TJie Action of Acetic Acid on Indigo Blue. 163 



well, the only objection being that it is nearly impossible to 

 remove traces of lead from the indigo preparation. The 

 oxides of manganese which are between the mono-oxide 

 and the di-oxide are quite suitable if of good quality ; a 

 necessary feature is that the specimen used must be perfectly 

 soluble in dilute acetic acid to which sulphurous acid is added- 



The amount of oxygen required is 1 at. to 1 mol. of 

 indigo. When the operation is successfully carried on with 

 permanganate of potash the reduction of the permanganic 

 acid to manganous acetate is complete, no manganic acetates 

 being formed ; under certain conditions of temperature and 

 dilution abundant crystals of manganous acetate are visible 

 in the finished mixture. They are perfect rhomboids ; these 

 crystals are more frequently visible when using a manganic 

 oxide as the converting agent, the manganous acetate being 

 but sparingly soluble in strong acetic acid. 



Sometimes the operation with permanganate does not 

 go well, the minimum quantity does not complete the 

 operation, more has to be added, and the already formed 

 manganous acetate is converted into a manganic acetate, 

 which, in the subsequent washings, is decomposed, and the 

 indigo product would be contaminated with manganic oxides 

 if it were not treated before the final washings with dilute 

 acetic acid, containing sulphurous acid, which completely 

 removes the impurity. 



The indigo blue used should be of a high degree of 

 purity; with commercial indigo of the best sorts, the 

 crystals are formed pretty well, but it is vain to expect to 

 obtain in any clear or certain manner the products of 

 decomposition to be presently described. No commercial 

 indigo that I have ever had in my hands contained more 

 than 75 °/ Q of indigo blue, and the 25 °/ Q of matters not 

 indigo blue are unconquerable impediments in the way of 

 getting a good product. The indigo precipitate obtained 

 by oxidising the dissolved indigo contained in a reduction 



