INDIGO-CARMINE                      85
selective preference for nuclei or protoplasm, it possesses to a
ihigh degree the property of imparting different hues and
intensity of stain to different tissues; and the nuclei being
brought out by carmine, preparations are obtained of a dia-
grammatic clearness that is not afforded by carmine alone.
Combined with oxalic acid (Thiersch), indigo-carmine can
be made to afford a nuclear stain.
Indigo-carmine is found in commerce in the form of a blue
paste, soluble in   water.   It not unfrequently    contains
impurities, and it is, therefore, better to prepare it oneself.
The following statements concerning commercial indigo-
carmine are taken from Heidenhain's paper on the kidney, in
'Arch. Mik. Anat.,' x (1874). The article known in com-
merce as sulphindigotate of soda is a variable mixture of
different substances, of which the chief are generally these
three: the soda (or potash) compounds of sulphindigotic
acid (sulphocoruleinic acid), of hyposulphindigotic acid
(byposulphocoruleinic acid), and of sulphophoenicinic acid.
The sulphindigotate of soda is freely soluble in water,
slightly soluble in weak alcohol, and as good as insoluble in
absolute alcohol. It is precipitated from its solutions by
concentrated solutions of salts.
The hyposulphindigotate of soda is equally soluble in water
and in absolute alcohol, and is not precipitated by neutral
salts. This combination is frequently present as an impurity
in the sulphindigotate of commerce, which then appears partly
soluble in absolute alcohol.
The sulphophcenicate of soda is much less soluble in water
than the sulphindigotate, is easily soluble in alcohol, and is
easily precipitated by small quantities of salts.
The following method of preparing the pure sulphindigotate
is recommended (after Crum and Berzelius) by Maschke :
102. Indigo-carmine.2-One part of best powdered indigo
is gradually shaken into a large flask containing 7 to 8 parts
I ' Arch. Mik. Anat.,' x (1874), p. 32.