Polarographic Determination of Acrylonitrile WILMER L. BIRD, Esso Standard Oil Co., Baton Rouge, La., CECIL H. HALE', Esso hboratories, Baton Rouge, La.
N T H E manufacture of Paracril, a butadiene-acrylonitrile
1 copolymer, a routine analytical control method was required
for the determination of acrylonitrile. The first method (2) used consisted in hydrolysis of the acrylonitrile to ammonia with sodium hydroxide, distillation of the ammonia into standard acid, and back-titration with standard base. This method requires 2 hours for analysis of an aqueous solution and is not specific for acrylonitrile.
45
40 35
AND
graphic cell is emptied and the cellulose cap is stripped off, The tube is m s h e d and a new Celon cap is slipped over the end and allowed to dry. This cell is easily prepared, economical, and readily cleaned, and does not require a new mercury pool for each sample. The tetramethylammonium iodide was purchased from the Eastman Kodak Go. and purified by recrystallization. Acrylonitrile, ethylene cyanohydrin, and lactonitrile \\el e obtained from the American Cyanamid Co. and redistilled, the latter two at 10-mm. pressure. Middle fractions were used in each case. Other nitriles and cyanoacetamide were purchased from the Eastman Kodak Co. and used without purification. Calibration. Weigh about 2 ml. of acrylonitrile into a 100-nil. volumetric flask and dilute to volume with water. As acrylonitrile polymerizes on standing, even at subzero temperatures, only freshly distilled material should be used. Add aliquots of this solution to 100-ml. volumetric flasks containing 10 ml. of 0.2 M tetramethylammonium iodide. Polarograph the solutions a t suitable sensitivities. The relationship between concentration and wave height was found to be a straight line in the concentration range investigated, 0.003 to 0.013 M. The teni-
Lu VI
5 30
1 a.
P5
Table I.
Polarographic Reduction of Various Nitriles
u
I f 0 Compound Propionitrile p2hloropropionitrile Cyanoacetamide Acrylonitrile Lactonitrile Ethylene cyanohydrin
15 IO
E I , ~Volts ,
Formula CHiCHzCK CICHGHzCN NCCHICONH, CHz=CHCK CHiCHOHCN HOCHKHzCN
No red. No red. No red. -2.1 -2.1
No red.
I d . Microarnperw moles/I.. C.hlilli-
--_
-
-
-
8.54 0.10 -
Table 11. Polarographic Analysis of Samples 1.8
P.0 2.P P.4 APPLIED POTENTIAL VS. S.C.E.
Acrylonitrile, Wt. %Added Found
~
Type of Sample
Sample No.
Synthetic Samples
Figure 1. Typical Polarogram of Acrylonitrile Water
0.02 M tetramethylammonium iodide
A polarographic procedure was devised which requires only about 15 minutes for analysis of an aqueous solution and does not measure other nitriles that might be present. It has been applied t o the determination of acrylonitrile in water, in butadiene, and in air, and for measuring the purity of crude acrylonitrile. T h e polarographic method is based on the fact that acrylonitrile is reducible a t about -2.05 volts against the saturated calomel electrode with tetramethylammonium iodide as the supporting electrolyte. As shown in Figure 1, one w-ell-formed wtve is obtained.
