V O L U M E 28, NO. 6, J U N E 1 9 5 6 ( 2 ) Callear, A. B., Cvetanovic, R. J., Can. J . Chem. 33, 1256 (1955). (3) Cropper, F. R., Heywood, A , , .Tuture 172, 1101 (1953). (4) Ibid., 174, 1063 (1954). (5) Glueckauf, E., Barker, K. H., Kitt. G. P., Discussions Faradall SOC.7, 199 (1949). (6) Griffiths, J. H., James, D. H., Phillips, C . 9. G., Analyst 77, 897
(1952).
( 7 ) Griffiths, J . H., Phillips, C. S. G . , J . C h e m . SOC.1954, 3446. (8) Harvey, D., Chalkley, D. E., Fuel 34, 191 (1955). (9) James, A . T., Biochem. J . 52, 242 (1952). (IO) James, -1.T., Research 8 , 8 (1955). (11) James, A. T., Martin, A . J. P., Analyst 77, 915 (1952). (12) James, A. T., Martin, A. J. P., Biocheni. J . 50, 679 (1951). (13) James, D. H., Phillips, C. S.G., J . Chem. Soc. 1953, 1600. (14) Ibid., 1954, 1066.
983
(15) James, D. H., Phillips, C . S.G., J . Sci. Inst. 29, 362 (1952). (16) Janak, J., Collection Czechosloc. Chem. Conimuns. 18, 798 (1953). (17) Ihid., 19, 684 (1954). (18) Ihid., p. 700. (19) Kremer, E., Aluller, R., Mikrochini. Acta 36, 533 (1951). (20) Lichtenfels, D. H., Fleck, S. d.,Borow, F. H., A x . 4 ~ .CHEX 27, I510 (1955). (21) AIcXesby, J. It., Drew, C . 31., Gordon, -1.S.,J . P h y s . Chern. 59, 988 (1955). (22) Nelkonian, G. A , , Reps, B., 2 . Elektrockem. 58, 616 (1954). (23) Patton, H. W., Lewis, J. S., Kaye, W. I., ANAL.CHEW27, 170 (1955). (24) Phillips, C. S.G., Discussions Faraday S G C .7, 241 (1949). (25) Ray, K.H., J . A p p l . Chem. 4, 21 (1954). RECEIVED for review September H , 19.55. Accepted February 27, 1956.
Polarographic Determination of Methyl Methacrylate Monomer in Polymers RENE J. LACOSTE, ISADORE ROSENTHAL, and CARL H. SCHMITTINGER Rohm
&
Haar Co., Philadelphia, Pa.
A polarographic method is described for the determination of methyl methacrylate monomer in polymers and poly-esters at concentration levels as low as 0.1% relative. The method is based on the reduction of the a$unsaturation of the acrylate in a benzene-ethyl alcoholwater solvent system with a tetraalkylammonium salt aslsupporting electrolyte. Some problems of technique which might deter the application of polarography to this type of problem are discussed. Data are also presented to show the applicability of the procedure to monomer determinations in methacrylate-styrene copolymers.
D
OUBLE bonds which are activated by electronegative groups or conjugation are, in general, polarographically
reducible. T h e potentials a t which reduction takes place vary from about -0.4 volt for compounds such as fumaric acid to about -2.5 volts for butadiene. T h e authors were confronted with the problem of determining residual amounts of acrylatetype monomers in polymers and of analyzing these compounds in the presence of materials t h a t interfere with conventional assay procedures such as bromination. The pyridine sulfate-dibromide method or the mercaptan addition method ( 3 ) can be used to determine residual monomer in terms of total unsaturation. However, these methods are subject t o interferences from some inhibitors, catalysts, and plasticizers, and are not specific for the niononier iinsaturation. A polarographic method which is based on the reduction of the a,p-iinsaturation in methyl methacrylate has been successfully applied to this problem. Amounts of monomer as little as 0.17; of the polymer have been determined with a precision within =t3'C relative. T h e limiting factor in sensitivity is the slight solubility of the polymer. This method is applicable to other acrylate esters and the procedure is suitable for determining other reducible groups in polymers. This paper reports on the procedure and the selection of reagents t h a t can be convenientl?. used in a variety of situations for the analysis of acrylates in the presence of other monomers and polymers, and indicates the solution to some problems of technique Tvhich might deter the :ipplication of polarography to this type of problem. Experiments by the authors have shown that the douhle bond in xci,ylic nrids, unlike the eaters, is not reducible poinrographi-
