§ 75. UEIC ACID. 105 



QaantitatiTe estimation.— This acid in the free state 

 may be determined by a volumetric process, the same as 

 for the determination of acetic acid. The solution, free 

 from acetic or other free acid except lactic acid, is titrated 

 with the standard sodic solution. Each cubic centimetre 

 of sodic solution required corresponds to 0.81 grra. of the 

 anhydrous acid, CgH^^O^. 



To remove acetic acid as well as carbonic from the so- 

 lution, before estimating the lactic acid, evaporate a por- 

 tion of the liquid in the water-bath with the addition of 

 pure quartz sand and with constant stirring towards the 

 end of the operation ; continue to heat the dry residue 

 until no more acid odor is given off, then treat it with 

 water, filter, and wash the sand on the filter as long as the 

 washings are acid, and determine lactic acid in the filtrate 

 with the standard sodic solution as above. 



URIC ACID. H3C5H2N4O2 + 4aq. 304. 



75. This acid is but slightly soluble in water, and is 

 insoluble in alcohol. Alkaline urates are soluble in water ; 

 others are insoluble. 



Reactions. — If uric acid or a* urate is heated with mod- 

 erately strong nitric acid, the mixture filtered if not clear, 

 the filtrate carefully evaporated to dryness, and the resi- 

 due moistened with ammonia, a beautiful purple color 

 (murexide) appears. 



In urinary sediments, uric acid may often be recognized 

 under the microscope by the rhombic six-sided plates, or 

 right-angled four- sided prisms of a brown to a golden yel- 

 low color, which it forms. 



Quantitative estimation. — Precipitate the uric acid 

 from the solution containing it by the addition of hydro- 

 chloric acid, if no albumen is present ; in case it is pres- 

 ent, use acetic or phosphoric acid instead of hydrochloric. 

 Let the mixture stand 36-48 hours, and collect the pre- 



