centration of collected fosm/concentration of the original solution is used for the 1-naphthylamine, some measure of the concentrating effect can be obtained. With this Foncentration ratio, the in this work go from 40 at Ier0 reflux to loo at recovery. If a volume smaller than 3 ml. is collected, the concentration ratio would be higher. LITERATURE CITED
(1) Brunner, C. A., Lemlich, R., Ind.
Eng. Chem. Fundamentals 2, 297 (1963).
(2) DuBose, B., Holland, v. B., Am. DyestugRe?J.34, 321 (1945).
(3) Karger, B. L., Rogers, L. B., ANAL. cHEY. 33, 1165 (1961). (4) Kenrick, F. B., J . Phys. Chem. 16, 513 (1912). ( 5 ) Lemlich, R., Lavi, E., b’cience 134, 191 (1961). (6) Rogers, L. B., Olver, J. W., Ibid., 135, 430 (1982). \ - - - - I
(7jRuben, E., Gaden, E. L., Jr., “Separation Techniques,” H. M. Schoen, ed., D. 319. Interscience. New York. 1962. (8) Schnepf, R . W., kt al., Chek. Eng. Prog. 5 5 , 42 (1959). (9) Schne f , R. W., Kevorkian, V., personal 8on~munication, Eldib, I. A,,
“Advances in Petroleum Chemical Refining,” Vol. 7, K. A. Kobe, McKetta, J. J., Jr., eds., p. 98, Interscience, New York, 1963. RUSTOM P. POHCHA B.4RRY L. KARQER Department of Chemistry Northeastern University Boston, Mass. 02115 This work was supported by the Basic Research Fund of h’ortheastern University and the Food Division of the U. S.Army Natick Laboratory under Contract No. DA 19-129-AMC-302(N ).
Ultraviolet Spectrophotometric Determination of Trace Quantities of Phosgene in Gases SIR. Ai spectrophotometric method for the determination of phosgene in gases has previously been described ( 1 ) . This method depends upon the absorption of 1,3-diphenyIurea formed when phosgene is allowed to react with aniline in aqueous solution. It is possible, by incorporation of a n extraction step, to increase the sensitivity fold. Aside from a color reaction of phosgene with 7-(nitrobenzyl)pyridine, described by Lamouroux ( d ) , the modified method is the most sensitive spectrophotometric one yet available for traces of phosgene. APPARATUS A N D REAGENTS
A Cary recording spectrophotometer, Model l l M S , with matched 1-em. silica cells, was used for absorbance measurements. A manually operated spectrophotometer may be used, if enough points are plotted. The 1,3-diphenylurea was a recrystallized product, melting point 235.5-7O C. Freshly distilled aniline was used to prepare solutions in water containing 2 mg. of aniline per ml. Phosgene (99.5 mole %) was obtained from the Matheson Co. The 1-pentanol was Eastman practical grade. The chlorine, nitric oxide, and carbon monoxide mere commercial grade gases. All other reagents were of analytical quality. CALIBRATION
Prepare a solution of 100 mg. of 1.3-diphenylurea in 100 ml. of methanol. Dilute a IO-ml. aliquot to 1000 ml. with water. Add a 1-ml, aliquot of the aqueous solution to a separatory funnel containing 25 ml. of t h e aniline solution. Add 1 ml. of concentrated sulfuric acid and approximately 9 ml. of a 1 : l mixture of 1-pentanol and nhexane. Shake the separatory funnel for 3 minutes, allow the layers to separate, and make the organic layer to volume in a 10-ml. volumetric flask by washing the separatory funnel with a small volume of the solvent mixture. 424
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ANALYTICAL CHEMISTRY
Scan the spectrum of a portion of this solution in the ultraviolet from 290 to 215 mp, using the hexane-pentanol mixture as a reference solution. Draw a base line tangent to the curve at approximately 280 mp and 220 mp. Read the absorbance at 257 mp and divide the diphenylurea concentration by the absorbance to calculate a coefficient C. This should have a value of about 68.5 pg. per 10 ml. per absorbance unit when a‘ 1-cm. light path is used. Bubble the gas under investigation through 25 to 50 ml. of the aqueous aniline solution at such a rate that not more than 2 mg. of phosgene are passed per minute ( 1 ) . Transfer the solution to a 125-ml. separatory funnel; add 1 ml. of concentrated sulfuric acid and about 9 ml. of hexane-pentanol solvent mixture. Extract and make to a 10-ml. volume. If necessary, take an appropriate aliquot and dilute to volume with the solvent mixture. Scan the spectrum of a portion of this solution in a 1-cm. absorption cell from 290 to 215 mp. Determine the net absorbance at 257 mp as above. Call this quantity A . Milligrams of phosgene in 10 ml. of solution = A X C X 0.466. The factor 0.466 represents the molecular weight ratio of phosgene to diphenylurea. RESULTS A N D DISCUSSION
