The mother liquors and washings lose carbon dioxide on boiling,
and the insoluble trinitrobenzene separates see preparation XXIV);
after filtering, washing, and drying, it weighs 15-20 g.
(4 to 6 per cent of the theoretical amount).
It is essential that the stirring should be most efficient,
so that when the mixture becomes thick the dichromate will be evenly
distributed throughout the liquid, as rapidly as it is added.
If the stirring is not efficient, local reactions of extreme violence
(in certain cases leading to conflagration) will occur.
An iron stirrer may be employed in the oxidation reaction,
but not in the purification.
Technical sodium dichromate generally contains a certain amount
of chlorides, and the chlorine liberated from these tends
to cause a troublesome foam towards the end, of the reaction.
Only a very efficient stirrer, which draws down the surface of the liquid,
is able to combat this difficulty. The amount of solid sodium
dichromate given is for the dry crystalline compound containing
two molecules of water of crystallization.
Great care should be taken in dissolving the crude acid in the alkali.
If an excess of alkali persists for any length of time, a permanent
color is produced which will discolor the final product.
The acid is fairly soluble in cold water and should be washed with care.