By the action of caustic potassa, the reds are divided into four groups:
1, those which turn to a violet or blue; 2, those which turn brown;
3, those which are changed to a light yellow or gray; 4, those which
undergo little or no change.
The first group comprises madder, cochineal, orchil, alkanet, and
murexide. Madder reds are turned to an orange by hydrochloric acid,
while the three next are not notably affected. Cochineal is turned by
the potassa to a violet-red, orchil to a violet-blue, and alkanet to a
decided blue. Lac-dye presents the same reactions as cochineal, but
has less brightness. Ammoniacal cochineal and carmine may likewise be
distinguished by the tone of the reds obtained.
A characteristic of madder reds is that, after having been turned yellow
by hydrochloric acid, they are rendered violet on treatment with milk
of lime. A boiling soap-lye restores the original red, though somewhat
paler. Artificial alizarine gives the same reaction. Turkey-reds,
however, are quite unaffected by acid. Garancine and garanceux reds, if
treated first with hydrochloric acid and then with milk of lime, turn to
a dull blue.
Madder dyes are sometimes slow in being turned to a violet by potassa,
and this shade when produced is often brownish. They might thus be
confounded with the dyes of the fourth group, i.e., rosolic acid,
coralline, eosine, and coccine. None of these colors gives the
characteristic reaction with milk of lime and boiling soap-lye. If
plunged in milk of lime, they resume their rose or orange shades, while
the madder colors become violet. Murexide is turned, by potassa, gray
in its light shades and violet in its dark ones. It might, then, be
confounded with orchil, but it is decolorized by hydrochloric acid,
which leaves orchil a red. Moreover, it is turned greenish by stannous
chloride.
A special character of this dye (murexide) is the presence of mercury,
the salts of which serve as mordants for fixing it, and may be detected
by the ordinary reagents.
The second group comprises merely sandal wood or sanders red, which
turns to a brown. On boiling it with copperas it becomes violet, while
on boiling with potassium dichromate it changes to a yellowish brown.
The third group includes safflower, magenta, and murexide (light
shades). If the action of the potassa is prolonged the (soft) red woods
enter into this group. Safflower turns yellow by the action of potassa,
and the original rose shade is not restored by washing with water.
Hydrochloric acid turns it immediately yellow. Citric acid has no
action. Magenta is completely decolorized by potassa, but a prolonged
washing in water reproduces the original shade. This reaction is common
to many aniline colors. These decolorations and recolorations are easily
produced in dark shades, while in very light shades they are less easily
observed, because there is always a certain loss of color. Stannous
chloride turns magenta reds to a violet. Hydrochloric acid renders them
yellowish brown (afterward greenish?). Water restores the purple red
shade.
The fourth group comprises saffranine, azo-dinaphthyldiamine, rosolic
acid, coralline, pure eosine and cosine modified by a salt of lead,
coccina, artificial ponceau, and red-wood.
Saffranine is detected by the action of hydrochloric acid, which turns
it to a beautiful blue; the red color is restored by washing in water.
Azo-dinaphthyl diamine is recognized by its peculiar orange cast, and is
turned by hydrochloric acid to a dull, dirty violet. Rosolic acid and
coralline, as well as eosine, are turned by hydrochloric acid to an
orange-yellow: the two former are distinguished from eosine by their
shade, which inclines to a yellow. Potassa turns rosolic acid and
coralline from an orange-red to a bright red, while it produces no
change in eosine. If the action of potassa is prolonged, modified eosine
is blackened in consequence of the decomposition of the wool, the
sulphur of which forms lead sulphide. Coccine becomes of a light
lemon-yellow on treatment with hydrochloric acid. Washing with water
restores the original shade. It affords the same reactions as eosine,
but its tone is more inclined to an orange.
Artificial ponceau does not undergo any change on treatment with
hydrochloric acid, and resists potash. Red wood shades are turned toward
a gooseberry-red by hydrochloric acid, especially if strong. This last
reaction not being very distinct, red-wood shades might be mistaken
for those of artificial ponceau but for the superior brightness of the
latter. If the action of potassa is prolonged, the red-wood shades
are decolorized, and a washing with water then bleaches the tissue.
Rocelline affords the same reactions as artificial ponceau, but if
steeped in a concentrated solution of stannous chloride it is in time
completely discharged, which is not the case with artificial ponceau.