Polarographic Determination of Monoand Dinitroxylenes CECIL H. HALE', ESSOLaboratories, Baton Rouge, La.
A n analytical control method was needed for the rapid determination of monoand dinitroxylends in a commercial process for hydrogenation of nitroxylene to xylidine. A polarographic method was devised based on the reduction of mononitroxylene at about 0.9 volt and of dinitroxylene at about 0.5 and 0.9 volt in ethyl alcohol-benzene solvents. Sodium acetate was employed as the supporting electrolyte. Only a few minutes are required for a determination and the accuracy is within 4 and 10% for mono- and dinitroxylene, respectively. The procedure was satisfactory for control of plant operation and determination of purity of nitroxylene feeds and xylidine products. The advantages over the titanium trichloride titration method are that dinitroxylene can be measured in the presence of a large excess of mononitroxylene; the method is rapid and very adaptable to routine control, and does not involve use of an unstable reagent.
R
ECENT publications (8, 8) have described a high pressure
polarographic reduction of alkaline solutions of the mixed isomers of mononitroxylene obtained by nitration of commercial xylenes produces one well-formed wave. In alkaline solutions, the dinitroxylenes are reducible with the formation of two waves of equal height. The second wave occurs a t about the same potential as the one resulting from the reduction of mononitroxylene. Under very carefully controlled conditions, the first wave can be distinguished in the presence of a large excess of mononitroxylene. No attempt was made to measure the exact half-wave potentials, but they are approximately 0.5 and 0.9 volt for the dinitroxylene and 0.9 volt for the mononitroxylene, measured in alcohol against a mercury anode, with sodium acetate as supporting electrolyte. Sodium acetate was used as the supporting electrolyte because of its relatively high solubility in the solvent, and as it gave a satisfactory wave for nitroxylene, no other electrolyte was tried. No significant differences in either half-wave potential or diffusion coefficient for a few individual isomers of mononitroxylene were found. HoN-ever, the diffusion coefficient is greater for monothan for dinitroxylene. Typical polarograms of mono- and of dinitroxylenes and of mixtures are shown in Figure 1.
catalytic method for the hydrogenation of nitroxylene t o xylidine. For analytical control of the project, the determination of mono- and dinitroxylenes in the feed to the hydrogenation unit and of unreduced nitroxylenes in the product was necessary. The titrimetric determination of nitro aromatics with titanium(II1) chloride is well known ( 1 ) . Some difficulties were encountered in the routine application of this method because of the instability of the reagent, the necessity of titration in an inert atmosphere, and a relatively slow reaction with the nitroxylenes.
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I-
REAGENTS AND APPARATUS
0 -.4
1
I I 6 74 -a -.4 -8 VOLTS VERSUS Hc ANODE -4
Sodium acetate, 50% aqueous solution of the C.P. trihydrate. Tetraethylammonium hydroxide, lo%, purchased from Eastman Kodak Co. Gelatin, 0.5% solution. Nitroxylene for calibration and for preparation of synthetic samples was a middle fraction from a vacuum distillation of material obtained by nitration of mixed isomers of xylene. Dinitroxylenes were obtained from the bottoms fraction frqm the distillation of mixed nitroxylenes. The bottoms material was dissolved in methanol and chilled with dry ice until crystallization occurred, The product was purified by recrystallization three times from methanol and four times from ethyl alcohol. A Sargent Model X I polarograph and Sargent electrolysis vessels were used.
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Figure 1. Typical Polarograms of Mono- and Dinitroxylenes Solvent t 40 % benzene, 53 9% ethyl alcohol, 7 9% HzO. Supporting electrolyte: 0.15 M sodium acetate A . Mononitroxylene, 0.15 m ./ml. E . Dinitroxylene, 0.081 mg./?ml. C. Dinitrorylene, 0.081 mg./ml., i n excess mononitroxylene
Furthermore, the method is inadequate for the determination of small amounts (0.5 to 10.0%) of dinitroxylenes in the monoderivative. The possibility of an explosion with materials containing dinitro compounds and the deleterious effects of the diamines on color stability of the xylidine ( 4 ) made their determination of particular importance. The reduction of nitro derivatives of various aromatics other than xylene at the dropping mercury electrode has been studied by Shikata and coworkers (6-7'). Investigation revealed that 1 Present
PROCEDURE
Calibration. Calibration curves were prepared by polarographing ethyl alcohol solutions of purified mono- and dinitroxylenes accordin to the procedures described for analysis of samples. Straight-fine relationships were obtained between wave height and concentration in each case. Dinitroxylene. Weigh a 0.5-gram sample into a 50-ml. volumetric flask and dilute to volume with 95% ethyl alcohol. Pipet an aliquot (1 to 10 ml, depending on the dinitroxylene content) into a 25-ml. volumetric flask containing 0.5 ml. of 50% sodium
address, Southwestern Analytical Chemicals, 1107 West Gibson
St., Austin 4, Tex.
