The amount of hydrochloric acid indicated must not be reduced;
otherwise, diazoamino compounds are formed.
It is essential to keep the temperature low while unreacted diazobenzoic
acid remains in solution, in order to avoid decomposition.
If this precaution is not taken, the yields are considerably diminished,
through the formation of tarry by-products.
The use of a capillary tube for the addition of sodium nitrite
prevents loss of nitrous acid by local reaction at the surface
of the acid solution. The tube should not be tightly connected
to the dropping funnel, but should be so arranged that air is sucked
through with every drop. In this way, the entrance of the acid
liquor into the capillary is prevented.
The formation of the azo compound takes place slowly on the addition
of the dimethylaniline, but the speed of the reaction is greatly
increased when the hydrogen ion concentration is lowered
by the addition of the sodium acetate. It is nevertheless
necessary to allow the reaction mixture to stand a long time;
if the product be filtered off after only twenty-four hours,
a further quantity of dye will separate from the filtrate on standing.
The hydrochloride of methyl red is only sparingly soluble in
cold water, and is apt to separate in blue needles if the acidity
is not sufficiently reduced.
The alcoholic filtrate, obtained on digesting and washing the crude
methyl red, contains a more soluble red by-product which gives
a brownish-yellow solution in alkali. The methyl alcohol may be
recovered with very little loss by distillation; it is, however,
impracticable to attempt to recover any methyl red from the residue,
owing to the tarry nature of the by-product. The proportion of this
by-product is greatly increased if the temperature of the mixture
is allowed to rise too soon after the addition of the sodium acetate.
Methyl red is described as crystallizing in needles from glacial
acetic acid; on recrystallization from toluene it separates in plates.
When the methyl red is crystallized from toluene, it sometimes
separates in the form of bright-red lumps, probably on account
of too rapid crystallization. Under these conditions it is advisable
to crystallize again, using a somewhat larger amount of toluene.
It is advisable to titrate the crude anthranilic acid with standard
alkali and phenolphthalein before starting the experiment.
In checking these directions, an 80 per cent anthranilic acid
was used; it gave a correspondingly lower yield of methyl red
(650-700 g.). The yield of methyl red is about 65 to 70 per cent
based on the dimethylaniline actually used up, but only 58-63 per
cent based on the anthranilic acid actually present in the technical
anthranilic acid employed.