1282 INDIGO-BLUE — SKATOLE. 



Indoxylsiilphuiic acid is not known in the free state ; its 

 most important salt is that with potassium, the form in which 

 it occurs in urine. When warmed in aqueous solution with 

 hydrochloric acid it decomposes into indoxyl and potassium 

 sulphate: — 



C 8 H 6 N . O . S0 2 . OK + H 2 = C 8 H 6 N(OH) + KHS0 4 . 



Indoxyl by oxidation is converted into indigo-blue: — 



2C 8 H 6 N(OH) + 2 = C 16 H 10 N 2 O 2 + 2H 2 0. 



The blue coloration which results from the above reaction 

 affords the one test for the presence of indican in urine. The 

 test is applied as follows (Jaffe). A small volume of urine 

 (10 c.c.) is mixed with an equal volume of strong hydrochloric 

 acid and 2 — 3 c.c. of chloroform. A strong solution of chloride 

 of lime is then added drop by drop, shaking after the addition 

 of each drop. If indican is present the layer of chloroform 

 which settles on standing will be coloured more or less brill- 

 iantly blue according to the amount of indican in the urine. 

 The formation of indigo-blue is also the basis for the quantita- 

 tive estimation of indican. The latter is converted into indigo- 

 blue by oxidation and the indigo-blue is estimated either by 

 weighing or colorimetrically or spectrophotometrically. 



3. Indigo-blue. C 16 H 10 N 2 O 2 . 



It is formed, as stated above, from indican, and gives rise 

 to tlie bluish colour sometimes observed in sweat and urine. 



It may, by slow formation from indican, be obtained in fine 

 crystals ; these are insoluble in water, slightly soluble, with a 

 fiint violet colour, in alcohol and in ether. Chloroform dis- 

 solves them to a slight extent, as also does benzol. Indigo is 

 soluble in strong sulphuric acid, forming at the same time two 

 compounds with the acid, indigo mono- and di-sulphonic acids. 

 The sodium salts of these acids are soluble in water and, mixed 

 with sodium sulphate, constitute the crude 'indigocarmine ' of 

 commerce, and in a purer form the sulphindigotate of soda used 

 in certain experiments on the nature of the excretory activity 

 of the kidney and other glands (see § 336). These soluble 

 sulphonates give an absorption band in the spectrum which lies 

 to the red side of and close to the D line. This may be used 

 to detect indigo. 



r / NH \ i 



4. Skatole. C 9 H 9 N. C 6 H 4 Q ^CH. Methyl-indole 



NH 



C 



I 

 ('II 



3 



