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and near the heated portion it appears melted, reddish yellow and trans-
parent. Indigo-red quickly heated in contact with the air, melts,
fumes, and burns with a bright smoky flame. In chlorine-water it
becomes yellow and plastic like wax; but on exposure to the air, re-
covers its hardness and regains its former colour almost completely.
(Berzelius.) By chlorine in presence of water it is converted into a
brown resin, which is insoluble in water, but soluble in alcohol.
(Erdmann, J. pr. Chem. 19, 329.) It dissolves in fuming nitric acid
with a beautiful purple colour, which, owing to decomposition, soon
passes into yellow. Water precipitates indigo-red, apparently un-
changed, from the red solution; from the yellow solution it precipitates
yellow flakes like those obtained in the same manner from a solution
of indigo-brown in nitric acid. The red solution of indigo-red in
alcoholic soda turns brown on exposure to the air, and deposits, when
evaporated, a substance which forms with water a brown solution,
depositing brown flakes on the addition of acids. (Fritzsche, J, pr.
Chem. 28, 196.)

Indigo-red does not dissolve in water, in dilute acids, or even in
heated concentrated solutions of the alkalis. It forms with sulphuric
acid a dark-yellow solution, which becomes yellowish red on dilution
with water, but without forming a precipitate. The solution is deco-
lorised by steeping silk or wool in it for several hours; while the fabric
is dyed yellowish brown or red. It dissolves slightly in alcohol, but
more abundantly in ether; the bright-red solutions deposit the indigo-
red on evaporation as a dark-red powder. (Berzelius.) It dissolves in
cold creosote. (Reichenbach.)

b. From the leaves of Polygonum tinctorium. The fresh leaves are
exhausted with ether in a digesting funnel, and after a part of the ether
has been distilled off, crystals of indigo-blue separate out, and the
remaining solution is evaporated to dryness. The residue is exhausted
with alcohol, the solution filtered from a slight brown residue, and again
evaporated to dryness; and the residue is purified by hot water, which
extracts a little yellow colouring matter.


Indigo-red thus prepared is a beautiful red, resin-like substance;
hard, brittle and easily pulverized. Burnt with oxide of copper, it
yields carbonic acid and nitrogen in the proportion of 1 6 : 1 . Heated
in contact with the air, it softens, melts, swells up, and burns with
flame, leaving a bulky mass of charcoal which burns without residue.
When heated in a tube, it gives off ammoniacal vapours. Distilled in
vacuo, it yields merely ammoniacal oil, and without any crystalline subli-
mate (probably because too little was employed). When boiled with
nitric acid, it evolves abundant red fumes and dissolves with yellowish
red colour, while a waxy substance rises to the surface; on evaporating
the solution, it leaves a residue, from which water extracts picric acid,
while the so-called commercial indigo-resin remains behind. Jt dis-
solves in sulphuric acid, imparting to it a yellow colour; becomes dark
red when acted upon by anhydrous sulphuric acid, and retains this colour
on the addition of water. Concentrated hydrochloric acid turns it
black but does not dissolve it.

Indigo-red is insoluble in water, slightly soluble in potash and
ammonia. The alcoholic solution turns red on addition of baryta-water,
lime-water, sub-acetate of lead, or nitrate of silver, and, dark red with
protochloride of tin. With solution of alum it forms a beautiful red lake,
which is not acted upon by alkaline carbonates.

It is very slightly soluble in acetic acid, but dissolves very readily in
alcohol and in ether; the alcoholic solution is precipitated by sulphuric
acid. (Hervy, J. Pharm. 26, 293; Pharm. Centr. 1840, 807.)

The indigo obtained from the flowers of Tankervillia cantonensis
colours alcohol of 35 per cent, first green, then violet ; and the solution
yields on evaporation a residue consisting of two substances, the one
green and insoluble in alcohol of 45 per cent, (probably chlorophyll),
the other of a beautiful red colour, soluble in alcohol, but insoluble in
water and ether. This latter substance forms with sulphuric acid, a
brown solution from which it is precipitated unchanged by water.
(Clamor Marquardt, Repert. 7, 1.)

