2. Notes

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