ANALYTICAL CHEMISTRY
410
The precision of the instrument has been determined by measuring the cholinesterase activity of 10 aliquot portions of a preparation of goat red blood cells. The standard deviation was found to be ~ t 0 . 0 4peq. per minute. DISCUSSION
Thus far, the instrument has been used only in studying the enzyme system cholinesterase. The enzyme activity of the blood of humans, dogs, rats, goats, and rabbits, and of rabbit brain has been studied. The instrument's operation has been equally satisfactory with these different tissues and it should be satisfactory with homogenates of other tissues. A particularly useful application of the instrument in the field of enzymes is the study of enzymatic activity as a function of pH, temperature, or ionic strength. The procedure for varying pH was described above. Its advantages with respect to time and sample economy are obvious. Temperature could be varied systematically if the electrolytic cell rn ere thermostated, and the change of enzyme activity with temperature could easily be determined. Ionic strength could be varied by adding calculated amounts of salts to the cell after each equilibration. Although the instrument has been designed to measure pseudozero-order reaction velocities, it could be used to measure firstorder reaction velocities. Rate constants could be evaluated in the usual way, except that the milliammeter readings would be substituted for concentrations of reactive species as follo5-s:
where CO z1and 2 2 u1 and u2 I I and I2 k
= initial concentration of reactant = amounts reacted a t time tl and t 2 , respectively = reaction velocities a t time tl and t2, respectively
= current readings a t time
tl
and
f2,
respectively
= specific rate constant
In this type of determination the pH must necessarily increase. However, the increase may be made so small that it will not appreciably affect the reaction nor the observed current. Dynamic processes other than chemical reactions, such m gaseous, ionic, or liquid diffusion or solution rates, could be studied in this instrument after suitable modifications of the electrolytic cell, the sensing element, and the electrodes. LITERATURE CITED
Austin, R. R., Turner, G. K., Persy, L. E., Instruments
22, 588
(1049). --, Cooke, W. D., Furman, K. W., ANAL.CHEM.22, 8 9 6 9 (1950). Eckfeldt, E. L., U.S.Patent 2,621,671 (Dee. 16, 1952). Eustein, J., Sober, H. 9., Silver, S. D., ANAL.CHEY.19, 675-7 (1947). Fleisher, J. H., Pope, E. J., Arch. Ind. Hug. and Occupational Med. 9, 323-34 (1954). Lingane, J. J., ANAL. CHEM.21, 497-9 (1949). Michel, H. O., J . Lab. Clin. Med. 34, 1564-8 (1949). Shaffer, P. A., Jr., Briglio, A , , Jr., Brockman, J. A, Jr., ANAL. CHEM.20, 1008 (1948). Tammelin, L. E., Low, H. E., Acta Chem. Scand. 5, 322 (1951). \ - -
RECEIVED for review May 18. 1955. Accepted December 3, 1955. Requests for reprints should be addressed to Hans J. Trurnit, e t RIAS, Inc., P.O. Box 1574. Baltimore 3, M d .
Ultraviolet Spectrophotometric Determination Phosgene with An iIine WARREN B. CRUMMET" M a i n Laboratory, The Dow Chemical Co., Midland, Mich.
An ultraviolet spectrophotometric method has been developed for the determination of phosgene based upon the absorption of the 1,3-diphenylurea formed when phosgene is allowed to react with aniline in aqueous solution. The method is sensitive, specific, and capable of good precision.