The purple-red solution which is obtained when the purple-blue flakes
arising from the decomposition of indican by acids are boiled with alcohol,
yields a brown residue of indirubin very similar to indigo-red. This
substance is insoluble in alkalis, and when heated in a tube, gives off
purple vapours and yields a crystalline sublimate. (Schunck, If. Phil.
Mag. J. 10, 85.)

2. Colourless Indigo-red.


The sublimate obtained by heating indigo-red in vacuo is separated
from indigo-red by digesting it with alcohol, in which the latter is more
soluble, and is then purified by re-subliming in vacuo.

It crystallises when sublimed, in brilliant, colourless, transparent
very fine needles ; it crystallises on evaporation from its alcoholic or
ethereal solution in transparent colourless grains. It is inodorous, tasteless
and neutral to test papers.

When colourless indigo-red is heated in vessels containing air, it


melts and turns yellow, but crystallises again on cooling; when more
strongly heated, it melts,, boils, and partly decomposes, without however
giving off an acid or ammonia. Heated in free contact with the air, it
smokes, burns with a bright smoky flame, and leaves a trace of difficultly
combustible charcoal. It dissolves in fuming nitric acid and forms a
beautiful purple-red solution, which becomes yellow from decomposition;
the solution is then identical with that obtained by dissolving indigo-red
in nitric acid. Colourless indigo-red is immediately turned red by
dilute nitric acid.

Colourless indigo-red forms two compounds with sulphuric acid, one
soluble, the other insoluble in water. In cold sulphuric acid, it dissolves
very slowly and but partially, imparting to it a lemon-yellow colour;
the undissolved portion is of a burnt yellow colour; the same compound
is precipitated by water from the yellow sulphuric acid solution. Con-
centrated hydrochloric acid also forms a similar insoluble burnt yellow
compound; of this only a trace dissolves in the acid, to which it imparts
a yellow colour; it is not precipitated by water. Colourless indigo-red
does not dissolve in water or in aqueous solutions of the alkalis, not
even when they are boiling and highly concentrated. It dissolves very
slightly in acetic acid, from which it is not precipitated by water; in
alcohol and in ether it dissolves slowly, with brownish yellow colour.

3. Indigo-brown,

CHEVREUL. Ann. Chim. 66, 5; also Schw. 5, 291.
BERZELIUS. ScJtrb. 3, 685.

Resinous Indigo-green. Discovered by Chevreul, in indigo from Guatemala and
Java ; more completely investigated by Berzelius, who found it in all kinds of indigo.

Preparation. 1 . Powdered indigo is repeatedly boiled with water,
and the solution evaporated to a small bulk. The reddish syrup thus
obtained, is extracted with alcohol of 36 B.; water is added to the red
alcoholic solution; and the alcohol distilled off. The indigo-brown, which
is combined with ammonia, and remains dissolved in the aqueous solu-
tion, is precipitated by exactly neutralising the ammonia with sulphuric
acid; then washed on a filter with a little water, dissolved off with
alcohol, and the solution evaporated. (Chevreul.) It retains some of
the sulphuric acid by which it was precipitated, and also indigo-blue.
(Berzelius.) 2. Powdered commercial indigo is extracted withjsulphuric,
hydrochloric, or acetic acid, and afterwards with water, then gently
heated with concentrated potash-ley, which causes it to swell up to a
light black paste; and the dark brown alkaline solution is filtered off
without washing, since the dilute liquid would take up indigo-blue. The
alkaline solution is slightly acidulated with sulphuric acid, and filtered
(the yellowish-brown filtrate neutralised with carbonate of lime and
evaporated, yields a little more indigo-brown when treated with alcohol);
the precipitate, which is bulky, half gelatinous, and almost black,
owing to an admixture of indigo-blue, is washed out with water and
dissolved in carbonate of ammonia; the solution is evaporated to dryness;
and the residue is dissolved in water, and filtered from the undissolved


