V O L U M E 27, NO. 4, A P R I L 1 9 5 5 lived daughters. I n principle, this separation is similar t o the separation of radium and its daughters by electrical migration in a gel described by Veil in 1934 ( I S ) . As shorm by numerous experiments like those illustrated by Figures 2 and 5 , the separations of radium from barium and of barium from strontium were usually incomplete. These separations were most effective a t low concentration and in washed filter paper wherein sorption plays a n effective role. By contrast with chemical methods of separation, barium ions do not serve as a carrier for radium ions or for strontium ions during the differential electrical migration. The ion exchange column separation of radium and barium has recently been improved by the use of suitable complex-forming anions such as citric acid ( 5 , 1 2 ) . B y analogy, the use of eimilar compley-forming substances in the electrolytic solution may facilitate the separation of barium and radium by electrical migration. ACKNOWLEDGMEiVT
The authors are indebted to T. W.Speckman, who determined the y-ray activity of the radium samples.
525 LITERATURE CITED
(1) Dickey, E. E., J . Chem. Educ., 30, 525 (1953).
Frierson, W.J., and Jones, J. W., A N A L . CHEM.,23, 1447 (1951). XIcDonaid, H. J., Lappe, R. J., Narbach, E. P., Spitzer, R. H., and Urbin, 11. C., Clin. Chemist, 5, 35, 51 (1953). (4) McDonald, H. J., JIarbach, E. P., and Urbin, >I. C., I b i d . ,
(2) (3)
5, 17 (1953). (5)
Power, W. H., Kirby, H. W., lIcCluggage, W. C., Nelson, G. D., and Payne, J. H., Jr., Suclear Sci. Abstr., 8 , S o . 3301, 396
(6)
Sato, T. R., Diamond, H., Sorris, W.P., and Strain, H. H.,
(1954).
J . A m . Chem. Sac., 7 4 , 6 1 5 4 (1952). ( i )Sato, T. R., Kisieleski, W.E., Xorris, K. P., and Strain, H. H., 4 s ~CHEM.. ~ . 25, 438 (1953). (8) Sato, T. R., Sorris, X T . P., and Strain, H. H., Ibid., 26, 267 (1954).
Strain, H. H., I b i d . , 23, 25 (1951). Strain, H. H., “Frontiers in Colloid Chemistry,” pp. 29-64, Interscience, Kern York, 1950. (11) Strain, H. H., Sato, T. R., and Engeike, J., AN.4L. CHEM.,26, (9) (10)
90 (1954). (12) (13) (14)
Tompkina, E. R., J . Ana. Chem. Soc., 7 0 , 3520 (1948). Veil, S., Compt. rend., 199, 1044 (1934). Woods, E. F., and Gillespie, J. l I , , Australian J . B i d . Sei., 6.
130 (1953). RECEIVED for reriew August 30, 1954.
Accepted December 8, 1954.
Polarographic Determination of S-(l,2-Dicarbethoxyethyl)0,ODimethy I Dithiophos phate (Malathion) WALTER H. JURA Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.
A polarographic method for the assay of technical grade malathion is described. Unreacted diethyl fumarate is first determined in 0.3N hydrochloric acid30y0 ethyl alcohol solution, in which malathion is polarographically inactive. On treatment with alkali, both unreacted ester and malathion are quantitatively hydrolyzed to fumaric acid which is also determined polarographically after acidification of the solution. The contribution of the unreacted ester to the fumaric acid wave may be calculated and the corrected fumaric acid wave computed to its equivalent malathion. Evidence of film formation at the dropping mercury electrode by malathion, with resulting displacement of the waves of maleic and fumaric acids, their ethyl esters, ox>-gen, and hydrogen peroxide towards more negative potentials, is discussed. The half-wave potential of thallous ion, however, remains unchanged in the presence of malathion.
T
HE discovery that S-( 1,2-dicarbethoxyethyl)-OjO-dimethyl
dithiophosphate, commercially known as malathion, was a potentially valuable insecticide, and the decision to investigate its manufacture and application, necessitated the development of methods of analysis applicable to the technical product and to its determination as residue on plant materials. For residue analysis, .Averell and coviorkers (1) have worked out a colorimetric method which is based upon the alkaline decomposition of the insecticide and reaction of copper(I1) with the 0,O-dimethyl ester of phosphorodithioic acid to form an intense yellow-colored complex in carbon tetrachloride. The colorimetric method has also been extended to the analysis of formulations and the relatively pure product. However, for
the analysis of crude samples, such as n-ere encountered in process development studies, the coloi imetric method was not satisfactory. I n this paper, a polarographic method for the determination of malathion is described. One of its greatest advantages has been its successful application, for more than two years, to a wide variety of samples. An advantage, in so far as technique is concerned, is the fact that the reaction conditions do not have to be as rigorously controlled as in the case of the colorimetric method. This is due to the fact that the polarographic method is based upon the determination of the stable breakdown product-i.e., fumaric acid-and not on the determination of the 0,O-dimethyl ester of phosphorodithioic acid, which is unstahle and undergoes partial decomposition in the alkaline medium uqed to break down the insecticide. 1Ialathion is prepared (2) by the reaction of the 0,O-dimethyl ester of phosphorodithioic acid n ith diethyl maleate (Reaction 1). 0
//
s
HC-‘2-OEt
7
+
(CH~O)?-P-S-H
11
I €IC-c-OEt
--f
\\ 0
s
Ualathion, A
0
0 c HC-C-OEt
Free ester, B
526
ANALYTICAL CHEMISTRY
Diethyl fumarate, resulting from the isomerization of the maleate, may be expected as an impurity in the technical product. This unreacted ester, henceforth referred to as "free ester," may, however, be readily distinguished from malathion because it is polarographically active in acid solution-a medium in which malathion is stable and gives no polarographic waves. Diethyl fumarate is hydrolyzed to disodium fumarate by sodium hydroxide and, under the same conditions, it was found that malathion was rapidly and quantitatively broken down to the sodium salt of the 0,O-dimethyl ester of phosphorodithioic acid and disodium fumarate by a base-catalyzed elimination reaction (Reaction 2).
