It is well, however, to keep on hand a standard sample of pure indigo, prepared from reduced or white indigo, as directed by Berzelius (vol. vi. page 3, French ed., 1832), and in Muspratt’s Chemistry applied to the Arts (Dyeing, Indigo).
In the analysis by reduction of indigo, the process is simply as follows: Reduce the indigo to fine powder, and weigh it; weigh out an equal quantity of pure quicklime (made from pure white marble). Measure in a graduated vessel a certain volume of water. Slack the lime with a portion of this water. The rest of this water is to be used in rubbing up the indigo in a mortar. Then the slacked lime is to be mixed with the indigo, rubbing the substances well together. Introduce the whole into a large flask; 1½ to 2 litres (about 3 to 4½ pints) of water is required for 1 gramme (or about 15 grains of indigo). The flask and contents are then to be exposed to a heat of from 176° to 190° F. for some hours. This is best effected in a water bath. By this digestion the lime is made to combine with the indigo brown, and the coloring matter is set at liberty. Dissolve in the liquor a little protosulphate of iron, exempt from copper, and reduced to a fine powder. The flask is to be corked and well shaken, and allowed to cool. When the sediment is settled, decant the clear solution by means of a syphon into a graduated glass. The coloring matter oxidizes by exposure to the air; and to favor this oxidation and to keep the lime in solution, add muriatic acid to the liquor. When the liquor has become clear, filter and collect the precipitate on a weighed filter, which wash with hot water, and dry at a temperature of 212° F. Thus we can learn, by weighing the filter again, how much indigotine is contained in the sample.
If we make use of 200 measures of water, and have drawn off 50 measures of the solution to oxidate, and this 50 measures has produced 10 grains of indigo, the whole sample evidently contained 40 grains of indigo blue.
This method serves both for an assay of the sample and the production of a standard sample of pure indigotine. The operation may be carried on upon a larger scale for the manufacture of a standard sample.
Yours truly,
C. T. Jackson.
Dr. Jackson adds the following note:—
In the processes given I have not referred to the qualitative analysis or testing for all the kinds of adulterations, but have given only valuation of the coloring power of indigo.
I have had occasion to search indigo for Prussian blue, an occasional adulterant. This is ascertained by caustic potash, which becomes in part an oxide if Prussian blue is present. This acidulates with muriatic acid, and, tested with sulphate of iron, proves, by formation of Prussian blue, the presence of the ferrocyanide of potash in the solution, and hence Prussian blue in the indigo. Lime and clay are the usual adulterants, and oxide of iron is often present accidentally or from the clay adulterants. Starch and flour are rarely used, as they add little to the weight.
C. T. J.