portion, which consists of blue indigo mixed with a little indigo-brown,
without washing out, since the residue would then dissolve. The indigo-
brown is precipitated from the filtrate by sulphuric acid; the precipitate
while still moist is digested with carbonate of baryta and water, whereby
an insoluble and a sparingly soluble compound of indigo-brown with
baryta are formed; and the solution is filtered and evaporated. Indigo-
brown is thus obtained in combination with a little baryta; it seems
impossible to obtain it perfectly pure. (Berzelius.)

Properties. Indigo-brown (2) forms a brown, shining, transparent
varnish, which is almost colourless and neutral to test papers; (1) has a
green colour (arising, according to Berzelius, from an admixture of indigo-

Decompositions. 1. Indigo-brown (2) yields by dry distillation, a
black viscid oil, and colourless, highly amraoniacal water. 2. Heated
in contact with the air, it softens, awells up, and evolves fumes,
smelling like burnt animal matter; then burns with flame, and Cleaves
a residue of difficultly combustible charcoal containing carbonate of
baryta. 3. It dissolves in nitric acid with evolution of nitrous fumes,
and forms a yellow turbid solution, from which water precipitates burnt-
yellow flakes, which give a dark yellow solution with ammonia. The
aqueous solution first deposits crystals of oxalic acid on evaporation, and
when of the consistence of syrup, solidifies into a flaky crystalline mass,
which tastes first acid and then bitter. When neutralised with
potash, it yields crystals of nitre and of a burnt-yellow bitter substance,
which is deliquescent, soluble in alcohol, and swells up when heated
without exploding. 4. Chlorine gas passed through a solution of
indigo-brown, turns it to a paler colour, and then precipitates brownish
yellow flakes of hydrochlorate of indigo-brown, but does not produce
any further decomposition. (Berzelius.)

Combinations. Indigo-brown (1) is slightly soluble in water. That
prepared according to (2), combines very readily with acids, forming
compounds which are slightly soluble in water. When an alkaline solu-
tion of indigo-brown is treated with an excess of acid, a compound of
indigo-brown with the acid is precipitated, which is brown, very bulky,
transparent only when viewed in thin layers, reddens litmus after it has
been washed with water, and is very slightly soluble in water, to which
it imparts a yellow colour. The precipitates produced by sulphuric
acid and hydrochloric acid, colour boiling water yellow, and shrink up
to a brittle mass, which may be pulverised in the solution. (Berzelius.;
According to Chevreul, (1) dissolves in aqueous solutions of acids more
readily than in water, and likewise in sulphuric acid, forming a green

Indigo-brown (2) unites readily with alkalis, and forms very dark-
brown compounds, which are soluble in water, and do not show any
alkaline reaction with reddened litmus paper. When the solution of
indigo-brown in carbonate of ammonia is evaporated, and the residue
dried at 70, it forms a black shining mass, which splits up into long
needle-shaped fragments; has a slight and very unpleasant taste; evolves
large quantities of ammonia when treated with potash, but no carbonic
acid when treated with acids; it dissolves readily in water and somewhat
less in alcohol. (Berzelius.) According to Chevreul, (I) dissolves in


ammonia, forming a red solution. When the solution of indigo-brown
(2) in aqueous potash is exactly neutralised with acetic acid, the liquid
evaporated to dryness, and the acetate of potash extracted by alcohol
(which takes up a little indigo-brown), the compound of indigo-brown
with potash remains behind. This compound dissolved in water and
evaporated to dryness, forms a black shining mass, which splits up
into needle-shaped fragments. The baryta-compound is difficultly
soluble in water. The lime-compound is insoluble: hence lime-water
precipitates indigo-brown from its ammoniacal or potash solution; and
when hydrate of lime is boiled with a solution of indigo-brown in potash,
it removes the former so completely, that a colourless ley remains behind.
The solutions of (2) in alkalis or acetic acid form dark precipitates with
acetate of lead, subacetate of lead and ferric sulphate; they are not preci-
pitated by ferrocyanide of potassium and corrosive sublimate, nor, with
the exception of the baryta compound, by infusion of galls. (Berzelius.)