T o test the method, small quantities of phosgene were weighed in sealed capillary tubes. These tubes were broken under n-hexane in which phosgene is very soluble. The resulting solution contained 18.4 pg. of phosgene per ml. The phosgene content mas initially 17.7 p g , per ml. by the extraction method. This decreased to 15.6 pg. per ml. in 3 days, after which it remained constant for a period of several weeks. This initial change in concentration was probably caused by reaction of the phosgene with solvent impurities. The limit of detection of phosgene per
10 ml. of solution is approximately 1 pg. I n air, 0.01 p.p.m. (1 p.p.m. = 4.05 pg. per liter of air) can be determined by blowing about 30 liters of air through the aniline solution and then using the extraction procedure. Two different techniques were used to allow the phosgene to react with aniline in the aqueous solution. First, portions of the standard solutions were vaporized and aspirated through the aniline solution. I n a second approach portions of the standard solutions were shaken with the aniline solutions. The results are summarized in Table I. Although the results are quantitative in only a few instances, they show better than 90% recovery in most cases. This is sufficient for practical applications. Difficulties encountered in handling phosgene solutions quantitatively and in assuring complete contact of phosgene with aniline account for the low results. They are not caused by the extraction which is shown in Table I1 to remove 1,3-diphenylurea quantitatively. The pH of the aniline solution may change when combustion products or vent gases are bubbled through it. The effect of pH on the reaction of phosgene with aqueous aniline was investigated by shaking 1.00 ml. of a n-hexane solution containing 15.6 pg. of phosgene with 50 ml. of the aqueous
Table I.
Determination of Phosgene
Aspirated samples Shaken samples Phosgene Phosgene Phosgene Phosgene taken, found, taken, found, rg.
rg.
6 2 6 2
5 8 5 9
7 7 7 7
6 9 7 1
rg.
1 3 9 13 26
3 1 4 2 4
PG.
1 0 2 8 9 1
13 2
26 3
Table II. Extraction of Diphenylurea from Aqueous Acidified Aniline Solution with 50: 50 n-Pentanol-n-hexane
1,3-Diphenylurea taken, p g . 0.00
6 0 13.7 13.7 13.7 27.4
1,3-Diphenylurea found, pg. 0.05 5.9 13.3 13.7 13.7 27.7
aniline solution and carrying this through the procedure. The results are shown in Table 111. The initial p H of the aniline solution was 7.4. Higher pH values were obtained b y adding ammonium hydroxide and lower ones by adding hydrochloric acid. The incomplete reaction ai, p H 9 and above is caused by hydroxyl ion concentration and not by a reaction between aniline, phosgene, and ammonia to form phenylurea or between phosgene and ammonia to form urea. This was shown by using other bases to obtain the same recovery at the same p H values, and b y
examining the reaction products for phenylurea. N t r i c oxide, if present in large quantities, will react with aniline to form a yellow-orange compound which interferes. Free chlorine will oxidize aniline to form a brown tar. These materials, however, are seldom present in concentrations high enough to interfere in practice. Large excesses of carbon monoxide (lo8) have no effect on the reaction. Samples of vent gases from gas burners produced aniline solutions of a p H from 4 to 8. They did not contain enough nitric oxide or chlorine to cause interference. Materials which are evtracted into the hexane-pentanol solution and interfere by absorbing ultraviolet light can often be eliminated by extracting the aniline solution with n-hexane before it is acidified with sulfuric acid. Diphenylurea is not appreciably soluble in n-hexane and remains in the aqueous layer. Such interferences are seldom encountered in combustion products, however.
