be now added the purple 

 changes to blue. Instead of nitric acid, bromine water may be employed. 



(2) SCHIFF'S TEST. If uric acid be dissolved in a little soda and a drop be 

 placed on filter paper previously moistened with silver nitrate, a yellow or brown 

 spot is produced. 



(3) On boiling uric acid with Fehling's solution for some time, a yellowish 

 precipitate of cuprous hydrate is produced. 



(4) An alkaline solution of uric acid on treatment with a few drops of a 

 solution of phosphomolybdic acid gives a dark blue precipitate with a metallic 

 lustre, consisting of microscopic prismatic crystals. 



(5) With sodium hypobromite uric acid is decomposed, giving off about half 

 of its nitrogen as the free gas. 



URATES. Of the four hydrogen atoms in uric acid two can be 

 replaced by metallic radicals. Uric acid thus acts as a weak dibasic 

 acid. It forms three orders of salts, namely, the neutral urates, the 

 bi-urates, and the quadri-urates. The neutral urates, M' 2 U, are very 

 unstable, and only exist in the presence of caustic alkalies. They are 

 decomposed even by the carbonic acid of the atmosphere. The bi- 

 urates, MHU, are the most stable of the urates. They may be pre- 

 pared by dissolving uric acid with the aid of heat in weak solutions 

 of the alkaline carbonates, from which they separate, on cooling, in 

 stellar crystals. 



The quadri-urates have the formula H 2 U, MHU. They may 

 be prepared by boiling uric acid with dilute solutions of potassium 

 acetate. On cooling the mixture the quadri-urates separate as an 

 amorphous precipitate or in crystalline spheres. The quadri-urates 

 are extremely unstable, and in the presence of water are broken up 

 into the bi-urates and free uric acid. It is probable that under 

 normal conditions the greater part of the uric acid in the urine is 

 present in the form of a quadri-urate (Roberts), and the so-called lateri- 

 tious deposit, the brick-red amorphous precipitate of urates which 

 occurs in concentrated urine on cooling, consists of these quadri-urates. 



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