Valuation of Indigo.—As a great deal oi indigo is used for dyeing in our country, and is the imported kinds (Bengal and Guatemala) are very high in price, a method of esti mating the comparative value of different samples, must be very acceptable. The fol lowing is a method for estimating the same proposed by Dr. Penny, the eminent chemist in Glasgow:— Ten grains of the sample very finely pow dered are carefully rubbed with 2 measured drachms of fuming sulphuric acid, and the mixture allowed to digest 12 to 14 hours with occasional stirring, the air being excluded.— A small flat bottomed flask, with a tight cork answers best for this operation. Some frag ments of broken glass should be added to pre vent the indigo Irom clotting. The temperature should be from 70 to 80 Fah., if it rises higher sulphurous acid may be generated and the whole operation render ed worthless. When the indigo is perfectly dissolved the solution is gradually poured (constantly stirring) into a basin containing a pint of water; by measure f of an ounce of hydrochloric acid is instantly added. An al-cimetre of 100 equal parts is made up with 7J grains of pure dry bichromate of potassa dis solved in it, and this is gradually added to the indigo in the basin, until a drop of the mix ture, let fall upon a slip of filter paper pre sents a light brown or ochre shade, without any mixture of blue or green. The number of measures of bichromate solution used, is then read off, and this shows the comparative value of the sample. In applying the test drop to the paper, the best results are obtain ed by bringing the end of a glass rod in con tact with the indigo solution, and then gently pressing it against the surface of the paper. It is advisable t keep the indigo solution gently warmed while the bichromate is being added, and the mixture should be well stirred after each addition. Towards the conclusioa the bichromate should be added very slowly and carefully, as one or two drops then pro duce a great effect. The changes of color in the mixture clearly indicate the advance of the operation. The original blue color of the sul phate of Indigo becomes lighter and lighter, then acquires a greenish shade, then greenish brown, and almost immediately after an ochre brown. Ten grains of pure indigo re quire nearly 7J grains oi bichromate of potash. For dyers and color-makers in print works, the above mode of testing good indigo (we have not tried it) by Dr. Penny, if correct, is invaluable, at the same time we can say, that long experience enables a good practical che mist to judge very closely ol the quality of indigo by the eye.
This article was originally published with the title "Events of the Week" in Scientific American 8, 29, 229 (April 1853)