T
HE aniline method for the determination of phosgene was first described by Kling and Schmutz (6) and shortly afterwards was successfully used by Biesalski ( 1 ) in studying the thermal decomposition of carbon tetrachloride. The method consisted in bubbling the gas to be tested through water saturated with aniline and weighing the 1,3-diphenylurea (carbanilide) formed. Olsen and coworkers ( 7 ) studied this method and found that 100 ml. of the aqueous aniline solution dissolves about 5.5 mg. of 1,3-diphenylurea. They therefore modified the procedure by first saturating the solution with diphenylurea to eliminate solubility errors. Comparing this technique with the sodium hydroxide (8), silver nitrate ( 7 ) , and sodium iodideacetone methods (4), these investigators recommended their procedure and that involving the use of sodium iodide and acetone. General acceptance of the aniline method by industry testifies to its practicality. However, it has some inherent weaknesses. Very small precipitates are difficult to handle. Some foreign materials may precipitate, giving high results. The solubility of diphenylurea may vary m-ith the conditions under which the sample is taken. On consideration of these variables it seemed
of interest to develop a new method which would be more sensitive and more specific. The variation in the absorption spectrum of aniline with pH has been studied by Tischler and Howard (9) from 305 to 255 mp. In acid solution the absorbance is much smaller than in basic solution and the spectrum is similar to that of benzene (3, 6). The ultraviolet absorption spectrum of l13-diphenylurea in ethanol has been reported by Schroeder and coworkers (8). The spectra of these compounds in methanol were studied in the present investigation (Figures 1 and 2). It was found that fairly small quantities of l13-diphenylurea can be determined in the presence of relatively large amounts of aniline in acidic methanol. Although both compounds exhibit absorption maxima a t 254.5 mp, the absorbance of 1,3-diphenylurea on a weight basis is 93.6 times as intense as that of aniline. Aniline also exhibits a sharp absorption peak a t 260.5 mp, which can conveniently be used to determine the quantity remaining after the phosgene has reacted. APPARATUS AND REAGENTS
A Cary recording spectrophotometer, hlodel 11M5, with matched 1-cm. silica cells, was used for absorbance measurements. The slit control was set to produce a slit width of 0.12 mm. at 254.5 mp. A manually operated spectrophotometer may be used, if enough points are plotted. The spectrum of 1,3-diphenylurea was determined on solutiona of a recrystallized product, melting point 235.5-7' C . Fresh1 distilled aniline was used to prepare solutions in water, in sucg concentrations that 50 ml. contained about 2 mg. per mg. of
411
V O L U M E 2 8 , NO. 3, M A R C H 1 9 5 6 phosgene anticipated, lus an excess of 50 mg. Phosgene (99.5 mole %) was obtaineffrom The Matheson Co. All other reagents were of analytical quality. CALIBRATION
Dissolve approximately 0.5 gram of 1,3-diphenylurea (weighed accurately to the nearest milligram) in methanol. Add 3 ml. of 36% hydrochloric acid and dilute to 100 ml. Take aliquots and make dilutions in such a way that the final dilution contains about 0.5 mg. per 100 ml. of methanol. Scan the spectrum of a portion of this solution in the ultraviolet from 300 to 210 mp, using acidic methanol as a reference solution. Read the absorbance a t 254.5 mp and divide the diphenylurea concentration by the absorbance to calculate a coefficient, C. This should have a value of about 0.615 mg. per 100 ml. per absorbance unit when a 1-cm. light path is used.
1.2001
I
Table I.
Determination of Phosgene
Phoegene Found, Mg. Recovery, % ' Teken, 1st 2nd 3rd 1st Mg. bubbler bubbler bubbler Total bubbler Tot81 3.91 0.04 95.4 99.0 0.10" 3.95 3.77 3.91 97.5 0.02 98.6 3.97 0.02 3.87 6.05 95.2 0.02 96.7 6.26 5.96 0.07" 6.86 99.0 0.04 0.04 100.1 6.85 6.78 7.55 99.8 0.04 101.0 0.05 7.47 7.46 7.58 96.3 0.02 96.9 0.03 7.83 7.53 Results high becsuae gas wa8 passed through bubblers too rapidly.
