INDOLE AND ITS DERIVATIVES 345 



Indoxyl consists of yellow crystals which dissolve in water with a 

 green fluorescence, also in alcohol, ether, acetone. It melts at 85. 

 Dilute acids convert it into a red substance and an unpleasant smell 

 is produced. In alkaline solution it oxidises in the air to indigo. 



Detection in Urine. 



Indoxyl is detected by conversion into indigo blue : 



(1) An equal volume of concentrated hydrochloric acid is added 

 to 10 c.c. of urine and 2 or 3 c.c. of chloroform. A very dilute solu- 

 tion of bleaching powder is added, drop by drop, and the solution is 

 inverted after the addition of each drop. The chloroform becomes 

 bluish-violet owing to the formation of indigo blue. Excess of bleach- 

 ing powder must be avoided as the indigo blue undergoes further oxi- 

 dation to colourless compounds. 



(2) The further oxidation is to a large extent avoided by using 

 a fresh solution of ferric chloride in concentrated hydrochloric acid as 

 the oxidising agent (Obermayer's reagent). An equal volume of this 

 reagent and a few c.c. of chloroform are added to the urine. On mixing 

 thoroughly for 1-2 minutes by inverting the liquids, the chloroform 

 becomes blue. 



(3) Salkowski recommends the use of copper sulphate as the oxidising 

 agent to prevent further oxidation. An equal volume of hydrochloric acid, 

 i c.c. of copper sulphate solution and a few c.c. of chloroform are added to 

 10 c.c. of urine and the mixture shaken carefully as above. 



(4) The following other oxidising agents may be employed : 



One drop of a 10 per cent, solution of potassium persulphate to 5 or 6 

 c.c. of urine. 



One drop of a 3 per cent, solution of potassium chlorate to 10 c.c. of 

 urine. 



Indigo Blue or Indigotin. 



Indigo blue is formed by the oxidation of indoxyl in alkaline solu- 

 tion on exposure to the air. Two molecules of indoxyl combine in 

 this reaction : 



iC-OH HOC 



^CH HC 1 



NH NH 



Indigo blue is a dark blue powder and shows a metallic coppery 

 lustre on rubbing. It sublimes giving copper-red glistening prisms. 

 It is insoluble in water, alcohol, ether, dilute acids and alkalies, and has 

 neither smell nor taste. It dissolves in aniline and molten paraffin 

 with a purple-red colour, also in turpentine from which it crystallises. 