Table 111. Effect of pH of Aniline Solution on Recovery of Phosgene
PH Recovery, %" 0.5 0 1.97 9.3 4.04 79.4 4.95 98.6 102.0 6 07 7.03 100,4 8.06 92.0 9 01 63.4 10.42 12.4 a 15.6 pg. of phosgene; 50 ml. of aqueous aniline solution (2 pg./ml.). LITERATURE CITED
w.B., ANAL.CHEM.28, 410-12 11956). ( 2 ) Lamouroux, A., Mem. Poudres 38, 383-6 (1956). W. B. CRUMMETT (1) Crummett, \
,
J. D. M C L E A N ~ Special Services Laboratory The Dow Chemical Co. Midland, Mich.
Present address, Department of Chemistry, Michigan State University, Lansing, Mich.
Use of Highly Acid Supporting Electrolytes in Polarography Observed Changes in Polarographic Waves of Selenium(IV) upon Standing SIR: The polarographic characteristics of selenium(1V) in a number of electrolytes were recently reported ( I ) . I n acid medium two waves are found, only the second being reversible. The authors have since had occasion to employ high concentrations of sulfuric acid supporting electrolytes for polarographic determinations of selenium in biological samples. I n 3144 sulfuric acid the second selenium wave decreased in height upon standing in the poiarographic cell under nitrogen with mercury dropping through the solution; the wave eventually disappeared completely. The wave also shifted to slightly more negative potentials as it decreased in height. Sometimes a milky precipitate occurred with these changes but this was not always the case. This behavior has been investigated further and the observed changes have been shown to be due to reaction of free mercury ions with intermediate reduction products of the selenium; the free mercury ions are formed from acid dissolution of the mercury electrode. EXPERIMENTAL
Reagent grade chemicals were throughout. Stock selenium(1V) tions were prepared by dissolving trograde black selenium powder
used soluspecin a
small amount of nitric acid and diluting to volume with water. All polarographic measurements were made with a Sargent Model XV polarograph. The polarographic cell was equipped with a mercury pool anode unless otherwise stated. Solutions were deoxygenated with Seaford nitrogen (Southern Oxygen Co.) for 10 minutes prior to the running of the polarogram. During the course of the polarogram a nitrogen atmosphere was maintained above the sample solution. All polarograms were obtained on a sample thermostated a t 23' ==! 0 . 2 O C. RESULTS
A polarogram was obtained on a solution of 10 pg. of selenium(1V) per ml. in 3M sulfuric acid. The original height of the second wave (after 10 minutes in the cell) was 0.90 pa. The wave almost completely disappeared upon standing for 1 hour under nitrogen in the cell [in contact with the mercury pool anode and the dropping mercvry electrode (DME)]. This decrease in the second wave height was accompanied by a change in the first wave from a fairly smooth wave to one with several maxima and minima. On further standing (after wave 2 disappeared) wave 1 became even more distorted. The maxima and minima could be eliminated by the addition of gelatin.
The effects of nitrogen and mercury were studied by transferring aliquots of the same solution (10 pg, Se/ml. in 3M HBSOI) to two flasks, one containing a pool of mercury. Each solution was bubbled for 8 hours with nitrogen. A polarogram was then obtained immediately after adding each of the solutions to the polarographic cell. The solution which contained no mercury yielded a height of 0.98 pa. for wave 2 and wave 1 was not distortedi.e., no change. After 1 hour in the cell, the height of wave 2 in this solution decreased to 0.85 pa. accompanied by a slight distortion of wave 1. The second solution, which had been over mercury, gave a height of 0.78 pa. for wave 2 and wave 1 was slightly distorted. Solutions of selenium(1V) in 3M sulfuric acid in the presence of air and the absence of mercury showed no change on standing, even after several weeks. T o test whether the observed phenomena were associated with the sulfate species or with the high acidity of the medium, the same studies were made in 4M perchloric acid. Essentially the same results were found. The original polarogram, after deoxygenating for 10 minutes, showed wave 1 to he more drawn out in this medium (almost straight). After 3 hours in the cell, VOL. 37, NO. 3, M A R C H 1965
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