BLUE LAKES.
=Indigo.=—The colouring matter of the indigo plant is distinguished by
great resistance to the action of light, air, and chemical reagents.
It is thus often cited as a particular example of a fast and durable
colour. It is very largely used in dyeing, and in certain combinations
is important as a painters’ colour.
Indigo comes into commerce in a great variety of qualities, all
imported from the tropics, in which alone the plant grows. The
principal varieties are Indian, Java, Egyptian, American and African,
but in these merchants make numerous subdivisions which are often to
be distinguished from one another only by very small differences. Good
indigo, whatever its source, has the following characteristics: It
appears in almost cubical lumps, but may also form irregular masses.
The lumps must show a small specific gravity, a higher might be caused
by intentional additions of sand or earth. It is without taste and
smell, and the colour of the finest varieties is not particularly
bright, but when rubbed with a hard substance, or even by the pressure
of the finger nail, it acquires a peculiar coppery lustre, which is the
more intense the purer the indigo, and is thus regarded as a common
test of quality. Indigo is completely insoluble in almost all solvents;
in hot oils a small quantity dissolves, which again separates on
cooling. The following are the principal signs by which the quality
of indigo is recognised in commerce: It forms a dull-coloured, loose,
light mass of earthy nature, a fresh fracture of which has a completely
uniform appearance. It must be so porous that it at once adheres to the
tongue. Uniformity of colour, light weight and fine fracture are the
most important of the practical tests for good indigo. The composition
of indigo is well known; we shall glance rapidly over the most
important substances which occur in indigo in varying quantity.
=The Constituents of Indigo.=—Indigo contains the following compounds:
Indigo blue, indigo red, indigo brown, indigo gluten, water and salts.
The single important constituent for the purposes of the colour maker
is indigo blue, or indigotin.
Pure indigo blue can be obtained from indigo only by a lengthy
process. The indigo is boiled in turn with dilute sulphuric acid,
caustic potash, and strong alcohol. The gluten dissolves in the acid,
the indigo brown in the potash solution, and the indigo red in the
alcohol. By these operations indigo blue is finally obtained in a state
of almost complete purity, it now contains only a small quantity of
inorganic substances. The quantity of pure indigo blue obtained in this
manner varies, according to the purity of the indigo employed, from 20
to 75 per cent. Indigo blue can also be obtained by direct sublimation
from indigo, but the yield is very small, since a large proportion is
decomposed by dry distillation.
The most important constituent of indigo, indigo blue, can be obtained
in quite pure form by a peculiar chemical process. When finely powdered
indigo is brought in contact with a reducing agent in the presence
of a strong alkali, the indigo blue is converted into indigo white,
which forms a soluble compound with the alkali, which compound is
stable only so long as it is protected from the action of the air. In
contact with air indigo white is at once oxidised, and is reconverted
into indigo blue, which separates from the liquid as a soft deep blue
powder; after washing and drying it is quite pure. Whilst sublimed
indigo blue forms needles with a splendid coppery lustre, when
precipitated from a solution of indigo white by the action of oxygen it
is obtained as an amorphous powder, which acquires the coppery lustre
only when rubbed.
The process of the reduction of indigo blue by a reducing agent in the
presence of a strong base is not only of theoretical interest, but is
largely used in practice. Fabrics are generally dyed with indigo by
immersing them in a solution of indigo white (known as the indigo vat),
and then exposing them to the air. The oxidation of the indigo white
then takes place on the fibre, upon which the indigo is deposited in a
very finely divided form; the blue colour is not immediately formed,
there is an intermediate green stage.
Indigo is used principally in dyeing. Certain of its compounds are
used as pigments; the starting point in the preparation of these is a
compound of indigo blue with sulphuric acid. When indigo is treated
with sulphuric acid, according to the duration of the reaction and the
temperature at which it is performed, either the monosulphonic acid
or disulphonic acid is formed. The reactions proceed according to the
following equations:—
C₁₆H₁₀N₂O₂ + H₂SO₄ = H₂O + C₁₆H₉N₂O₂.HSO₃
C₁₆H₁₀N₂O₂ + 2H₂SO₄ = 2H₂O + C₁₆H₈N₂O₂.(HSO₃)₂
When indigo is treated with eight times its weight of strong sulphuric
acid, and the solution considerably diluted, indigotin monosulphonic
acid separates as a fine blue precipitate, which dissolves with
difficulty in water and alcohol, and is insoluble in dilute acids. In
order to obtain the disulphonic acid 1 part of indigo is digested with
15 partsof sulphuric acid at 50° to 60° C. for several days. If fuming
sulphuric acid be used the solution is effected in a shorter time.
The solution is diluted with a large quantity of water, when a little
monosulphonic acid separates; the clear solution is drawn off and a
piece of wool immersed in it. The sulphonic acid is soon fixed by the
animal fibre; the wool is washed with water and then treated with a
solution of ammonium carbonate, when a deep blue solution of ammonium
indigotin disulphonate is produced. This solution is evaporated at a
low temperature and treated with strong alcohol, which leaves a residue
of the pure disulphonate, which is dissolved in water, the solution
precipitated with lead acetate and the lead indigotin disulphonate
treated with sulphuretted hydrogen. The colourless solution becomes
blue on evaporation; finally pure indigotin disulphonic acid is left as
an amorphous deliquescent mass. The solution became colourless owing to
the reducing action of the sulphuretted hydrogen; when this was driven
off by heating the blue colour reappeared.