cally. The explanation for this is similar to that given for chloroacetic acid ( 4 ) . The reduction of the double bond 111 acrylate esters is independent of p H over the range from 6 to 10. This indicates, therefole, that hydiogen ions do not enter into the potential-determining step. Studies of other acid-ester pairs (4)indicate that the ester and flee acid should reduce at about the same potential, a i t h the ester being slightly easier t o reduce. However, the acrylate anion could be expected to reduce as much as 1 volt more negative. Consequently, at p H values low enough for the acrylic acid to be undissociated, the hydrogen wave discharges before the double bond wave ( -2.0 volts); at p H values where the acrylic species is present predominantly as the anion, the double bond viave is shifted to potentials beyond the discharge of the alkyl ammonium salts. T h e net effect is t h a t a wave for free acrylic acid cannot be obtained under an) of the various conditions tried. This can be used to advantage if one wishes to analyze for the ester in the presence of the acid. Then, by running a bromination in addition to the polarographic determination, both components can be determined. Sieman and Shubenko ( 2 ) described a polarographic method which they used for the determination of methacrylic ester in polymerization studies. Their data r e r e obtained in 0 . 1 S lithium chloride and in 0.1S tetramethylammonium iodide with 25% alcohol. Although the3 did not specify the remaining 75yo of the solvent, it is assumed to have been water. These authors reported a half-xive potential of - 1.92 volts us the normal calomel electrode in lithium chloride. Hon ever, this solvent system could not be adapted to a situation in which the polymer is the predominant constituent. Consequently, a benzene-ethgl alcohol system, which ie more amenable for dissolving polymers arid polvesters, \vas investigated and subsequently adopted. T h e method has also been applied to the determination of monomeric material in methacrylate-styrene copolymers and is applicable to the determination of residual monomer in other acrylic-type polymers and copol) mers. Although the problem of quantitatively handling 1,3-butadiene under these conditions has not been specifically woiked out, this compound should also be determinable in copolymers with the acrylates. APPARATUS AYD REAGENTS
A Leeds & S o r t h r u p Model E Electro-Chemograph was used a t a damping of 1; average currents were used throughout The dropping mercury electrode (Corning marine barometer tubing)
984
ANALYTICAL CHEMISTRY
was not the same capillary throughout in this study. Further discussion of the capillary is included below. The polarographic cell ( I ) was maintained a t a temperature of 25.0' & 0.2' C. When filled with the benzene-ethyl alcohol-base electrolyte, the cell resistance was about 5000 ohms. The reference electrode consisted of a conventional calomel half-cell, with 0 . l K tetrabutylammonium chloride (Southwestern Analytical Chemicals, Austin, Tex. ) replacing the usual potassium chloride electrolyte. The nitrogen used for deoxygenating the test solutions was Airco Seaford grade. An all-glass system was used between the nitrogen-solvent equilibrating tube and the cell, in place of the usual plastic tubing, because the benzene-ethyl alcohol solvent tended to leach out reducible materials from the plastic tubing. Occasionally, samples of reagent grade benzene which would not give suitable base solution waves were encountered. It was found t h a t this was due to the presence of volatile reducible materials and could be remedied by degassing the contaminated benzene with nitrogen before using it as a solvent or in the nitrogen-equilibrating tube. The mercury was purified in the usual way.
I
t IUA.(CURVES A 8 C )
I-
z W
K K
and the monomer content is calculated from the i d / C value obtained for the standard. Curves B and C in Figure 1 represent two runs on the same test solution of methyl methacrylate (2.30 X 10--4LTl)in the presence of 0.45570 polymer a t different current sensitivity settings on the Electro-Chemograph. Curve A is a typical base solution wave without monomer or polymer. These curves illustrate a phenomenon which, a t first sight, might appear to negate the reliability of results but which can be compensated for in calculations. The phenomenon referred to is the rather sharp change in capillary characteristics \Thich occurs a t -2.35 volts a t 0.5 pa. per inch sensitivity, and a t -2.50 volts a t 1.0 pa. per inch sensitivity. The apparent potential a t v-hich this change occ~irsseems to be a function of the current sensitivity setting. The curves which are obtained with a particular capillary and test solution are reproducible, regardless of the sensitivity setting a t which previous polarograms were recorded. The phenomenon might be attributable to a lag in balancing the current-measuring circuit and can be minimized by decreasing the sensitivity and decreasing the damping to zero. S o t all capillaries shon this effect, and, in any case, accounting for the change in drop time eliminates any discrepancies betaeen sections of the cuIve. The influence of such changes in capillary characteristics on id/C values is shown in column 4 of Table I. This column lists z d / C values a t varioiis concentration levele of
3 0
-1.8
-2.0 Figure 1.
