THE AROMATIC OXY-ACIDS. 281 



its melting-point, 72 C. Should smaller amounts, however, be 

 present, crystallization does not occur. In this case the distillate is 

 extracted with ether by shaking. Traces of benzoic acid and the 

 phenols are thus likewise extracted, but if then the residue of the 

 ethereal solution is washed with water other impurities are removed, 

 and the crotonic acid remains. 



Quantitative Estimation (according to Darmstaedter). The method 

 is based on the decomposition of the /9-oxybutyric acid with the for- 

 mation of a-crotonic acid and the estimation of the latter. 100 c.c. 

 of urine are rendered feebly alkaline with sodium carbonate and 

 evaporated on a water-bath almost to dryness. With the aid of 

 150-200 e.c. of sulphuric acid (50-55 per cent.) the residue is 

 transferred to a liter flask, which is closed with a doubly perforated 

 stopper. Through the one aperture a drip-tube passes, while a bent 

 glass tube passes through the other to a condenser. Heat is applied, 

 at first mildly, so as to avoid foaming ; then vigorously. Water is 

 allowed to enter through the drip-tube as fast as the distillate passes 

 over. The distillation is interrupted when from 300 to 350 c.c. have 

 been obtained, which usually takes from two to two and one-half 

 hours. The distillate is extracted two or three times with ether. 

 The ether is distilled off, the residue heated for a few minutes on a 

 sand-bath to 160 C. in order to drive off any fatty acids that may 

 be present, and then dissolved on cooling with 50 c.c. of water. 

 The solution is filtered, and the filter washed with a little water. 

 The aqueous solution of the crotonic acid is now titrated with a 

 decinormal sodium hydrate solution, using phenolphthalein as an 

 indicator. 1 c.c. of the soda solution corresponds to 0.0086 gramme 

 of crotonic acid. The corresponding amount of oxybutyric acid is 

 obtained by multiplying by 1 .21 . Sugar does not interfere with the 

 process. 



From what has been said above, it is clear that every urine which 

 contains /9-oxybutyric acid will probably also contain diacetic acid. 

 On the other hand, it will also be understood that diacetic acid may 

 occur in the urine in the absence of /9-oxybutyric acid, and this is 

 indeed more common. 



Diacetic Acid. 



Tests. As diacetic acid is rapidly decomposed on standing, it is 

 necessary that the urine should be as fresh as possible, when it is to 

 be examined in this direction. To this end, several direct tests are 

 available. 



ARNOLD'S TEST (as modified by Lipliawski). Two solutions 

 are employed, viz., a 1 per cent, solution of para-am ino-aceto- 

 phenon and a 1 per cent, solution of potassium nitrite ; 6 c.c. 

 of the first solution and 3 c.c. of the second are added to an 

 equal volume of urine, together with a drop of concentrated am- 



