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
ANALYTICAL CHEMISTRY
588 rent (corrected for blank) w i t h the calibration chart gives a quantitative measure of methacrylonitrile in original sample. Tests for interfering substances showed that moderate amounts of isobutyronitrile (equimolecular) do not affect either the halfwave potential or the diffusion current. Even a 13t o 1 molecular ratio of isobutyronitrile-methacrylonitrile caused only a 6% depression in diffusion current and no effect was observed on the half-wave potential. a,8-Unsaturated aldehydes do not interfere because they are reduced a t niuch more positive potentials than niethacrylonitrile ( I , 3 ) . *4crylonitrile was found to interfere, but, a very limited study indicatcd that it can also be determined polarographically in the absence of methacrylonitrile. Acrylonitrile appears t o be slightly easier t o reduce (about 50 mv.) and t o give a slightly higher diffusion current. Time did not permit further investigation of this problem. Removal of oxy-
gen prior to analysis is unnecessary. However, quantit'at ive det,ermination of methacrylonit,rile should be conducted soon after the nitrile is dissolved. Xoticeably lower diffusion currents were obtained aft,er samples had stood for 4 hours. ACKNOWLEDGMENT
The author is grateful t o the Allied Chemical and Dye Colp. for permission t o publish this work. LITERATURE CITED
(1) Fields and Blount, J . A m . Chein.
SOC.,70,930 (1948). (2) Lingane and Kolthoff, Ihid.. 61, 825 (1939). (3) Moshier, IND.ENG.CHEM.,Axax.. ED., 15, 107 (1943). RECEIVEDfor review Soveinber 8. 1950. Accepted July 13, 1951. h e sented before the Division of Analytical Chemistry at t h e 118th Meeting of the AMERICANCHEYICALSOCIETY,Chicago, Ill.
Polarographic Determination of low Concentrations of Silver G. C. B. CAVE' AND DAVID N. HUME Laboratory f o r Nuclear Science and Engineering and Department of Chemistry, Massachusetts Znstitute of Technology, Cambridge 39, Mass.
K T H E course of determining the solubility of silver thiocyanate in potassium nitrate-pot'assium thiocyanate mixtures, i t was necessary t o develop methods for determining small amounts of silver in the presence of relatively large amounts of thiocyanate. By the application of unusual precautions it was possible to do the determination polarographically over a range of 5 X 10-eM to 0.01 M silver with a precision such that duplicate samples agreed nithin 0.5% except, at the very lowest concentrations. A Sargent Model XXI polarograph with a conventional dropping electrode and H-cell assembly, thermostated a t 30' f 0.1' C., was used. The numerical value of m*/3t1/Gatthe potential of the saturated calomel electrode was 1.68. Repurified nitrogen was used t o remove oxygen. The supporting electrolyte was a mixture 1 M each in potassium nit,rate and potassium thiocyanate, no maximum suppressor being required. All measurements were made a t -0.400 volt PS. the saturated calomel electrode, the potential being set with the aid of an auxiliary potent,iometer. The diffusion current was recorded for several minutes a t this potential and the average of the maximum pen excursions was determined. Because silver, even in thiocyanate medium, is more noble than mercury, the half-wave potential of t.he silver reduction is more positive t.han the dissolution potential of mercury and therefore is not observable.
1 .If both in potassium nitrate and thiocyanate, i t became evident that the degree of uniformity of the individual spikes waa dependent on the applied voltage. If the applied voltage were such that the residual current was anodic or essentially zero, the individual spikes were erratic-e.g., variations of 0.02 pa. at - 0.20 volt. With small cathodic residual currents, however, the variations were much less-0.001 pa. a t - 0.4 volt. The working potential should be chosen to give as small a cathodic residual current as is feasible.
Table I.
Calibration Data for Determination of Silver
Residual Diffusion Ag Current, Current, Sensitivity. Concn., Ma Mm. h1m.b pa/Mm. 0.999 x 10 -5 3.9 14.3 0,003 3.9 41.2 0.003 2.998 x 10 -5 3.9 66.2 4.997 x 10-5 0.003 133.1 0.999 x 10-4 3.9 0.003 135.5 4.997 x 10 - 4 0.9 0.015 135.2 0.03 0.4 0.999 x 10-3 135.0 0.15 0.1 4.997 x 10 - 8 134.9 0.9993 X 10 - 2 0.0 0.3 Mean of all 8. ed/C = 4088,c = 79.0,or 1.9%. Mean of runs 3 t o 8. id/C = 4049,c = 26.2,or 0.65%. B y dilution of stock 0.9993 X 10-2 M solution. b Mean values of triplicate runs, corrected for residual current +
Run SO.
id/(?.
pa./M
No unusual difficulties were observed in the determination of concentrations greater than M , but a t extremely low concentrations, with correspondingly high inst>rumentSensitivity settings, a number of ordinarily minor instrumental variables became significant. The immediate problem was to extend the lower limit of concentration to 5 X 10-8 31 silver while maintaining t,he error a t not over & 5 % . The solution was achieved through ~tcareful study of the factors that affect the constancy in the height of the individual "spikes" (pen excursions for the individual drops) at constant applied voltage. I t vias found that grounding the water bath, the metal framework holding accessories in the thermostat, and finally the dropping electrode itself eliminated several sources of fluctuat,ions in t,he diffusion current. Further undulations of about f 0.006 pa. in the diffusion current tracings were shown to he due to high atmospheric humidity and were satisfactorily eliminated by working in an air-conditioned room or keeping the recorder interior dry with a large charge of desiccant. It was also found advisable to use no damping on the polargraph; otherwise a t high recorder sensitivities (0.003 pa. per nini.) the recorded diffusion currents became completely steady only after 5 or 6 minutes. During measurements of the residual currents for solutions
. I short study was made of the effect of drop time on the rcproducibility of the diffusion current. The reproducibility appeared to be independent of drop time between 4 and 8 seconds a t low concentrations of silver, but in 10-2 Jf solutions the diffusion current was erratic unless the drop time was greater than 5 qeconds. The concentration of 5 X 10-6 fir appears to be near the practical lower limit of the method using conventional equipment. This correqonds to a diffusion current of 0.020 pa. after correction for a residual current of 0.012 pa. For adequate reproducibility at this level, the composition of the supporting rlectrolyte had to be constant within + l o % and the potential of the dropping electrode. JTithin fl mv. The sensitivity and reproducibility of the method a l e indirated by the calibration data in Table I I t is seen that above 3 x 1 0 6 AI, the deviations from constancy of i d / C are very small, and even a t the lowest calibration concentration the error is only of the order of 5%. By interpolation in the empirical calibration curve it was found possible to determine the concentration of silver in solutions 5 X 10-6 M with an accuracy of +5%.
1 Present address, Provincial Department of Mines, Victoria, B. C . , Canada.
RECEIVED for review August 16,1951. Accepted November 5, 1951. Work supported in part by Atomic Energy Commission.