-2.2 -2.4 POTENT 1 A L I V O L T S )
-26
-2.8
Typical polarograms of methyl methacrylate
A . 0.1M methyltri(n-buty1)ammonium chloride in benseneethyl alcohol-water, 5 to 5 to 1 by volume E , C. Same as A , with methyl methacrylate: 2.30 X 10-4M, monomer&4.55 X 10-0 gram per ml., polymer
The methyl methacrylate monomer was a redistilled laboratory preparation (Rohm & Haas Co.), while the styrene was plantgrade material of better than 99y0 purity. The supporting electrolyte stock solution was made by adding concentrated hydrochloric acid (Baker and Adamson, c.P.) dropwise, to a 1.OJI aqueous solution of methvltri(n-buty1)ammonium hydroxide (Southwestern Analytical Chemicals, Austin, Tex.) until the solution indicated pH 8 with Hydrion paper (Micro Essential Laboratory, Brooklyn, X. Y.). Reagent grade benzene (Mallinckrodt) was used as solvent with anhydrous ethyl alcohol (Formula ZB,Publicker Industries, Inc., Philadelphia, Pa.).
Figure 2. Concentration function of diffusion current for methyl methacrylate o x
Absence of pol>-mer Presence of polymer
PROCEDURE
This procedure is intended for those polkmers which are sufficiently soluble in a benzene-ethyl alcohol-water system which is approximately 15, 45, and lOyc,respectively. Experiments have shown, however, that as much as 6570 benzene may be used in those instances when the solubility of the polymer so requires. The sample is prepared by dissolving 1 gram of polymer in benzene and diluting the solution to 100 mi. Depending on the nature of the polymer, solution map require 1 day to 1 week. A blank stock solution should be prepared by dissolving, in 100 ml. of benzene, all the components other than methacrylate monomer and polymer which are contained in a 1-gram sample of polymer. The aliquots of stock solutions and diluents which are pipetted into the electrolysis cell are taken so that the final ratio of solvents is 5 parts of benzene by volume, 5 parts of ethyl alcohol, and 1 part of aqueous stock solution of supporting electrolyte. Sufficient polymer stock solution is taken so that the final concentration of monomer is about 2.5 X 10-4M. If turbidity develops in the electrolysis cell upon adding the ethyl alcohol, it is usually eliminated after degassing for 5 minutes with nitrogen. If turbidity persists, a higher benzene-ethyl alcohol ratio should be tried, Polarograms are obtained for the polymer blank solution and standard monomer. The sample curve is corrected for the blank
0 -k8 Figure 3.
I
-20
I
I
I
-2.4. -2.6 POTENTIAL IVOLTSI -2.2
I
-28
Typical polarogram of methyl methacrylate and styrene
1.77 X 10-4 M styrene, 1.83 X 10-4 M methyl methacrylate: 7.99 X 10-4 gram per ml. of copolymer in benzene-ethyl alcoholwater, 5 to 5 to 1 by volume; 0.LW methyltri(n-butyl)ammonium chloride
V O L U M E 28, N O . 6, J U N E 1 9 5 6 Table I. Monomer Concn.,
M X 10-4 2.29 2.29 3.99 3.99 3.99 6.89 6.89 6.89 9.18
985
Effect of Instrument Sensitivity on id/C
Sensitivity, #a./Inch 0.5 1.0 0.5 1.0
2,0 0.5
1 .o
2.0 2.0
Drop Time, Seconds 1.30 1.43 1.36 1.50 2.30 1.36 1.50 2.50 2.50
id/C,
id/C, Normalized #&./Mole t o t = 1.30 0.585 X 104 0.585 X 104
0,604 0.608 0.614 0.692 0.584 0.620 0.680 0.668
0.593 0.602 0.599 0.629 0.579 0.605 0.610 0.599
Ella -2.05 -2.07 -2.06 -2.07 -2.07 -2.07 -2.08 -2.09 -2.09
methacrylate under the conditions iiidicated in columns 2 and 3. As is expected, the i d / C values are somewhat higher a t the longer drop times. In column 5 these same i d / c values have been normalized to a drop time of 1.30 seconds by means of the relation
Sensitivity variations indicated a t any one concentration represent the only change which xvas made in the experiment between polarogranis on the test solution. T h e half-wave potentials ivliich are listed in the last column have not been corrected for I K drop. Whenever a sharp capillary characteristic change occurred before the attainment of a measurable diffusion plateau, the height of the wave \vas obtained by extrapolation of that portion before the capillarj- change. From this, the half-wave potential was measured. Figure 2 demonstrates the linear relationship between the diffusion currrnt and concentration of ester in both the presence and the absence of polymer. Comparison of the data obtained in the presence of polymer with that obtained in its absence s h o w that t,he polymer does not affect the id/C value. The selection of p H 8 for the solvent medium was based on preliminary experiments which demonstrated t h a t this was the condition under which the best base solution FT-ave could be obtained. I n addition, hydrolysis effects are negligible at this pH but become appreciable at p H 10. The effect of temperature on the diffusion current was found to be about IY0 per degree Centigrade. This is based on one experiment where the temperature differential was 12.5” C. T h e reduction current was found t o be proportional t o h”2, indicating that the electrode process is diffusion controlled. 3 CAPILLARY
