214 PETTENKOFER'S REACTION. 



f urol 1 by the action of the sulphuric acid upon the sugar, 

 the colour arising from the interaction of furfurol with cholalic 

 acid. 2 



It is important to remember that an extended series of sub- 

 stances other than cholalic acid and the bile-acids (pigments and 

 other substances which are charred by sulphuric acid) either 

 interfere with the brilliancy of the reaction or else themselves 

 yield a purple colour which closely resembles that due to the 

 bile-acids. Among the latter those of chief importance are pro- 

 teids, amyl-alcohol, oleic acid, the higher fatty acids, and cho- 

 lesterin. 3 A further element of uncertainty is introduced by the 

 fact that if the suspected solution be extremely dilute no charac- 

 teristic colour is obtained although bile-acids may be present. 

 All the above militate against the detection of bile-acids in fluids 

 such as urine, in which their determination is a matter of not in- 

 frequent importance. The application of Pettenkofer's reaction 

 in its original form has hence been modified in details by many 

 observers with a view to rendering it more decisive and delicate. 

 The decisiveness of the reaction is ensured by careful spectro- 

 scopic examination of the absorption spectrum of the coloured 

 solution, since the colours produced by the majority of those 

 substances which yield a reaction resembling that produced by 

 cholalic acid, show no absorption bands in their spectra. Some 

 few however do exhibit absorption bands which fortunately oc- 

 cupy a different position in the spectrum from those shown by 

 cholalic acid (Udranszky). If the suspected solution is ex- 

 tremely dilute it may frequently be made to yield Pettenkofer's 

 reaction directly by a previous concentration on the water-bath. 

 A further modification which is applicable to dilute solutions is 

 the following. A little cane-sugar is dissolved in the solution 

 and a strip of filter-paper dipped into it and then air- dried. 

 When dry one drop of concentrated sulphuric acid is applied to 

 the paper with a glass rod. If bile-salts are present (even to the 

 extent of -03 p. c.) a distinct violet stain may be observed on the 

 paper after standing for a quarter of a minute : the stain is most 

 easily seen by transmitted light. 4 Instead of sugar an aqueous 

 (O'l p. c.) solution of furfurol may be used to great advantage as 

 follows. One drop of this solution is added to 1 c. c. of the sus- 

 pected solution, either aqueous or alcoholic, in a test tube. To 

 the above is then added 1 c. c. of concentrated sulphuric acid and 

 the mixture is cooled under water so that its temperature does 

 not exceed 50 60 C. To detect bile-acids in urine with ab- 

 solute certainty it is essential to separate them from this excre- 



1 Also known as furfuraldehyde C 4 H 3 O . COH, the aldehvde of pyromucic acid 

 C 4 H 3 O . COOH. 



2 Mylius, Zt.f. physiol. Chem. Bd. xr. (1887), S. 492. 



3 For a complete list of these see Udranszkv, loc. rit. S. 358. 



4 Strassburg, Pfliiger's Arch. Bd. iv. (1871)', S. 461. 



