530 TRANS ACTIONS OF SECTION fi. 



The process as recommended for the estimation of pure indigotin did not prove 

 satisfactory. But as a result of experiments made, it was found that the foUowiug 

 points must be attended to to "insure quantitative results : — 



(a) The strengths of the two solutions must be adjusted. 



(6) Certain relations must exist between the tartaric and mineral acids present. 



Under these conditions titanium trichloride can be safely used as an adjunct to 

 the new analytical process described below. 



Knecht, however, claimed further that in the case of crude indigo, by employ- 

 ing Grossmann's treatment with calcium carbonate he could use titanium tri- 

 chloride with success. But in view of the present results Kuecht's claim cannot 

 be sustained. 



The Potassium Tetrasulphonate Process for the Estimation of Indigotin. 



A new process of analysis was now evolved on the following lines: In the case 

 of pure indigotin it was found that if sulphonation was effected with 20 per cent, 

 fuming sulphuric acid, the mixture being heated for forty-five minutea at 98° C, 

 the product was indigotintetrasulphonic acid. And that this solution, at a 

 suitable dilution, yielded, on the addition of solution of potassium acetate, the 

 whole of its indigotin as crystalline potassium indigotintetrasulphonate. This 

 precipitate was filtered off and redissolved in water to a definite volume. On this 

 solution estimations could be made of the indigotin, with quantitative accuracy, 

 by titration with — 



(rt) Solution of potassium permanganate; 



(6) Solution of titanium trichloride, employing the precautions previously 

 stated. 



Furthermore, with suitable modification of the mode of sulphonation, the 

 process could be applied to the estimation of indigotin in crude cake indigo. 



For it was found that the impurities of the crude cake were, after sulphona- 

 tion, more soluble than indigotintetrasulphonate, thus permitting the quantitative 

 separation of all indigotin as the potassium tetrasulphonate, whilst the concentration 

 of potassium acetate was too low to determine the precipitation of the impurities. 



As a proof of the accuracy of the potassium tetrasulphonate process two 

 considerations are advanced:—* 



(rt) After separating the potassium tetrasulphonate from the sulphonation 

 product of a crude indigo it yielded on titration with both potassium permanganate 

 and titanium trichloride an end-point identical with that observed when pure 

 indigotin is similarly treated. 



{b) Pure indigotin was mixed with known weights of the brown impurities of 

 cake indigo (these having been extracted from crude Bengal cake by the action of 

 pyridine and dried at 140° C.) ; on sulphonating these mixtures and separating 

 the indigotin as potassium tetrasulphonate, the indigotin percentage obtained on 

 titration was precisely that of the mixture made. 



In the report on the work done for the Government of Bengal at Dalsingh 

 Terai the author expressed the opinion that the 'efficiency ' of the present method 

 of indigo manufacture was very low.* 



As the result of a large number of analyses now made, employing the potassium 

 tetrasulphonate method, of the finished cake produced by the Pemberaudah factory 

 during the season 1903-1904, the author's prediction wasi verified. 



For it was found that if the indigotin content for the whole green plant 

 (ordinary Indian varieties) be taken, as seems reasonable, at 06 pel' cent-, then 

 the highest efficiency attained duriug careful manufacture does not reach 50 per 

 cent, of the total indigotin obtainable, whilst the average efiicieccy of the whole 



• Lie. cit., Section 11., p. 7. 