572
V O L U M E 23, NO. 4, A P R I L 1 9 5 1
573
Table I. Polarographic Determination of Nitroxylenes in Synthetic Samples Weight % ’ h’itroxylene Added Found 0.49 0.51 0.50 1.72 1.70 1.67 4.3 4.2 3 8 12.8 13.3 13.3 0.26 0.27 0.25 0.43 0.44 0.42 1.16 1.10 1.03 91 91 89 74 71 71 82 86 86 92 91 89
Diluent Cyclohexane Mineral oil
Mineral oil
+ xylidine
Ethyl alcohol Dinitrox ylene
Table 11. Polarographic Determination of Dinitroxylene in Mononitroxylene Sample 1
2
Weight % ’ Dinitroxylene Added Found 1.2 1.3 1.2 2.5 2.1 2.6 12.2 11.3 18.4 25.8 51.3
11.1 19.4 19.0 26.3 26.3 49.4 51.6
taken of the fact that the diffusion coefficient of dinitroxylene is smaller than that of mononitroxylene. The correction factor was calculated from the mono- and dinitroxylene calibration curve^ t o be 1.29, as follows: At concentrations of 0.35 mg. per ml. the heights of the one wave for mononitroxylene and of the two waves for dinitroxylene were 68 and 88 mm., respectively, a t a galvanometer sensitivity 88 of 1/200. The correction factor = - = 1.29. 68 The amount of mononitroxylene in a mixture with dinitroxylene is calculated from the total apparent nitroxylene content by the following correction:
yo mononitroxylene = % nitroxylenes (total, apparent) - 1.29 X % dinitroxylene RESULTS AND DISCUSSION
Three types of synthetic samples were prepared and analyzed polarographically:
1. Known amounts of mononitroxylene in xylidine and in various inert diluents 2. Known amounts of nitroxylene in dinitroxylene 3. Known amounts of diriitroxylene in nitroxylene The results, some of which are shown in Tables I and 11, indicate that nitroxylene and dinitroxylene can be determined by this method to within *4 and + l o % , respectively. S o attempt was made to control the temperature of the cell during electrolysis. The method was also used to determine the purity of nitroxylenes, as received for hydrogenation, in which cases the temperature of the electrolysis cell was controlled a t 25’ * 0.5”C. The precision that was attained in the analysis of such samples is shown by the typical results in Table 111.
Table 111. Analysis of Typical Samples of Nitroxylenes wt. 70 Pt. % Sample 1
2 3
acetate solution and 1 ml. of 10% tetraethyl ammonium hydroxide. Add 10 ml. of benzene and dilute to volume with 95% ethyl alcohol. For materials that contain less than 5% dinitroxylene, use the total sample without preliminary dilution with ethyl alcohol. The optimum concentration of dinitroxylene in the solution to be electrolyzed is 0.1 to 0.3 mg. per ml. Mix the solution thoroughly, transfer a portion to an electrolysis.vesse1, and remove dissolved oxygen by scrubbing with nitrogen for 10 minutes. Place a gas-washing tube containing 20% benzene in 95% ethyl alcohol on the nitrogen line before the electrolysis vessel to prevent vaporization of the sample solution. Record the polarogram between 0 and 1.5 volts, measured against the mercury anode. The rate of change of applied potential should be as low as possible in order to obtain a more distinct wave for the dinitroxylene. The dinitroxylene content is found by comparison of the height of the first wave with a previously prepared calibration curve. -411 wave heights were measured by the extrapolation method illustrated by Kolthoff and Lingane ( 3 ) . Mononitroxylene. Prepare the solution in the same manner as for the determination of dinitroxylene, but add 1 ml. of sodium acetate solution, 5 drops of 0.5y0 gelatin, and no tetraethyl ammonium hydroxide to the 25-ml. volumetric flask before addition of the benzene and ethyl alcohol. Transfer a portion of the solution to an electrolysis vessel, and polarograph as for the dinitroxylene, except that rate of change of applied potential should be fairly high to obtain a well-formed wave. Under these conditions, the primary wave for dinitroxylene is not resolved and is included in the single wave for both mono- and dinitro derivatives. The total apparent nitroxylene content, uncorrected for included dinitroxylene, is found by comparison of the wave height uith a previously prepared calibration curve for mononitroxylene. A correction must be applied for samples that contain dinitroxylene, because both waves of the dinitro compounds are included in the measurement. I n making this correction, account must be
4
5
h’itroxylene 99.5 100.0 92.6 92.1 88.0 88.0 93,5 94.0 88.6 88.1
-
Dinitroxylene 0 1.4 1.6
1.5 1.6 2.1 2.0 9.2 8.8
The procedure was satisfactory in the control of plant operation and determination of purity of nitroxylene feeds and xylidine products. Dinitroxylene can be measured in the presence of a large excess of nitroxylene; the method is rapid and very adaptable to routine control, and does not involve the use of an unstable reagent. LITERATURE ClTED (1) (2)
Becker, W.W.,ISD.ENG.CHEM.,ANAL.ED.,5, 152 (1933). Brown, C. L., Smith, W.hl., and Scharmann, W. T., Ind. Eng. Chem., 40, 1538 (1948).
Kolthoff, I. M.,and Lingane, J. J., “Polarography,” p. 57, New York, Interscience Publishers, 1941. (4) Kunc, J. F., Jr., Howell, W.C., Jr., and Starr, C. E., Jr., Ind. (3)
Eng. Chem., 40, 1530 (1948). (5) Shikata, M., Trans. Faraday Soc., 21, 4 2 (1925). (6) Shikata, hI., and Hoaaki, K., Mem. Coll. A g r . K y o t o Imp. Univ., 17, 1, 21 (1931). (7) Shikata, &I., and Watanabe, hl., J . A g r . Chem. Soc. Japan, 4, 924 (1928). (8)
Voorhies, A., Jr., Smith, W. A I . , and Mason, R. B., Ind. Eng. Chem., 40, 1543 (1948).
RECENEDMarch 27, 1950. Presented before the Division of Analytical Chemistry at the 117th Meeting of the AVERICAXCHEUICALSOCIETY, Houston, Tex.