Indigo-brown (2) treated with a little acetic add, forms two com-
pounds, the one soluble, the other insoluble in water. The first is obtained
by acidulating with acetic acid a solution of indigo-brown in potash,
so that it decidedly reddens litmus; the solution is then evaporated to
dryness, and the acetate of potash extracted with alcohol. This com-
pound reddens litmus, arid dissolves very readily in water, from which
it is not precipitated by boiling ; it is slightly soluble in alcohol, but
when boiled with the latter, becomes almost insoluble in water. The more
acid compound is precipitated by strongly supersaturating the potash
solution with acetic acid; when washed with water, a small portion is
converted into the first compound; the wash-water continues to dissolve
the precipitate, bat becomes turbid when mixed with the first acid
filtrate. (Berzelius.)

Indigo-brown (1) readily forms with alcohol, even when cold, a solu-
tion which appears green by reflected light, and red by transmitted
light; it is not rendered turbid by water, but becomes green, and on the
addition of a little ammonia, red. (Chevreul.)

The green thick oily substance extracted from fresh woad, by
Chevreul (Schio. 5, 310), behaves in a similar manner; it diffuses an
aromatic and empyreumatic odour when placed on glowing charcoal, and
leaves a residue of tumefied charcoal. It is almost insoluble in water, is
coloured red by alkalis, and becomes green again on the addition of an

According to Hervy (J. Pharm. 26, 293), a substance similar to
indigo-brown is obtained from Polygonum tinctorium. The fresh leaves are
bruised and exhausted with cold water, and the reddish solution is filtered
and evaporated to dryness. A yellow colouring matter is extracted by
ether from the residue, the remainder of which is dissolved in alcohol
of 40, filtered, and evaporated to dryness. The product is rose-coloured,
and dissolves with green colour in dilute acids; in concentrated acids it
turns green without dissolving. It dissolves in alkalis, with evolution of
ammonia. It is soluble in water, from which it is precipitated green by
concentrated acids ; also in alcohol, but not in ether.

When an aqueous solution of indican, which has become brown by
long boiling, is boiled with sulphuric acid or hydrochloric acid, dark
brown flakes separate out, and may be washed on a filter with water.
The precipitate boiled with alcohol, forms a brown solution which
leaves on evaporation, a dark brown, shining, resinous residue of
Indiltumin. This substance melts in boiling water, is decomposed and


dissolved by boiling nitric acid, and dissolves with brown colour in
ammonia; the ammoniacal solution forms brown precipitates with chloride
of calcium and chloride of barium, and the alcoholic solution is completely
precipitated by acetate of lead. The composition of indihumin corre-
sponds most-nearly to the formula C 16 NH 8 5 . (Schunek, N. Phil. Mag. J.
10, 86.)

C 16 NH 5 4 = C 16 NH 5 2 ,0 2 .

LAURENT. Compt. rend. 12, 539; /. pr. Chem. 24, 2; Pharm. Centr.

1841, 601; N. Ann. Chim. Phys. 3, 371; J. pr. Chem. 25, 434;

Ann. Pharm. 48, 262; Pharm. Centr. 1842, 246; Rev. sclent. 10, 295,

300; 18, 458; J. pr. Chem. 35, 108.

ERDMANN. J. pr. Chem. 24, 11; Pharm. Centr. 1841, 707.
A. W. HOFMANN. Ann.' Pharm. 53, 11.

Discovered simultaneously by Laurent and Erdmann, in 1841.

Formation. By the action of nitric acid or chromic acid upon indigo-
blue (p. 42).

