Manufacture of Indigo Carmine.

Scientific American 11, 9.9.1868

A recipe for a green color for confectioners' use, published in No. 10, current volume, has called forth inquiry as to the nature of indigo carmine, and its method of manufacture. The following description will be found a complete answer to these inquiries:

In the first place, the choice of the indigo on which to operate s not without importance. Its price is generally in proportion to the quantity  of pure indigotine which it contains, and it is most advantageous to employ the finest qualities, in order to avoid, in manufacturing a fine uality of indigo carmine, a number of purifications, washings, etc., which soon become costly operations. The manufacturer must not allow himself to be entirely guided by the external appearance of the indigo, the best qualities of which are porous, light, clear, etc. but he should ascertain by one of the known methods the quantity of pure indigotine contains.

Indigo carmine consists of a perfectly uniform paste of a fine copper color, without any granulations. Spread upon a sheet of glass, and viewed by transparency, it should give a pure blue color with a slight tint of violet.

In the manufacture of indigo carmine, the first operation is the pulverization of the indigo. The author places some 10 lbs. at a time in a wooden drum, properly closed, and in which have been previously placed three cannon balls of 6 lbs. weight each. This drum is fixed to a wooden case, which catches any of the powder that may happen to escape during the pulverization. The drum is turned on its oxis by means of a handle, and in about three hours the above quantity is completely pulverized. It is then withdrawn, and passed through a silken sieve containing 100 threads to the square inch. Whatever remains on the sieve is put aside, and replaced in the drum in a future operation.

The powder thus obtained must be completely dried, otherwise, when placed in contact with the sulphuric acid in the next operation, it would give rise to a degree of heat which would injure the product. The desiccation is operated at a temperature of 60° to 70° C.

When the pulverized indigo is dry and has cooled, its dissolution in the acid is proceeded with, and as this part of the process determines the result of the manufacture, it is impossible to operate with too much care. The author recommends that small quantities should be operated on at a time, for the work is thus facilitated, and if an accident happens the loss is comparatively slight.

It is best to add the acid to the indigo, rather than the indigo to the acid; the temperature rises less high, less sulphurous acid is produced, and diddolution is more complete. As to the quality of the sulphuric acid employed, it must contain no nitric acid; for complete safety it is best to add a little sulphate of ammonia, to neutralize the effects of any nitric acid that might be present. The concentration of the acid is another point of great importance. Acid at 66° did not yield good results; the stronger the acid, the more perfect the dissolution. It is best to use a mixture of 4½ parts of fuming sulphuric acid and 1 part of acid at 66° Baumé. The weaker the acid the more violet will be the indigo carmine produced when viewed by transparency.

The following is the method adopted by Herr Roesler:

One pound weight of the pulverized indigo is placed in earthenware dishes kept cool by water, and upon it is poured 2 3/4(?) lbs. of the mixture of acids above quoted, previously cooled. The mass is stirred with a thick glass rod, slowly at first, then more rapidly, so as to prevent the indigo from agglomerating. In the coarse of about half an hour the whole forms a dark smooth paste, almost black; it is stirred rather slowly, while a second quantity of acid, equal t othat already mentioned, is added. When the mixture forths considerably and evolves much sulphurous acid gas, it is a bad sign; on the contrary, the operation may be considered successful when, after the mixture is completed, the thick foam of little bubbles of gas forms upon the surface while the mass gradually thickens.

It is true that this manner of dissolveing indigo is somewhat slow, since one workman can scarcely operate upon hundred-weight per diem, but the results are always good.

The operation is not yet complete, however; the transformation of the indigo into sulphiadigotic acid is not entirely effected, and if the process is immediately continued at this point, a bad result can alone ensue. The earthenware vessels must now be covered to protect them from dust, and their contents allowed to remain in this state for about a fortnight, care being taken to stir up the mixture now and then during that interval, and to warm the vessels a little on the last few days. The whole product is thus transformed into a thick mass, covered by a thinner or more liquid layer.

The next operation is that of precipitation. The contents of five of the earthenware vessels are emptied into a large vat and 237 1/3 pints of pure cold water are added, and then, gradually, a concentrated solution of common salt (1,17 sp. gr.), until the whole of the coloring matter is precipitated. The author formerly used carbonate of soda instead of salt, but the cost is greater and loss of time ensues on account of the violent effervescence.

By the use of common salt a large amount of hydrochloric acid is generated, which attacks the ordinary suspended filters hitherto used. The filtration is therefore effected in cases provided with false bottoms pierced with holes, over which the well-soaked filtering material lies. The first portions which pass muct be passed again through the apparatus until the liquid filters clear. The clear solution which filters through is of a blackish green tint. When salt has been used, the clear liquid is afterwards evaporated to crystallize; when chloride of potassium is used, instead of salt, sulphindigotate of potassa is obtained, but this product is not so soluble as the soda compound, and is therefore less esteemed.

When the filtration is finished, the filter is doubled upon itself, and the product submitted to a careful pressure. The cakes of indigo carmine thus obtained are fit for certain purposes; but when it is desirable to furnish a product capable of giving very pure tints, the first yield must be submitted to a few more operations.

The precipitate yielded by 5 pounds of indigo, is mixed with 219 pints of boiing water, and 5 pounds of monohydrated sulphuric acid are added, while the whole is well stirred with large wooden spatules.

Although this quantity of acid is not sufficient to dissolve all the product, it is enough to bring it to a very fine state of division, and to keep in solution all the impurities during the subsequent precipitation. The latter is then operated with 5 pounds of s solution of soda at 90°, and an equal quantity of common salt for every 2 pounds of indigo. The mixture is caretully stirred, allowed to cood, and filtered on clothes about two square yards in size, streched on wooden supports. The mother water has a dirty green tint. The filered product is washed until the water which passes has a clear blue tint.

With impure qualities of indigo it is advantageous to repeat the latter operation to obtain a perfectly pure product.

The indigo carmine collected on linen filters is pressed, and finally a little glycerin is added to perserve a proper degree of moisture in the mass.

One pound of indigo yields about ten pounds of indigo carmine.