A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1923, 3, 13
DOI: 10.15227/orgsyn.003.0013
Submitted by W. Lee Lewis and H. C. Cheetham.
Checked by Oliver Kamm and A. O. Matthews.
1. Procedure
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. (10.9–14.5 per cent of the theoretical amount) (Note 6), (Note 7) and (Note 8).
2. Notes
1. Arsenic acid 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. It is necessary to air dry the crystals of arsanilic acid at least twenty-four hours before they come to constant weight.
7. 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.
8. 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.
3. Discussion
p-Arsanilic acid can be prepared from aniline and arsenic acid.1 Various temperatures ranging from 150–200° and various ratios of aniline and arsenic acid have been recommended. From most of the published methods it might be inferred that this is a smooth reaction; whereas an important side reaction consists in the oxidizing action of arsenic acid on aniline, with the production of a deeply purple-colored dye as well as much tarry material and some diarylarsonic acid. It can also be prepared by the reduction of p-nitrobenzenearsonic acid with iron and hydrochloric acid,2 or by reduction of the salts of p-nitrobenzenearsonic acid in the presence of a nickel catalyst.3
This preparation is referenced from:

References and Notes
  1. Fichter and Elkind, Ber. 49, 239 (1916); Kober and Davis, J. Am. Chem. Soc. 41, 451 (1919); Cheetham and Schmidt, ibid. 42, 828 (1920); Nijk, Rec. trav. chim. 41, 484 (1922).
  2. Les Etablissements Poulenc Frères, Ger. pat. 408,575 [C. A. 23, 849 (1929)]; Fr. pat. 636,660 [C. A. 23, 848 (1929)].
  3. Inoue and Kimura, J. Pharm. Soc. Japan 53, 1105 (1933) [C. A. 28, 3391 (1934)]; Stevinson and Hamilton. J. Am. Chem. Soc. 57, 1298 (1935).

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

oxides of nitrogen

arsenious acid

bromophenol blue

ethyl alcohol (64-17-5)

hydrochloric acid (7647-01-0)

aniline (62-53-3)

sodium hydroxide (1310-73-2)

iron (7439-89-6)

nitric acid (7697-37-2)

Arsanilic acid,
p-Arsanilic acid (98-50-0)

arsenic acid (1327-52-2)

aniline arsenate

sodium arsanilate (127-85-5)


nickel (7440-02-0)

decolorizing carbon,
Norite (7782-42-5)

p-nitrobenzenearsonic acid (98-72-6)