Phoagene
PROCEDURE
Bubble the gas under investigation through 50 ml. of the aqueous aniline solution (see Apparatus and Reagents) a t a rate such that not more than 2 mg. of phosgene is passed per minute. Transfer the solution and any precipitate vhich may have formed into a 250-ml. volumetric flask with methanol. Add 2 ml. of concentrated hydrochloric acid. Dilute with methanol. Take an appropriate aliquot and dilute it to 100 ml. with methanol. Scan the spectrum of a portion of this solution in a 1-cm. absorption cell from 300 to 240 mp. Determine the net absorbance a t 260.5 mp and multiply by the absorbance ratio, R. Subtract the result from the absorbance a t 254.5 mp to obtain the absorbance due to 1,3-diphenylurea. Call this quantity A . Milligrams of phosgene in 100 ml. of eolution = A X C x 0.466. The factor 0.466 represents the molecular weight ratio of phosgene to diphenylurea. RESULTS AND DISCUSSION
In order to test the method, small quantities of phosgene were weighed in sealed capillary tubes. These tubes were broken a t the stopcock in the apparatus shown in Figure 3 and the phos-
W 2h0
'
AIR PRESSURE FROM
,
I
O
I
260
240
AIR-ROW REGULATOR
300
280
WAVE LENGTH, Mp Figure 1. Ultraviolet absorption of aniline hydrochloride in methanol
55.9 mg. per 100 ml. 1.00-cm. cellm
In a similar way prepare a solution of 50 mg. of aniline (weighed accurately t o the nearest 0.1 mg.) in 100 ml. of methanol t o which 1 ml. of hydrochloric acid has been added. Scan the spectrum from 300 to 210 mp. Draw a base line through the minimum located a t 258 mp and tangent to the curve a t about 267 mp (see Figure 1). Read the absorbance a t 260.5 mp and subtract the base-line absorbance a t the same wave length to obtain a net absorbance. Read the absorbance a t 254.5 mp and determine the ratio, R, of the total absorbance a t 254.5 mp to the net absorbance a t 260.5 mp. This should have a value of about 2.80.
,800
W
VAPORIZATION CHAMBER
Figure 3.
Apparatus for determination of phosgene
.800C
I * O
: 220
240
260
280
WAVE LENGTH,M#
Figure 4. Ultraviolet absorption of acetanilide in methanol 0.70 m g . per 100 ml. 1.00-cm. cella I
230
Figure 2.
I
250 WAVE LENGTH, Mp
270
290
Ultraviolet absorption of 1,3-diphenylurea in methanol 0.517 mg. per 100 ml. 1.00-cm. cells
gene was bubbled slowly through three gas-absorption bubblera, each containing 50 ml. of the aniline solution. The results are tabulated in Table I . Although the results are not always quantitative, they all show better than 95% recovery in the first bubbler. In the rune
ANALYTICAL CHEMISTRY
412 Table 11.
Sample NO.
1 2
Comparison of Ultraviolet and Gravimetric Methods for Stack Gas Analysis Weight of Ppt., hIg. Gravimkric Method 54.3 9.6
1,3-Diphenylurea in Ppt., M g . , Ultraviolet Method 14.0 8.1
in which the gas flow was slowed down to rates such as those encountered in practice, the recoveries are ahove 97.5%. This is considerably better than could be espected from the gravimetric method. Acetyl chloride is very easily hydrolyzed and in the presence of water only a small amount reacts with aniline to form acetanilide. This compound s h o m an absorption maximum a t 240 mp in methanol (Figure 4). Chloroacetyl chloride quantitatively forms a-chloroacetanilide in aqueous aniline. It also shows an absorption maximum at 240 mp in methanol. APPLICATION TO STACK GAS ANALYSIS
The precipitation of materials other than 1,3-diphenylurea may cause high results by the gravimetric procedure. This TTas
emphasized by the analysis of two samples from a titanium chlorinator stack. The phosgene was precipitated as 1,3diphenylurea according to the procedure of Olsen and coworkers (’7) and the precipitate was weighed. The precipitate was then dissolved in methanol and the 1,3-diphenylurea was determined by the ultraviolet method. The results are shown in Table 11. -4s diphenylurea in the precipitate must inevitably be revealed by its absorbance, it is apparent that the gravimetric method mas giving high results on these samples, particularly in the first case. LITERATURE CITED
Biesalski, E., Z.angew. Chem. 37, 314 (1924). Fieldner, 8. C., Oberfell, G. G., Teague, M . C., Lawrence, J. X , J . Ind. Eng. Chem. 11, 519 (1919). Flesser, L. A , , Hammett, L. P., Dingwall, A., J . Am. Cheni. SOC.
57, 2103 (1935). Jahresber. (?hem.-Tech. Reichsdnstalt 5, 11-20 (1926) ; 6, 57-63 (1927).