Butadiene
1
0.52
0.53 0.53
2
0.93
0.97 0.98
3
1.58
1.59 1.60
4
1.88
1.88 1.88
5
2.57
2.56 2.57
6
3.14
3.10 3.11
I
3.78
3.72 3.73
1
0.99
1.01
2
1.95
1.95 1.96
3
1.98
1.99 2.00
Polarograph
Hydrolv.si*
APPARATUS AND REAGENTS
E. H. Sargent polarographs, Models X I and XX, were used in these studies, For routine analyses the Model X I was operated manually and the readings were made from the indicating scale. The polarographic cell found best suited to routine work is similar to the one described by Gawron ( 1 ) . A Celon cap, style TRSP, size 24 X 40 mm., sold by the Celon Co., 2034 Pennsylvania Ave., Madison, \Vis,, is used for the membrane. It is fitted over the bottom of a cup, made by cutting off the bottom of a 25-mm. test tube 45 mm. from the lip. After drying, the resulting cellulose bottom is water-tight but allows the passage of electric current. This small cell is fitted into a thin rubber stopper and placed in a 150-ml. tall-form beaker containing the mercury pool which is connected to the anode outlet of the polarograph. The beaker is filled to a suitable level with 0.02 M tetramethylammonium iodide for routine analyses or with potassium chloride solution saturated with calomel for measuring half-wave potentials. After completion of an analysis, the polaro-
1.01
Plant Samples Water
1.41 1.41 1.46 1.45
1.35 1.38
3
1.64 1.64
1.69 1.69
4
1.66 1.69
1.71 1.76
3
1.82 1.83
1.78 1.78
1
2.67 2.67
2.82 2.82
2
3.20 3.22
3.04 3.06
1 9
Butadiene
Present address, Southwestern Analytical Chemicals, 1107 West Gibson St., h u s t i n 4, Tex. 1
586
1.42 1.46
V O L U M E 2 4 , NO. 3, M A R C H 1 9 5 2 perature coefficient was found to be 1.6% per degree centigrade a t 20" to 30" C., and for precise work the temperature of the cell should be controlled. PROCEDURE
The procedure for analysis of aqueous solutions or determination of purity of acrylonitrile is the same as for calibration. The sample is prepared to contain 0.01 to 0.03 gram of acrylonitrile in 100 ml. of 0.02 Af tetramethylammonium iodide. For the determination of acrylonitrile in butadiene, pour a measured amount of butadiene into a flask containing 40 ml. of 95% ethyl alcohol which has been cooled in dry ice. Allow the mixture to come to room temperature over a period of several hours, transfer to a 50-ml. volumetric flask, and dilute to volume with alcohol. Withdraa a 2-ml. aliquot, transfer to a 100-ml. volumetric flask, and analyze as with an aqueous solution. Thr original volume of butadiene should be such that a 2-ml. aliquot of the alcohol solution contains about 0.02 gram of acrylonitrile. To determine acrylonitrile in air, pass a measured volume oi air through a series of three scrubbers, each containing 10 ml. of 95% ethyl alcohol a t 0" C. Determine the acrylonitrile content of the alcohol in each scrubber as in the analysis of butadiene. If acrylonitrile is found in the third scrubber, the air rate was too high and the determination should be repeated with a slower rate of scrubbing. DISCUSSION
T o determine the selectivity of the method, an attempt was made to reduce propionitrile, 8-chloropropionitrile, cyanoaceta-
587 mide, ethylene cyanohydrin, and lactonitrile in 0.02 M tetramethylammonium iodide. The results, shown in Table I, indicate the method to be very selective. I n the case of the lactonitrile, although every precaution was made to obtain a pure compound, a trace of impurity may have been responsible for the slight reduction. The accuracy and precision of the polarographic method n-ere established by the analysis of synthetic samples and plant samples. The plant samples Rere also analyzed by the hydrolysis method. The results, summarized in Table I1 indicate the polarographic method to be a t least as accurate as the hydrolysis method. The time required for an analysis is much less. The polarographic method has been used in plant control for over 6 years and found entirely 6atisfactory. Oxygen removal has not been required in the routine determination of acrylonitrile in water or alcohol-water solutions. However, if interference by dissolved oxygen is encountered, extreme care should be used in oxygen removal by nitrogen blowing. as acrylonitrile is very volatile. LITERATURE CITED
(1) Gawron, O., ANAL. CHEX. 22, 014 (1950) (2) Radziseewski, Ber., 18, 355 (1885). RECEIVED for review December 27, 1948. Accepted January 8 , 1952. Presented a t the Fourth Annual Southwest Regional Meeting of the A X E R I ~ % \ CHEXICAL SOCIF:TY, Shreveport, La., December 1948
Polarographic Determination of Methacrylonitrile
'
