200 INDIGO-BLUE. 



in urine. 1 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 O = 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 2 + 2H 2 0. 



The blue colouration 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 brilliantly blue accord- 

 ing to the amount of indican in the urine. 2 The formation of 

 indigo-blue is also the basis for the quantitative estimation of 

 indican. The latter is converted into indigo-blue by oxidation 

 and the indigo-blue is estimated either by weighing or colori- 

 metrically or spectrophotometrically. 3 



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



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

 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 

 faint violet colour, in alcohol and in ether. Chloroform dissolves 

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

 strong sulphuric acid, forming at the same 1 time two compounds 

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

 salts of these acids are soluble in water and, when mixed with 

 sodium sulphate, constitute the crude ' indigocarmine ' of com- 

 merce, 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 416). 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. 



Indigo as ordinarily seen possesses a pure blue colour ; it leaves 

 a reddish copper-coloured mark when pressed with a hard body, 

 and the crystals exhibit the same colour if seen in reflected light. 



Treated with reducing agents in strongly alkaline solution in- 



1 For its isolation and preparation from urine see Baumann u. Brieger, Zl. 

 f. physiol. Chem. Bd. in. (1879), S. 254. See also Baumann u. Tiemanu, Ber. 

 'd. deutsch. Chem. Gesell. xn. (1879), Sn. 1098, 1192 ; and xm. (1880), S. 408. 



2 Jafft', Pfluger's Arch. Bd. HI. (1870), S. 448. Cf. Senator, Centralb. f. d. med, 

 Wiss. 1877, S. 357. 



3 For details and literature see Xeubauer u. Vogel, Die Harnanalyse, 1890, 

 S. 492. 



