Colorimetric Determination of Primary Mononitroparaffins EUGENE W. SCOTT AND JOSEPH F. TREON Kettering Laboratory, University of Cincinnati, Cincinnati, Ohio
T
HE recent work of Hass and his associates (4) on the nitration of paraffins in the vapor phase has made the mononitroparaffins and their derivatives commercially available. I n the course of studies on the toxicity of certain of these compounds carried out a t this laboratory, a method for the determination of primary mononitroparaffins in air was developed, and was later modified for analysis of biological materials. The determination of these compounds in mixtures with air was first attempted by reduction of the nitro compound and determination of the amine formed. Air samples containing nitroethane were absorbed in dilute sodium hydroxide and this solution was added to reduction mixtures of sulfuric acid with iron, tin, and zinc. Quantitative yields of ethylamine were not obtained under these conditions, although aqueous solutions of nitroethane were reduced, with yields of from 95 t o 97 per cent. It was observed that when one of the alkaline samples was added to an excess of hydrochloric acid containing ferric chloride, a pink color was formed. This color reaction was made the basis of a satisfactory method of determining nitroethane and further experiments with reduction methods were abandoned. The method adopted consisted of adding an excess of hydrochloric acid to an alkaline solution of nitroethane, this being followed immediately by the addition of ferric chloride. When the final pH was between 1.25 and 1.30, a reddish brown color was formed which rapidly changed to deep red. After a study of the factors which influenced the intensity and stability of the color, a procedure was adopted which gave reproducible results with amounts as low as 0.5 mg. in 25 ml.
concentration in order to lessen its color interference. The color was always developed a t room temperature. When the colored solution was heated it lost color rapidly. Nitroethane solutions which had not been made alkaline did not react with ferric chloride, and alkaline solutions after the addition of acid lost their ability to react if they were allowed to stand for a short time. The evidence points convincingly to the theory that only the aci form of the nitro compound reacts with ferric chloride. By analogy to similar structures such as those of phenols, enols, and aliphatic acids which also give colors with ferric chloride, it might also be assumed that the ferric chloride
Reagents Sodium hydroxide, 20 per cent, hydrochloric acid (concentrated diluted 1 to 7), 10 per cent ferric chloride, and a standard solution of nitroethane containing 1 mg. per ml.
Procedure An aliquot of the sample containing 1 to 20 mg. of nitroethane in 1 to 15 ml. is neutralized and treated with 1.5 ml. of sodium hydroxide in a 25-ml. volumetric flask. After standing for 15 minutes the solution is acidified with 6.0 ml. of hydrochloric acid, and 0.5 ml. of ferric chloride is added immediately. A standard solution containing approximately the same amount of nitroethane is treated similarly. After standing for 15 minutes the solutions are diluted to the mark and compared in a colorimeter equipped with a 1.58-cm. (0.625-inch) Wratten filter KO.65A in B glass (Eastman Kodak Co.).
Study of the Reaction The final p H of this solution was found to be 1.25 to 1.30, which was the optimum for the greatest intensity and stability. Solutions which contained more acid faded gradually, while those which had a higher pH failed to give the change from the original brownish red to a deep red. The amount of acid necessary t o obtain a pH of 1.25 to 1.30 was determined each time fresh solutions of hydrochloric acid and sodium hydroxide were prepared. With larger amounts of nitroethane (15 t o 20 mg.) it was necessary to use the indicated amount of ferric chloride, but with very small quantities of nitroethane it was desirable t o reduce the ferric chloride
FIGURE1 189
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Application of h l e t h o d The method as given above wa5 first used to determine the amount of nitroethane in air samples taken from a cage into which a constant stream of air and nitroethane was flowing. The latter was foIced by means of a special plunger geared to a synchronous motor into a stream of air which vaporized the nitroethane. The air flow was measured with a rotameter ( I ) . The air samples were taken from the cage by means of large evacuated bulbs and the nitroethane was removed by shaking the sample with alkali placed in a smaller bulb connected to the large bulb by a stopcock. In one experiment in which a mixture calculated to contain 0.05 per cent of nitroethane was used daily for 23 days, the results obtained on 23 samples taken 2 hours after the start varied from 0.043 to 0.055 per cent with a mean value of 0.0485 * 0.0004 per cent.
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8 0 combines with the hydroxyl group formed by the shift of a proton from the adjacent carbon atom to an oxygen atom attached to the nitrogen. Stable colored complexes with ferric chloride were also formed by 1-nitropropane and 1-nitrobutane and these colors were used to determine these substances in the same manner. Ferric chloride reacted with 2-nitropropane and 2-nitrobutane to give colors, but they faded rapidly. Kitromethane did not react and was determined by means of the color which it formed with vanillin and ammonia ( 2 , 5'). I n the case of nitroethane the colored complex was isolated in an impure state by mixing relatively large amounts of nitroethane and ferric chloride and evaporating a t room temperature in a vacuum desiccator. This complex could not be purified for analysis, however, because of its instability. The colors given by the primary nitro compounds and ferric chloride were examined in a photoelectric spectrophotometer and the absorption curves of the complexes in the range of 420 to 690 mp were obtained by determining the density of the solutions a t 10 mp intervals. The curves shown in Figure 1 were obtained by plotting wave lengths against the molecular extinction coefficient, E = D/Cl, where D is the observed density, 1 is the length of the cell in centimeters, and C the molar concentration of the absorbing material. The latter value was assumed to be the same as the concentration of the nitro compound used. This assumption was sufficiently correct for the authors' purposenamely, comparison of the absorption of the three nitro compounds on a n equivalent basis-but the true values of E may vary somewhat from these figures. The absorption of all three compounds a t their maxima was examined over two ranges of concentration with the use of a 1.25-cm. (0.5-inch) and a 5-cm. (2-inch) cell. K h e n density was plotted against concentration the resultant curves were in straight lines, which indicates that in these ranges the absorption followed the law of Lambert-Beer. The results are given in Figures 2 and 3.
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The method with slight modifications has also been applietl to the determination of nitroethane in tissues and in most instances the recovery of added nitroet'hane has been better than 95 per cent. Summary X simple colorimetric method has been described for the determination of the primary mononitroparaffins 11-ith the exception of nitromethane. The absorption curves of the colored complexes of nitroethane, 1-nitropropane, and 1nitrobutane with ferric chloride in acid solution have been determined. A study of the reaction indicates that these complexes are formed only by the aci forms of the nitro compounds. Literature Cited (1) Machle, W. F., Scott, E. IT., and Treon, J. F., J . I d . Hug. To.c:icol., 21, 72 (1939). (2) hlachle, W.F., Scott, E. W., and Treon, J. F., in press. (3) Manaoff, C. D., 2. Sahr.-Genussm., 27, 469 (1914). (4) Seigle, L. IT., and Hass, H. B., IBD.Exc. CHEW,31, 648 (1939). PRESENTED before the Division of Physical and Inorganic Chemistry a t the 08th Meeting of the dmerican Chemical Society, Boston, 3Iass.