L. J. SPILLANE' Allied Chemical and Dye Corp., Morristown, N . J .
need for a rapid and reasonably accurate method of determining methacrylonitrile in the presence of unsaturated aldehydes, saturated nitriles, and water indicated application of polarographic analysis. Accordingly, an analytical procedure \\as developed which shoaed a precision within about 3%. Limited tests also indicated the method could be applied to estimation of acrylonitrile. APPARATUS AND REAGENTS
JIeasurements were made on a Fisher Elecdropode modified to permit immersion of the electrolysis cell ( 2 ) . Calibration of the galvanometer scale showed a sensitivity of 0.01935 microampere per unit deflection. For analytical purposes measurements were performed a t one twentieth of the maximum sensitivity and a t 25' i 0.02" C. Under a pressure of 25.0 cm. of nicwury the capillary possessed a drop time of 4.72 seconds in distilled water and 3.98 seconds in 0.1 J f tetramethylammonium hivmide (open circuit). The mass of mercury flowing per second i t 1 distilled water was 1.988 mg., resulting in a value of 2.048 for m 2 : 3f1 In. The diffusion current for 1.1 X .If potassium iodate in 0.1 .If potassium chloride measured with this capillary was 25 niici,o:lmperes; the calculated value from the IlkoviE equation is 2(i ( 2 ) . JIethacrylonitrile used in calibration was purified by fract'ional distillat,ion of commercial material through a 4-foot Fenske-type colunin packed with stainless steel helices. Only the middle fi,:ic.tion boiling a t 90-90.5" (755 mm., n2,0, 1.4005) was emploj.ed for analytical purposes.
could be used without purification, but if any current-voltage curve was noticed between -1.5 and -2.1 volts the material was recrystallized from ethyl alcohol-water. The half-wave potentials and diffusion currents shox that methacrylonitrile can he determined polarographically with a precision of about 3%. Data shonn in Table I may be converted into a convenient calibration chart b) applying a standard dilution technique for analysis. For routine purposes all the data shown in the table need not be collected. Instead, a plot can be prepared of galvanometer deflection (corrected for blank) against milligrams of methacrylonitrile dissolved in 50 ml. of 0.1 M tetramethylammonium bromide.
Table I.
Polarographic Properties of Methacrylonitrile
in 0.1 M Tetramethylammonium Bromide Concn., Millimoles,' liter 2.39
a
pa. 15.49 1.80 11.63 1.66 10.66 7.75 1.200 900 6.00 5.81 0.881 Saturated calomel electrode. Id,
Id
x
/c
10J
6.48 6.46 6.46 6.48 6.28 6.59
111,2,
( N g pool)
-2.02 -2.02 -2.02 -2.02 -2.02 -2.02
Volt (S.C.E.a) . - 2 07 -2.07 -2.07 - 2 07 -2.07 - 2 07
4NALYTICAL PROCEDURE CALIBRATION PROCEDURE ( 'urrent-voltage
curves were determined for weighed quantities of freshly distilled nitrile dissolved in 0.1 IM tetramethylaiiiiiionium bromide. Experimental results are ,summarized in Table I. Removal of dissolved oxygen prior to electrolysis is unnecessary, but a blank determination was always made on fresh batches of the supporting electrolyte. Occasionally, cominerc>ialtetramethylammonium bromide (Eastman White Label) I
Present address, Lion 011 Co., El Dorado, irk
-4 sample containing an unknown quantity of methacrylonitrile is weighed into a 100-ml. volumetric flask, using about 5 ml. of 95% ethyl alcohol to aid in transfer, and diluted to the mark x i t h distilled water. ,4 1.00-ml. aliquot is removed and diluted to the mark in a 50-ml. volumetric flask with 0.1 .If tetramethylammonium bromide. Determination of the complete current-voltage curve fiom to -2.2 volts (us. the mercury pool) is not always necessary but is advisable, in order t o reduce chances of error and to point out proresr: changes. Comparison of the diffusion cur-1.5
