Colorimetric Determination of Certain Primary and Tertiary Aromatic Amines F. L. Eh-GLISH .Miscellaneoirs Intermediates Luboratory, Chambers W'orks, E. I . dit P o n t de Semours & Go., Deepwater Point, 4.J . Procedures are presented for the estimation of aniline in the presence of monoethyl- and diethylaniline or monomethyl- and dimethylaniline; otoluidine in the ethylated o-toluidines; m-toluidine in its ethyl derivatives; and a-naphthylamine in its ethyrated products. .111 these methods are based upon treatment of the sample with nitrous acid, coupling the diazotized primary amine with Chicago acid, and reading the concentration of the resulting azo dye with a photoelectric colorimeter. Methods for the determination of dimethllaniline, diethylaniline, and diethyl-rn-toluidine in the
presence of their corresponding primary and secondary amines depend upon the formation of the highly colored p-nitroso derivatives when the sample is treated with nitrous acid, the nitrosamines produced by the secondary amines being substantially colorless, as are the diazo derivatiies of the primary amines. Duplicate determinations of any of these compounds should agree within about *l scale ditision on the colorimeter, corresponding roughly to *3% of the amount present. h single determination can be run in 10 to 15 minutes.
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sents an additional source of error, since it is very difficult to obtain two cells with the same sensitivities throughout the spectrum. IVith this instrument, very poor curves were obtained with yellow solutions, the graphs being high1 curved even with solutions such as potassium dichromate whicx closely follow the Lambert-Beer dilution law. Another serious objection to the Fisher type of instrument, which precludes its use in most of the analyses detailed below, is that a highly colored comparison (blank) solution cannot be used. It is thus impossible with this instrument to detect, for example, a small amount of red color in a strongly yellow solution, as is necessary in the estimation of primary amines in the presence of tertiary.
Ll\IEROLTSmethods have been proposed foil the estimation of primary, secondary, and tertiary aromatic amines, either singly or in t,he presence of each other. Of the more important, some depend upon the evolution of a gas: methane, from the reaction with methyl magnesium iodide (Y),or nitrogen from decomposition of the diazonium derivative of the primary amine (e). In others, the diazo is titrated with p nitroaniline ( 4 ) , J-acid (8, 9 ) , or, in microbiological work when only primary amine is present, by coupling with a suitable compound and measuring the intensity of the resulting azo dye colorimetrically (1, 5 ) . The reaction of picryl chloride with primary amines ( 3 , 6 )has also been utilized. Some of these methods are longer than is desirable for routine application; others are of too restricted application for the purpose. The development of rapid colorimetric procedures was therefore undertaken.
.4 calibration rurve \van p l r p a r d for each determinat,ion by running known amounts of purified material through the analytical procrdures detailed below. These standard materials were prepared as follows: Aniline. Commercial aniline was fractionally distilled, the middle 60% fraction being retained. Monomethylaniline. Crystallized three times from water as the hydrochloride, converted to the free amine by means of sodium hydroxide, and fractionally distilled, the middle 50% portion being retained. This portion was converted to the oxalate crystallized from water, washed with ethanol and acetone, dried in vacuum a t room temperature, and used in this form. Dimethylaniline. Crystallized three times from water as the picrate, converted to the free amine by means of sodium hydroxide, and fractionally distilled; the forerun and residue were discarded. Freezing point, 2 . 4 " C. Monoethylaniline, Diethylaniline, m-Toluidine, and o-Toluidine. crystallized three times from water as the hydrochloride, based, and fractionally distilled. o-Naphthlamine. Crystallized tnice from water as the hydrochloride, based, fractionally distilled, and crystallized twice from alcohol. Freezing point 4 8 . 7 " C. Monoethyl-a-naphthylamine. Crystallized twice from water as the hydrochloride, based, and twice fractionally distilled. Diethyl-a-napthylamine. The ethylation of a commercial sample was completed with a large excess of ethyl bromide and the product purified by repeated fractionation. X o salt of this base was found that crystallizes readily. Monoethyl-m-toluidine and Monoethyl-0-toluidine. Crystallized as the hydrochloride successively from water and ethanol. Used without ~basing. Diethyl-rn-toluidine. Purified by treatment with phthalic anhydride followed bv crystallization as the picrate twice from' ethanol and once from benzene, based, and fractionally distilled. Diethyl-0-toluidine. Ethylation completed with excess ethyl bromide. fractionallv distilled. treated with formaldehvde. dissolved in hydrochloAc acid, extracted with chloroform t6 remove formaldehyde-toluidine compounds, based, and dried.
When a mixture of primary, secondary, and tertiar amines is treat,ed with sodium nitrite in acid solution, the primary forms the colorless diazo derivative; the secondary, the substantially colorless nitrosamine; and t'he tertiary, the usually int'ensely yellow p-nit,roso compound. Preliminary experiments .showed that the yellow nitroso color can be utilized for the estimation of the tertiary amine content and that the diazo, whpn coupled with a suitable agent, produces a red color, the int'ensity of which i$ proportional to the primary amine, any y e l l o ~color due To tertiary being compensated by using it (without the addition of the coupling agent) in the comparison cell of the phot-oelectric colorimcBtcr. I n t,he early work, €€-acid (l-amino-8-naphthol-3,6disulfonic acid) was used as t.hc coupling agent but it was later abandoned in favor of Chicago acid (1-amino-9-naphthol-2,idisulfonic acid) because the latter, although yielding slightly less tinctorial poiwr, is much less subject to oxidation and photochemical decomposition in solution. I n the course of this vork, three photoelectric colorimeters were used: 1. Cenco-Sheard-Sanford Photelometer, Catalog KO. 12,335, provided with 1-em. flat absorption cells and Cenco light filters. This combination was found satisfactory for all purposes for which it was tried and i t was used in the development of all the methods detailed below. 2. A later, smaller Cenco Photelometer, Type B-2, Catalog S o . 12,346, provided with cylindrical cells. This instrument was found to be distinctly less accurate than the above, chiefly bei'ause the cylindrical cells cannot be made so uniform as the flat ones. 3. A Fisher Electrophotometer, Catalog No. 7-101. This instrument also uses cylindrical cells and hence is subject to the Jame errors as the small Cenco. It uses two barrier-layer type photocells, instead of one as in the Cenco machines, which pre-
The reagents required in the various primary amine dt,terniinationi are: 457
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Sodium Nitrite. Dissolve 3 grams of sodium nitrite and 5 grams of sodium bromide in Lvater and dilute to 100 ml. The purpose of the bromide is t o increase the diazotization velocity and ensure completion of the reaction. Sitrite solutions so prepared are sufficiently stable for periods of months. Dilute Hydrochloric Acid. Dilute 10 ml. of 377, hydrochloric acid to 100 ml. Sulfamic Acid. Dissolve 10 grams in water and dilute to 100 ml. Sodium Acetate. 50% by w i g h t in water. Chicago Acid. Dissolve 1 gram of purified material in water containing 2 ml. of concentrated hydrochloric acid and dilute to 100 ml. Prepare this solution fresh daily. The Chicago acid may be purified as follows: Dissolve 100 grams (lOOyo basis) of commercial material in 800 ml. of water, add 20 ml. of concentrated hydrochloric acid, and 40 grams of Darco or other decolorizing charcoal, heat to 70" C., filter, cool to about lo", add 200 ml. of concentrated hvdrochloric acid, continue cooling to 0" to So,filter \vith suction, \
