CHEMICAL BASIS OF THE ANIMAL BODY. 201 



Indoxyl-sulphuric acid is not known in the free state ; its most 

 important salt is that with potassium, the form in which it occurs 

 in urine 1 . When warmed in aqueous solution with hydrochloric acid 

 it decomposes into indoxyl and potassium sulphate : 



C 8 H 6 N . O . SO 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 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 (lOc.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 according 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 

 colorimetrically 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 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 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 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. 



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

 /. phijsiol. Chem. Bd. in. (1879), S. 254. See also Baumann u. Tiemann, Ber. 

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



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

 Wlss. 1877, S. 357. 



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

 S. 492. 



