TRANSACTIONS OP SEdTtON B» 529 



Dalsin^h Terai Research Station (under subsidy of the Government of Bengal) 

 from 1903 to March 190i,' by VV. P. Bloxain and 11, M. Leake, with the assistance 

 of R. S. Finlow (Calcutta : The Bengal Secretariat Book Depot, 1905).» 



In the Report to the Government of Bengal the author expressed the opinion 

 that no further progress was possible in the chemistry of plant indigo until an 

 accurate process was available for the estimation of indigotin (a) in the pure state, 

 (b) in the green leaf, (e) in the crude calte. 



In October 1905 the author was appointed by the Government of India to 

 continue this work in the Clothworkers' Research Laboratory of the University of 

 Leeds, under the general supervision of Mr, A. G. Perkin, F.R.S. 



A revision of the methods previously proposed for analysis of pure indigotin 

 and of crude indigo was first attempted. 



1. The original method of Mohr'' involved sulphonation and subsequent titration 

 with permanganate. To avoid the excessive consumption observed, due to inter- 

 action of the impurities present, modification of this process had been proposed by 

 Rawson and by Grossmann. The former first proposed salting-out the sulphona- 

 tion product with sodium chloride, resolution and titration. Later Rawson 

 proposed removal of the impurities by the addition of solution of barium chloride, 

 and subsequent titration of the claritied sulphonate. 



Grossmann ^ states that Rawson's process is unsatisfactory, and claims that a 

 satisfactory purification is effected by treating the sulphonation product of a crude 

 indigo with calcium carbonate, and making a titration of the clear liquid after 

 subsidence of the precipitate. 



A prolonged and careful examination of these methods proved them to be 

 inaccurate ; for either (a) a loss of indigotin was experienced, it being carried down 

 by the precipitate designed to purify the solution, or (6) the impurities were not 

 removed. 



Next followed a study of the process of sulphonation and the preparation of 

 the following salts : Potassium indigotinmonosulphonate, potassium indigotin di- 

 sulphonate, potassium indigotin trisulphonate, potassium indigotintetrasulphonate. 



Investigation was then made of the conditions under which the sulphonation 

 of pure indigotin could be effected without loss. It was found that, contrary to 

 existing statements,* that fuming sulphuric acid could be employed for sulphona- 

 tion without involving destruction of indigotin. Thus pure indigotin could be 

 heated to 98° 0. for three-quarters of an hour in a water oven, with 20 per cent, 

 fuming acid, without loss of indigotin. for crude cake indigo the acid used must 

 be 5 to 10 per cent, stronger in order to attain the same degree of sulphonation. 



It now became clear that the processes recommended by Rawson and Grossmann 

 failed because the degree of sulphonation attained was too low. In consequence, 

 on addition of salts of barium or calcium the difficultly soluble disulphonates were 

 thrown down with the impurities, thus escaping estimation. 



As, however, addition of calcium carbonate did not produce precipitation with 

 solutions of the tri- and tetra-sulphonic acids when added at Grossmann'e 

 prescribed concentration (1 : 1000), it seemed possible to use Grossmann 's prescrip- 

 tion at the higher degrees of sulphonation. 



But here a new difficulty arose ; for although on sulphonating crude cake indigos 

 to the stage of tri- or tetra-sulphonic acids, addition of calcium carbonate gave 

 no precipitation of the blue calcium disulphonate, little of the impurity was 

 removed. The conclusions previously drawn regarding these processes were thus 

 confirmed, and the processes were abandoned. 



2. Examination was now made of the method proposed by Knecht for the 

 estimation of indigotin by means of titanium trichloride.'' 



' ' Our Present Knowledge of the Chemistry of Indigo,' by W. P. Bloxam, diem 

 Soc. Trans., 1905, 87, p. 975. 



* Dingl. polytech. Jourii., vol. cxxxii., p. 303. 

 » Joxirn. Soc. Chem. Ind., 1905, p. 308. 



* Wangerin and Vorlander, Zeits.f. Farben- und Textilchemie, 1902, vol. i., p. 281. 

 » Joum. Soc. Dyers and Col, 1903, pp. 66, 169 ; 1904, p. 97 ; 1905, p. 292. 



1906. M M 



