CH. XII.] URIC ACID. 293 



Although the aqueous solutions of uric acid react 

 neutral, it behaves like a disbasic acid C 5 H 2 N 4 O 3 .H 2 and 

 can form two series of salts, C 5 H 2 N 4 O 3 .Na 2 (neutral, normal, 

 or di-sodium urate) and C 5 H 2 N 4 O 3 .HNa (biurate, acid urate 

 or mono-sodium urate). It is also possible that there is a 

 third form of salt, C 5 H 2 N 4 O 3 .HNa.C 5 H 4 N 4 O 3 (quadriurate 

 or hemi-sodium urate), though this may be merely a mixture 

 of its two constituents. The di-sodium salts are more 

 soluble than the mono-sodium, but are only stable in 

 markedly alkaline solutions. In the blood and urine 

 urates exist as mono-sodium salts, which react neutral. 



It is interesting to note that there are two modifica- 

 tions of the mono-sodium salt, called the a- and /3-form. 

 The a-form is more soluble than the /3-form, but is un- 

 stable, and slowly passes over into the other form. They 

 are probably the salts of the two tautomeric modifications 

 of uric acid described by Fischer : 



NH - CO N = C.OH 



CO C-NH^ HO.C C-NH^ 



> || || CO 



- C - NH^ N - C - 



Lactam modification forming Lactim modification forming 



unstable a-urate. stable /?-urate. 



It is of great interest to observe that in gout the 

 amount of urate in solution in the blood is in excess of the 

 amount of the /3- urate that can be held by normal blood. 

 So that in gout it must be present at least, partly, in the 

 unstable a-form. The deposition of urates in the tissues 

 during an acute attack may be due to the conversion of the 

 unstable a- into the stable, less soluble ^-modification. 



Urates are completely precipitated as amorphous 

 ammonium urate by saturation with ammonium chloride. 



They exert a reducing reaction on Fehling's solution 

 and towards alkaline silver solutions, this being the basis 

 of Schiff's test. 



They yield a characteristic colour reaction when 

 evaporated with nitric acid, the so-called murexide test. 