Preparation. 1 kilogr. of finely powdered good commercial indigo
is stirred up in a large dish to a thin paste with water; it is then placed
over a moderate fire, and commercial nitric acid is gradually added
(which causes a violent effervescence, without evolution of nitrous acid),
until the blue colour has disappeared, for which, from GOO to 700 grin, of
nitric acid are necessary. The solution is boiled with several quarts of
water, and filtered boiling as rapidly as possible; after 12 hours the
isatin separates in reddish crystalline nodules. The mother-liquor is
boiled with the undissolved residue and filtered, the operation being two
or three times repeated: the last mother-liquor still yields isatin on
evaporation. The crystals are moistened with water containing a little
ammonia, in order to remove a resinous matter; they are then washed
with cold water, and finally recrystallised several times from boiling
alcohol (100 pts. of indigo yield 18 pts. of isatin). (Laurent.) When an
excess of nitric acid is employed, nitrosalicylic acid is formed. After
each addition of nitric acid, it is necessary to wait until the effervescence
is over. If no effervescence takes place, in consequence of the indigo
being mixed with too much water, and if the boiling is continued, whilst
more and more nitric acid is added, a violent reaction suddenly ensues
when the solution has reached a certain concentration, the mass over-
flowing the vessel even when it is very capacious, and the residue
contains generally nitrosalicylic acid. (Hofmann.) 2. Indigo is heated
with chromic acid, properly diluted, until the whole nearly boils, when it
is filtered hot; isatin then crystallises out on cooling, and is (purified by
repeated recrystallisation from water and lastly from alcohol. If the
chromic acid is so concentrated that it causes an evolution of carbonic
acid, the solution deposits little or no isatin on cooling, but a small
quantity on spontaneous evaporation or in vacuo; on evaporating the
solution by heat, only sequioxide of chromium and a brown powder
separate. (Erdmann.) Laurent drops a dilute aqueous solution of
chromic acid upon powdered indigo, and warms the mixture, the jiddi-

" K 2


tion of chromic acid being discontinued as soon as the blue colour has
disappeared, and the process continued as in (1). In order to purify
crude isatin which still contains resin, Hofmann dissolves it in potash,
and carefully adds hydrochloric acid to the solution, as long as it
forms a black or brown precipitate; when a portion on filtering is of a
pure yellow colour, and gives a deep red precipitate with hydrochloric
acid, the whole solution is filtered off, decomposed with hydrochloric acid,
and the precipitate washed with a little water.

Properties. Isatin crystallises in large transparent brownish red
prisms, or in smaller yellowish red prisms, belonging to the right prismatic
system; the crytals are very brilliant, especially when obtained from an

alcoholic solution. ooP. ooP oo. Poo ; (Fig. 65) with predominating faces t;
u' : u = 133 50' 55'; i : i' = 127 15' 30', G. Rose; ooP.

ocPoo ."Poo ; (fig. 68) without p and i; u' : u = 45 56'; y : y = 126 D 44'.
(Schabus.) Isatin is inodorous and has a bitter taste. (Schabus.) It melts
when heated, and solidifies on cooling into a crystalline mass of needles;
when heated above its melting point in contact with the air, the greater part
volatilises without decomposition, in yellow and very irritating vapours.

16 C

96 ....


65-61 ....



14 ....


9-44 ....


5 H

5 ....


........ 3-43 ....


4 O



21-52 ....


