URIC ACID. 163 



then gives with ammonia a reddish-purple salt, which 

 serves to detect and identify the acid. 



The urates as found in the urine are either in solu- 

 tion or form a sediment. The latter is generally amor- 

 phous and is always colored yellow to brown. Acid sodium 

 urate may occur in spherical aggregations of microscopic 

 acicular crystals. Ammonium urate, formed when urine 

 becomes alkaline by fermentation, may be found as 

 brownish spherules covered with irregular spicules, the 

 so-called "thorn-apple" crystals. (Plate II, 9 and 11.) 

 The amount of uric acid in urine is sometimes found by 

 precipitating from a measured volume of urine by hydro- 

 chloric acid, the albumin having first been removed if it 

 is present. After washing the crystals they are weighed. 

 The results thus obtained are too low, because of the slight 

 solubility of the crystals in water. Volumetric methods 

 may also be employed. These are as accurate and no more 

 difficult. 



351. Prepare uric acid from urine by quite strongly 

 acidifying a beakerful with hydrochloric acid. In twenty- 

 four hours the uric acid will have separated. Examine the 

 crystals under the microscope. It can be purified and 

 gradually freed from the color which it derives from the 

 urine by repeatedly dissolving in concentrated sulphuric 

 acid and reprecipitating by diluting with water. 



352. Precipitate from urine the uric acid with acids 

 of varying concentration, acting for different times. Sketch 

 the principal forms obtained. 



353. Dissolve a few of the crystals of the acid in 

 sodium hydrate and add a few drops of Fehling's solution. 

 Boil and the red cuprous oxid will be formed, best seen 

 by the use of a dark background. 



