Thorough washing of the crude aldehyde is particularly desirable,
as it removes a reddish impurity which tends to distil over and
color the product lemon yellow or sometimes even brownish yellow.
When such a brownish product is obtained, it is quite necessary to make
a second precipitation, as well as to observe the directions mentioned
in the purification of the crude aldehyde, namely, to precipitate
the first few grams and the last few grams of the aldehyde separately.
The precaution of rejecting the first and last portions
of the precipitate is unnecessary in the reprecipitation.
In the reprecipitation of a deeply colored product, the portion
of aldehyde at the end may be even purplish in color and particular
care must be taken to keep this separate.

Vigorous mechanical stirring must be employed during the precipitation
of the crude aldehyde, as otherwise large lumps are formed which
make washing difficult.

A previous investigator has mentioned that the crude product must
be dried before distilling. This, however, is unnecessary.
If the aldehyde is dried before distilling, it is possible to use
a 500-cc. distilling flask instead of a 1-l one.

In purifying the aldehyde by dissolving in acid and reprecipitating,
it is essential not to use stronger acid than that specified
(1:6), as stronger acid causes a deepening of the color of the solution.
If the concentrated acid, which is to be diluted and used in this
procedure, does not have a sp. gr. of 1.19, it will be necessary
to add the equivalent amount of weaker acid in order to dissolve
the _p_-dimethylaminobenzaldehyde. In purifying the aldehyde,