THE CHEMISTRY OF URIC ACID 505 



HN-C=0 HN-C=0 



HCJ-C-NH O=0 NH 



I I > H > c=a 



gL_6_jf HN-C-NH 



Hypoxanthin Uric acid 



(6-oxypurin) (2-6-8-trioxypurin) 



H 3 C-N = 



>H 



H S C N-0- 



Caffein Theobromine 



(1-3-7 trimethyl-2-6 dioxypurin) (3-7-dimethyl, 2-6 dioxypurin) 



Properties of Uric Acid. Uric acid, when pure, is 

 white, and crystallizes in rhombic tablets. Its solubility 

 is very slight; at room temperature (18) it dissolves but 

 about one part to 40,000 of water, so that a saturated solution 

 contains but 0.0253 gram to the liter. It is much more solu- 

 ble in blood-serum, dissolving in 1000 parts, 1 probably held 

 in some complex combination. His and Paul have shown 

 that in a saturated solution only 9.5 per cent, of the molecules 

 are dissociated, the dissociation occurring in two steps ; the first 

 and chief dissociation is into H and C 5 H 3 N 4 O 3 , which then 

 undergoes further dissociation into H and C 5 H 2 N 4 O 3 , the latter 

 dissociation being very slight. If any other acid is present 

 in the solution, its dissociation and liberation of free hydro- 

 gen ions interferes with the dissociation of the uric acid, and as 

 the undissociated uric acid is extremely insoluble, the amount 

 dissolved in an acid solution is much less than in a neutral 

 solution. 



With alkalies uric acid yields two series of salts, correspond- 

 ing to these two steps in dissociation : one, in which one atom 

 of the base enters, is called the biurate or monobasic urate ; the 

 other is the so-called " neutral " or bibasic urate. 2 Of the two, 

 the latter is much the more soluble. The monosodium urate 

 forms colloidal solutions in water, from which the crystalline 

 salt gradually falls out. 



In the urine the uric acid and the urates are kept in solution 

 by the phosphates, the disodium phosphate preventing the 

 decomposition of the urates into uric acid by the acid salts of 

 the urine. Possibly other constituents of the urine, especially 



Baylor, Jour. Biol. Chem., 1906 (1), 177. 



2 As a matter of fact, both salts give a slightly alkaline reaction when dis- 

 solved in water (Taylor). 



