188 



HIPPURIC ACID. 



boiled with milk of lime in slight excess, by which means the acid is 

 fixed as a hippurate of calcium. It is then filtered, the filtrate concen- 

 trated to a small bulk and treated when cold with hydrochloric acid in 

 slight excess ; this decomposes the calcium salt, liberating hippuric acid, 

 which separates out at once owing to its comparatively slight solubility. 

 It is then purified by several recrystallisations from boiling water, but 

 it is extremely difficult to obtain it colourless. 



When rapidly separated out from its aqueous solutions, as in the 

 above method of its preparation, it assumes the form of fine needles. 

 By slower crystallisation it yields long foursided prisms or columns with 

 pyramidal ends ; these are frequently arranged in groups and present 

 a semitransparent, milky appearance. 



FIG. 31. HIPPURIC ACID CRYSTALS. (After Funke.) 



When pure they are odourless and of a somewhat bitter taste. They 

 require 600 parts of water for their solution at.0, are very readily 

 soluble in hot water, also in alcohol and to a less extent in ether. 

 They are conveniently insoluble in petroleum-ether, in virtue of which 

 hippuric acid can be readily separated from benzoic acid which is 

 soluble in this reagent. Its solutions redden litmus-paper. 



Hippuric acid is monobasic, and forms salts which (except the 

 iron salts) are readily soluble in water ; from these solutions, if suf- 

 ficiently concentrated, excess of hydrochloric acid precipitates the acid 

 in fine needles. When heated with concentrated mineral acids it is 

 resolved into benzoic acid and glycin. The same decomposition occurs 

 readily in presence of putrefactive organisms. 



Apart from the characteristics already stated the acid may be 

 recognised by the following reactions. When gently heated in a small 

 tube the acid does not at once sublime as does benzoic acid, but melts 

 and solidifies again on cooling. If more strongly heated it melts as 



