December 16,1871.] THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
495 
•ug a phenyle-carbamic acid. Anthranilic acid again, 
being heated to 570° F., gives satycilic acid, which may¬ 
be considered as a compound of carbonic with carbolic 
-acid. Lastly, indigotine, like carbolic acid, yields on 
treatment with excess of nitric acid, picric or trinitro- 
phenic acid. 
These facts have produced on the minds of chemists a 
conviction that indigotine will one day be artificially 
prepared from carbolic acid, and recently MM. Emmer- 
ling and Engler have accomplished the scientific arti¬ 
ficial production of indigotine from a compound acetone, 
•discovered in 1857 by M. Friedel, to which these gentle¬ 
men have given the name of acetophenone. Their pro¬ 
cess consists in acting on this compound with fuming 
nitric acid, when two nitro-compounds are produced, one 
crystalline, the other syrupy. To the latter, after its 
evaporation to the state of a resinous mass, they add ten 
parts of reduced zinc and one part of soda-lime. The 
mixture is heated in small tubes, when a little water 
evaporates, and a dark-coloured substance sublimes, 
which contains indigotine in small quantities. This 
.result is certainly a triumph of scientific chemistry, and 
•brings us another stage nearer the commercial artificial 
production of a most valuable dye. 
A great variety of substances, by adding one equiva¬ 
lent of hydrogen, convert blue indigo into white indigo, 
which is a colourless substance, without taste or odour, 
insoluble in water, but soluble in alcohol and ether, and 
in solutions of the alkalis and alkaline earths. On this 
point I may observe that, many years ago, I devised a 
process by which many pounds’ worth of indigo were 
recovered from the refuse bottoms of the blue dip vats, 
the preparation of which I shall explain further on. I 
found that indigo was susceptible of forming insoluble 
compounds with the lime and protoxide of iron. White 
indigo dissolves freely in strong sulphuric acid, giving a 
•dark purple-blue liquor. 
Many substances are employed commercially to con¬ 
vert blue indigo into the white indigo soluble in alka¬ 
lies. Thus, I may mention lime and protoxide of iron, 
caustic alkali and protoxide of tin, or sulphuret of tin or 
arsenic, and zinc and caustic alkali; also organic sub¬ 
stances, such as grape sugar and bran, which enter easily 
•into fermentation. The following formulae show the 
•difference of composition of the two indigos:—■ 
2(C 8 H 5 NO) + 2H = C 16 H 12 NO, 
Indigotine. Hydrogen. White Indigo. 
I shall now have the pleasure of proceeding to de¬ 
scribe some of the methods employed by chemists to 
determine the relative commercial value of samples of 
!his expensive dye. The best qualities of indigo from 
Bengal, Java and Guatemala are light, have a uniform 
texture, and a fine coppery hue, which is increased by 
friction. The following analysis, made by M. Chevreul, 
.shows the composition of a fair sample of commercial 
indigo: — 
Indigotine.45 
Matters soluble in alcohol . . . . 30 
Matters soluble in ether.12 
Resin soluble in hydrochloric acid . . 6 
Mineral matters. 7 
100 
■Commercial indigos are often adulterated with mine¬ 
ral matters of various kinds. This fraud is easily 
detected by calcining a known weight of the sample, 
which ought not to leave a residue of more than 10 
per cent. The most common adulteration, however, is 
ithe addition of starch. This can be detected by boiling 
;some of the pulverized indigo with a weak solution of 
^hydrochloric acid. The insoluble starch is thus con¬ 
verted into soluble dextrine, which yields a beautiful 
purple colour with iodine. 
There are several processes employed to determine 
I the amount of indigotine in commercial indigos. I shall 
here only give the outline of three. The first consists 
in dissolving one gramme of the dry pulverized indigo 
in twelve grammes of concentrated sulphuric acid, and 
heating the whole at a temperature not exceeding 120° F., 
, when the indigo combines with the sulphuric acid, and 
( becomes perfectly soluble in w^ater. It is then diluted 
with water, so that the whole occupies one litre. The 
operation is repeated with one gramme of pure indigo¬ 
tine, which serves as a standard of comparison. A solu¬ 
tion of bleaching powder, or bichromate of potash—the 
first proposed by M. Chevreul, the latter by Dr. Penny 
—is prepared of such a strength that one hundred 
volumes of the solution will completely destroy the 
whole of the colour produced by the gramme of indigo¬ 
tine. Part of the same liquor is then applied to the 
solution of the commercial indigo, and the number of 
volumes required to destroy the colour represents the 
percentage of indigo. Thus, if 60 divisions are re¬ 
quired, it is assumed that there is 60 per cent., if 70 
divisions, 70 per cent. This method of estimating the 
indigotine is not to be relied on, as it always gives a 
much higher percentage of colour-giving principle than 
exists in the indigo, owing to the hypochlorous acid of 
the bleaching-powder, and the chromic acid of the bi¬ 
chromate not only decomposing the indigotine, but act¬ 
ing on several colouring matters which are included in 
the portion soluble in alcohol in M. Chevreul’s analysis. 
The second process gives better results; it is due to 
Professor Fritzsche, and consists in introducing one part 
of indigo, finely pulverized, and one part of grape sugar, 
into forty-eight or fifty parts of boiling alcohol, to which 
are added two parts of a concentrated solution of caustic 
soda. The whole is put into a bottle exactly large 
enough to hold it, and left to cool. By exposing the 
colourless liquid to the atmosphere, the reduced indigo 
which is in solution absorbs oxygen, and the indigotine 
is precipitated under the form of beautiful prismatic 
crystals. 
The third process, devised by myself, I have found to 
give satisfactory results. It consists in introducing into 
a flask one part of finely pulverized indigo, two parts of 
green copperas,, and two hundred parts of water con¬ 
taining 10 per cent.,of caustic soda. The whole is kept 
at the boil for a short time, and allowed to cool. The 
clear liquor is exposed in shallow vessels to the atmo¬ 
sphere, when the soluble indigo is oxidized, and precipi¬ 
tates as pure indigotine. The residue in the flask is 
submitted to the treatment three times. The whole of 
the indigotine thus obtained is collected on a filter, 
diied, and weighed. 
A commercial process, founded on the above method, 
is carried on by Messrs. Haas and Co., who sell indigo 
so purified under the name of refined indigo. An infe¬ 
rior quality is also prepared by heating indigo, at a 
moderate temperature, with weak muriatic acid, which 
dissolves lime and other mineral matters, as well as any 
starch it may contain; it is slightly washed and boiled 
with weak caustic soda, to dissolve the chlorophyl and 
other resinous impurities. 
I shall now call your attention to three commercial 
preparations of indigo, obtained by the action of sul¬ 
phuric acid on it. The first is called sulplio-purpuric acid 
or phenicine , which is made by adding one part of indigo 
to four parts of highly concentrated sulphuric acid, heat¬ 
ing for a short time, varying from half an hour to an 
hour, or until a small quantity of it mixed with a large 
quantity of water gives a deep purple colour. Great 
care must be bestowed on this part of the operation, so 
as to avoid the formation of a compound, to which I 
shall have again to call your attention, sulpho-indigotic 
acid. The acid mass produced is thrown into abput 40 
or 50 parts of water, when a beautiful purple precipitate 
is produced, which is collected on a filter, and slightly 
washed with weak muriatic acid. To dye wool with 
this sulpho-purpuric acid it is necessary to add to the 