Kling, A , Schmutz R., Compf. rend. 168, 773, 891 (1919). Kutnler, W.D., Strait, I,. A., J . Am. C h e n . SOC.65, 2349 (1943). Olaen, J. C., Ferguson, G. E., Sabetta, T’. J., Scheflan, L., IND. ENG.CHEX.,.\XAL ED.3, 189 (1931).
Schroeder, W.A, Wileox, P. E., Trueblood, K. S . ,Dekker, A . O., i i s a ~CHEM. . 23, 1740 (1951). Tischler, A. O., Howard, J. N., Xatl. Advisory Cornm. Aeronautics, A.R.R. No. E5H27a (1945). RECEIVED for review July 2 5 , 1955. Accepted Sovember 25, 1955.
Determination of Water Content of White Fuming Nitric Acid Utilizing Karl Fischer Reagent M. L. MOBERG, W. P. KNIGHT, and H. M. KINDSVATER Aerojet-General Corp., Azusa, Calif.
A method which utilizes Karl Fischer reagent has been developed for the direct determination of the water content of white fuming nitric acid. The w-eighed acid sample is first neutralized by the use of pyridinedimethylformamide solution to prevent reaction with the reagent. .4n excess of Fischer reagent is then added and a standard methanol-water solution is used for the back-titration. The method has shown a relative accuracy within 1% between the calculated and experimentally determined values in the absence of dissolved metallic salts. Approximately 5’70 variation is found in the presence of the solids. Nitrogen dioxide concentrations of less than 1.5% do not interfere with the determination.
A
DIRECT method for the determination of the water content of white fuming nitric acid, using Karl Fischer reagent, was developed in this laboratory in 1950. Mitchell and Smith (2) had reported the use of this method for the determination of water in concentrated nitric or sulfuric acids, but the method had not been extended to the white fuming nitric acid system. Later, Eberius (1)used the Karl Fischer reagent for the determination of water in mixed acid. The following procedures were developed for the determination of the water content of white fuming nitric acid containing small quantities of nitrogen dioxide and dissolved metal salts. APPARATUS AND REAGENTS
Apparatus. Covered tall-form Berzelius beaker with openings for two burets, stirrer, and end-point indicator.
Dead-stop end-point indicator (3). Reagents. All reagents were chemically pure or better. White fuming nitric acid was prepared by reaction of concentrated sulfuric acid and sodium nitrate. Distillation was conducted under moderate vacuum, and dry nitrogen was passed through the distilled product to remove nitrogen oxides. Acids of 0.08 to 0.6% water content were made by this method; in the presence of phosphorus pentoxide, acids of so-called negative water contents have been prepared which contain small quantities of nitrogen pentoxide. Karl Fischer reagent. To prepare 2 liters of Karl Fischer reagent, use 538 ml. of pyridine, 169.4 grams of iodine, 1334 ml. of absolute methanol, and 90 ml. (128 grams) of sulfur dioxide. Any of the materials Tyhich contain over 0.1% water by aeight should be dried and distilled before using. Methanol-water solution, prepared by adding weighed quantities of nater to anhydrous methanol. Pyridine-dimethylformamide solution, 2 to 1 bg volume. Standard sodium hydroxide solution, checked with standard hydrochloric acid solution standardized against sodium acid phthalate. Standard ceric sulfate solution, prepared from ceric ammonium sulfate and standardized against arsenous oxide. Standard ferrous sulfate solution, prepared from ferrous ammonium sulfate and standardized against ceric sulfate solution. Liquid nitrogen dioxide, 98.0%, Matheson Co., Inc. Ferric nitrate, Fe(SO& 9HzO. Chromic acid. Xickel nitrate, Ni( NO8)*,6H20. Aluminum nitrate solution,, prepared by dissolving aluminum (99% pure) in white fuming nitric acid of known composition. PROCEDURE
The classical method for the analysis of the distilled white fuming nitric acid was utilized. Nitric acid was determined by direct titration with standard base, nitrogen dioxide by reaction with excess ceric sulfate and back-titration with ferrous sulfate