14 ; _ 100-00 100-00 .... 100-00

Decompositions. Isatin distilled in a retort, leaves a large residue of
charcoal. (Erdmann, Laurent.) When thrown upon glowing charcoal,
it diffuses the same odour as indigo. Heated in the air upon platinum
foil, it burns with a brilliant flame, and leaves a fused residue of difficulty
combustible charcoal. (Erdmann.) 2. When isatin is suspended in
hot water, and a current of chlorine passed through the solution, it is
converted into chlorisatin; no bichlorisatiri is formed, even if the solution
is exposed to sunshine. (Erdmann.) Chlorine gas passed into a solution of isatin
in potash-ley, produces a brown tarry liquid soluble in alcohol, but no bichlorisatin.
(Hofmann.) 3. Isatin shaken up with bromine-w&ter, in sunshine, yields
bromisatin ; in contact with anhydrous bromine in sunshine, it forms
bibromisatin. (Erdmann, Laurent, Hofmann.) 4. Moderately warm
nitric acid dissolves isatin without decomposition, but on boiling, a
violent action takes place, attended with evolution of nitrous fumes,
oxalic acid being formed together with a reddish brown resin, which
contains NO 4 and is soluble in ammonia ; no picric acid is formed.
(Laurent.) 5. Isatin heated with fuming sulphuric acid, dissolves with
brownish red colour; a strong effervescence then takes place, and on
treating the solution with water and alcohol, a yellow substance is
obtained, which has not been further investigated. (Laurent.) 6. Isatiti
dissolves in cold potash without undergoing decomposition, and imparts
to it a brownish red-colour ; after a time, or immediately on boiling,
the solution becomes yellow and is found to contain isatate of potash:
C 16 NH 5 4 + 2HO C 16 NH r 6 (Laurent, Erdmann). On pouring cold
potash on crystals of isatin, they first turn dark red, and on warming the


whole, an orange-yellow solution of isatate of potash is formed; if this
solution is concentrated by distillation, a reaction suddenly takes place,
colourless drops of aniline passing over with the steam, while hydrogen is
at the same time evolved:

Cir-NIPO 4 + 4(KO,HO) = C 12 NH" + 4(KO,CO-) + 2H.

(Hofmann.) 7. Ammonia forms with solutions of isatin, various pro-
ducts of decomposition, which vary according to the concentration of the
ammonia and the solvent of the isatin ; mixtures of various compounds
are always formed, all of which contain 1, 2, or 3 atoms of isatin, plus
1 or 2 atoms of ammonia, minus water. (Laurent.) Aniline behaves in
the same way. (Engelhardt.) Isatin with sulphurous acid in the
presence of alkalis, forms salts of isatosulphurous acid. (Laurent.)
Isatin dissolves in sulphite of ammonia with formation of isatyde.
(Erdmann.) 9. When a stream of sulphuretted hydrogen is passed
through an alcoholic solution of isatin, bisulphisatyde (C 32 N 2 H 12 4 S 2 ) is
formed. (Erdmann, Laurent.)

Combinations. Tsatin dissolves slightly in cold water, and more
abundantly in boiling water. The solution is of a dark reddish-yellow

It dissolves when gently warmed with nitric acid, and crystallises
unchanged from the solution on cooling.

Metallic Isatides. Isatin forms with the metals,. compounds in which
1 At. hydrogen of 1 At. isatin, is replaced by a metal (C U NH 4 M0 4 ).

Isatide of Ammonium is formed by dissolving powdered isatin in
ammonia, with which it forms a carmine-red solution. Acids precipitate
isatin from this solution, and nitrate of silver forms a carmine-red preci-
pitate. (Laurent.)

Isatide of Potassium is formed by dissolving isatin in cold potash. The
solution is of a dark violet colour, which, when diluted with water and
boiled, becomes pale yellow, and then contains isatate of potash.

Isatide of ammonium forms with salts of iron, cobalt and lead, precipitates which
are mixtures of isatin with the oxides.

A boiling solution of isatin, to which a little ammonia is added, forms
with protochloride of tin, a white precipitate, which becomes of a beautiful
carmine-red on boiling. The filtrate is rose-coloured, but turns yellow
in a few seconds; the precipitate on the filter is rose-coloured. (Laurent.)

Isatide of Cuprammonium forms a bright brown precipitate, which is
obtained by mixing an ammoniacal solution of isatin with a solution of

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