Organic Syntheses, Coll. Vol. 1, p.70 (1941); Vol. 3, p.13 (1923).
To 1035 g. of sirupy arsenic acid (80–85 per cent, sp. gr. 2.00/20°
; (Note 1)
in a 12-inch evaporating dish
, is added 828 g. (800 cc.) of aniline (Note 2)
in 100-cc. portions; meanwhile the lumps of aniline arsenate
which are formed are broken up by rapid stirring with a porcelain spatula
. When all the aniline
has been added, the powdered solid is transferred to a 3-l. round-bottomed flask
equipped with a mechanical stirrer
, a thermometer
reaching to the lower part of the vessel, and a condenser
arranged for downward distillation (Note 3)
; an additional 800 cc. of aniline
is added and the flask heated on an oil bath at 155–160° with stirring for four and one-half hours. The mass will have assumed an intense violet color.
The reaction mixture is poured into 700 cc. of water and the flask is washed out with a portion of a previously prepared solution of 330 g. of sodium hydroxide
in 1400 cc. of water, the washings being added to the reaction mixture. The remainder of the alkali is then added and the mixture agitated and cooled under the tap. At this point two distinct layers are present, a lower pink-colored alkaline water layer, and an upper strongly colored aniline
layer. The water layer is, while warm, carefully separated from the purple-colored oil by means of a separatory funnel
(heated to prevent the separation of sodium arsanilate
) and after treatment with 15 g. of decolorizing carbon
is filtered through paper.
The arsanilic acid
can be obtained from the aqueous alkaline solution either as the free acid or as the sodium salt. To obtain the free acid the solution is acidified with concentrated hydrochloric acid
until the purple color of tetrabromophenolsulfonphthalein
is changed to a faint yellow. Care should be taken in the addition of the acid not to overstep the end-point (Note 4)
. Crystallization is stimulated by scratching, and the flask is allowed to stand overnight to complete the precipitation. The crystals of pinkish-yellow acid (Note 5)
are filtered off, digested with 340 cc. of ethyl alcohol
, filtered, and recrystallized from 2.5 l. of boiling water, adding 5–6 g. of Norite
and filtering hot. This crystallization always produces a white product. The yield of thoroughly dry arsanilic acid
is 147–184 g.
per cent of the theoretical amount) (Note 6)
, (Note 7)
and (Note 8)
obtained by oxidizing arsenious acid
with nitric acid
is satisfactory, providing all oxides of nitrogen
are removed. If commercial arsenic acid
is used it should be evaporated first on the water bath and then with a free flame practically to the stage of the anhydrous acid, when most of the oxides of nitrogen
are driven out. water is added just before the material solidifies, and if necessary, the acid is again concentrated.
A good commercial grade of aniline
, light in color and boiling with not more than a 1° range, may be used. It is not necessary to use a freshly distilled product.
During the reaction a considerable amount of water escapes, together with aniline
vapors, from the reaction flask. The condenser is provided to condense and collect the distillate outside the reaction flask, and thus minimize any danger from the aniline
vapors. About 200 cc. of distillate will be obtained.
The precipitation of arsanilic acid
from the reaction mixture may cause difficulty if care is not taken to attain a definite hydrogenion concentration. The indicator consists of a 0.5 per cent solution of bromophenol blue
. Filter paper
dipped into this solution and dried forms a convenient test paper. That concentration of hydrogen ion at which the color of this indicator changes from purple to a faint yellow is considered the end-point. Congo red paper may also be used, but it is somewhat less sensitive.
A highly colored acid is occasionally obtained; under such circumstances it may become necessary to effect its purification by conversion into the sodium salt, followed by precipitation of the hot concentrated solution with alcohol, and regeneration of the acid. Colored impurities may also be eliminated by a process of fractional precipitation during the decomposition of the sodium salt, the impurities being separated with the first 25 g. fraction of the acid.
It is necessary to air dry the crystals of arsanilic acid
at least twenty-four hours before they come to constant weight.
Additional amounts of arsanilic acid
may be obtained from the various mother liquors. The products thus obtained are usually rather impure and hence are not considered in the yields quoted. On larger-scale production it would be economical to recover this material as well as excess aniline
In preparing arsanilic acid
upon a larger scale it was found that doubling the amount of aniline
used and continuing the heating at 155° during twelve to sixteen hours resulted in a yield of 190–240 g. of recrystallized acid. For small-scale work, when aniline
is not recovered and time is an important factor, the directions as presented seem preferable, particularly so since the arsanilic acid
prepared during a short heating period is purified more readily.
This preparation is referenced from:
Copyright © 1921-, Organic Syntheses, Inc. All Rights Reserved