1168 PHYSIOLOGY 



From urine uric acid may be obtained by adding one-fiftieth of its volume of concen- 

 trated hydrochloric acid and allowing to 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 by means of hydrochloric acid. 



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

 that ammonium urate is insoluble in concentrated solutions of ammonium 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 exist only 

 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 prepared 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 pre- 

 pared by boiling uric acid with dilute solutions of potassium acetate. On 

 cooling the mixture the quadri-urate separates 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 lateritious deposit, the brick-red amorphous precipitate of urates 

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

 urates. The exact condition of the urate however will depend on the 

 reaction of the urine. A bi-urate, with acid sodium phosphate, is decom- 

 posed with the formation of uric acid in the following way : 



MHU + MH 2 P0 4 = H 2 U + M 2 HP0 4 . 