Some difficulty was experienced TTith a tendency for the capillary to become erratic after extended use in this almost non-
aqueous system. This is probably attributable t o polymer deposition in the tip. It may be retarded t o some extent by never allowing the tip to dry with polymer solution adhering to it. T h e tip should be immersed in clean benzene whenever it is removed from the cell. Cleaning the tip with alcohol immediately on removal from a polymer solution is not atlvisable because the polymer is less soluble in this solvent than in benzene. Because of this difficulty, which necessitated the use of more than one capillary in this study: no effort was made to determine characteristics for every capillary. Of course, the capillary v a s never changed in the midst of a series of interdependent experiments. For example, all the data in Table I were obtained with the same capillary. T o furnish other investigators n-ith a basis for comparison of results, the characteristics of one capillary and also the i d / C value for methyl methacrylate in the absence of polymer with this capillary w r e determined. -4 drop time of t = 6.36 seconds and a value of m.= 1.62 mg. per second were obtained in distilled water, oprn circuit, at 25.0” i.0 . 2 ” C., with a mercury head of 45 em. T h e i(1,’C value (0.680 X lo4 pa. per mole) a-as determined at - 2 . W volts for a solution 2.30 X 10-4 M in methyl methacrylate a t a sensitivity of 0.5 pa. per inch. T h e drop time a t this potential was 1.25 seconds. The half-wave potential n-as measured a t -2.08 volts. STYREYE
Figure 3 is a typical polarograin of styrene and meth? 1 methacrylate a t equal concentration levels in the benzene-ethyl alcohol solvent and in the presence of the methacrylate styrene copolymer. T h e first wave, with a half-wave potential a t about -2.06 volts, is due t o the methacrylate. T h e second wave a t about -2.42 volts is the styrene wave. T h e i d / C value for styrene is independent of concentration and can be measured a t -2.6 volts. T h e methacrylate value ie also constant and can be measured a t -2.25 volts. Both of these constants do not change if they are measured in the ahsenre of the methac:ylatestyrene copolymer. LITERATURE CITED
(1) Komyathy, J. C., Malloy, F., Elving. P. J., dsat. CHEM.24,431
(1952). (2) Nieman, M. B., Shubenko, PI. -I., Zarodakaya Lab. 14, 394-6 (1948). (3) Riddle, E.H., “Monomeric Acrylic Esters,” Reinhold. Sen- York, 1954.
(4) Rosenthal, I., Tang, S. C., Elving, P.J., J . Am. Chenz. SOC.74, 6112 (1952). RECEIVED for review October 8, 1955. .4ccepted March 6, 1956. Delaware Valley Regional Meeting, ACS, Philadelphia, P a . , February 1956.
Polarograph with Electronically Controlled Cell Voltage ROBERT L. PECSOK and ROSS W. F A R M E R 1 D e p a r t m e n t o f Chemistry, University of California, Los Angeler
The accurate determination of half-wave potentials from recorded polarograms is improved with a new type of cell voltage control circuit. The applied voltage is continuously balanced against a reference voltage, which in turn is continuously compared to a standard cell. 411 adjustments are made automatically. The performance of the instrument indicates an accuracy within i.2 mv. 1 Present address, Atomic Energy Project. University of California, Los Angeles. Calif.
24, Calif.
T
HE accurate determination of half-wave potentials is essential for many applications of the polarograph. For
manual polarographs, the resistance-potentiometer technique ( 3 ) or the refined increment technique ( 5 ) provides the highest useful accuracy. Seither of these methods is easily adaptable to recording polarographs. There are two recording polarographs in common use in this country. T h e Sargent Polarograph, Xodel XXI, is supplied with lY0 voltmeters to adjust the span and initial voltages. Apparently with this instrument, potentials can be read with an accuracy within 4 ~ 3 0mv. Furthermore, the slide-wire used as a
