If the sodium sulfite solution contains an excess of alkali,
a black tar tends to form when the solution is warmed, and very
little phenylhydrazine is obtained. Great care must be taken
in determining the end point in the neutralization of the sodium
hydroxide by the sulfur dioxide.
If the sodium sulfite-diazonium salt mixture is acidified before
warming or before becoming dark, the red color of the solution does
not disappear on heating, and the precipitated phenylhydrazine
hydrochloride obtained is colored red.
The benzene solution of phenylhydrazine should be well dried
before distilling, since the presence of moisture causes an increased
amount of foaming to take place just after the benzene has distilled off.
When the distillation is carried out carefully, practically no
phenylhydrazine distils with the benzene or other low-boiling impurities.
In order to obtain the maximum yield, it is necessary to cool the
hydrochloric acid solution of the phenylhydrazine hydrochloride from 20'0
to 0'0, before filtration. From 5 to 10 per cent of product separates
between these two temperatures. When this is done, no more phenylhydrazine
hydrochloride is obtained by concentration of the mother liquor.
An increase in the amount of hydrochloric acid above 2 l.
for the precipitation of the hydrochloride produces no increase
in yield of product.
Most published directions for the preparation of phenylhydrazine
specify the use of zinc dust and acetic acid following the reduction
with sodium sulfite. No improvement in the quality or quantity
of the product was obtained by using zinc and acetic acid.
It is best to use freshly prepared sodium sulfite for the reduction,
since the commercial quality is poor and gives a lower yield
of phenylhydrazine. A cylinder of liquid sulfur dioxide should,
of course, be available.
The rapid addition of the diazonium salt solution to the sodium
sulfite seems to be advantageous.
Pure phenylhydrazine dissolves in dilute acetic acid to yield
a perfectly clear solution.
The phenylhydrazine hydrochloride may be purified by crystallizing
from water. A 600-cc. portion of water is used for 100 g.
of crude hydrochloride, and the solution boiled a short time
with a few grams of animal charcoal. After filtering, 200 cc.
of concentrated hydrochloric acid are added, and the mixture cooled
to 0'0. Pure white crystals in a yield of 85-90 g. are obtained.
Rubber gloves should be worn when working with large quantities
of phenylhydrazine, since the product may cause serious injury
to the skin. The vapors of phenylhydrazine should not be inhaled.