Table 11. Calibration Data for Free Ester in Malathion Mg. Ester per 50 Ml. of Solution Fa. b per pa. 1 1.95 0.277 2.59 2 t j . 64 0,964 2.6.5 3 5,ll 12.5 1 966 2.60 a ME.of diethyl fumarate per 0,0359 gram of malathion per 50 nil. of 30% ethyl alcohol, 0.3.V hydrochloric acid. b Corrected for ireaidual. Standard
Diethyl Fumarate, 3Ig.n 0.716 2.56
Ester, %
0
/
S A
+ 3NaOH
7
+
(CH80)2-P-SNa
HC-d-O-Na
II
+ Ka-0-C-CH
\\
0
+ 2EtOH + H20
(2)
Utilizing the above chemical properties, the polarographic assay of technical malathion consists of: examining the sample in acid solution (0.3N hydrochloric acid, 30% ethyl alcohol) to determine the free ester, and hydrolyzing both the free ester and malathion with alkali, acidifying the solution, and determining the total fumaric acid. The determination of the free ester makes it possible to subtract its contribution to the fumaric acid wave. The corrected fumaric acid wave thus represents the acid resulting from the hydrolysis of the insecticide and may then be calculated to its equivalent malathion. APPARATUS
A Leeds and Northrup Electrochemograph, Q p e E, was used
throughout this work. Analyses were carried out in a Heyrovskj. cell. Connection to the external saturated calomel electrode (S.C.E.) was made via an agar-potassium chloride bridge. Oxygen was removed from the sample solution with Airco prepurified nitrogen which was saturated with the sample solution by passing it through a reservoir containing a portion of the solution in the Heyrovskj. cell. A Serfass conductivity bridge, Model RC-hI 15, was used to measure the cell resistance of all solutions studied. In every case, the I R drop was negligible.
Table I.
Calibration Data for Hydrolyzed Malathion
Malathion Hydrolyzed, Mn. Malathion per 100 Grama pa. 6 MI. of Solution per pa. 0.01212 1.25 9.74 9.77 0,021 16 2.17 9.79 0.02424 2.48 3.16 0.03096 9.85 3.59 0.03526 9.81 Final volume of solution is 100 ml. as per procedure outlined. b Corrected for ireaidusl.
REAGENTS
Ethyl alcohol, 95%. Hydrochloric acid, 3N. Sodium hydroxide, 0.1N. Diethyl fumarate, Eastman Kodak (white label). Malathion highly purified material obtainable from American Cyanamid do., Stamford, Conn. Purification procedure analogous to that used for parathion (7). Boiling point = 156-7' C. a t 0.7 mm. of mercury with decomposition; refractive index n*: = 1.4985; specific gravity = 1.2284 at 26.5/26.5' C. Complete physical data will be published a t a later date. PROCEDURE
Preparation of Typical Calibration Curve for Malathion. The malathion used in the preparation of the calibration curve was a highly purified sample prepared in these laboratories. It was subsequently shown that Eastman Kodak diethyl fumarate (white label grade) could be used for calibration purposes.
L, L.,u.+d
i .,
-01
-02
-03
-05 - 0 6 -07 Volts vs. S C. E.
-04
-08
-09
-10
-:I
Figure 1. Typical polarograms of malathion sample before and after hydrolysis a. h1,alathion sample hydrolyzed by alkali. Solution: 0.3.V hydrochloric acid, 25% ethyl alcohol, oxygen removed b. Malathion sample containing 1.1% diethyl fumarate. Solution: 0.3.V hydrochloric acid, 30% ethyl alcohol, oxygen reiiioved
Samples of approximately 0.1 to 0.35 gram (weighed to nearest milligram) were weighed, dissolved, and diluted to 250 ml. with 95% ethyl alcohol This gave solutions of malathion ranging from 0.001 to 0.004M. A 25.0-ml. aliquot of a given standard solution was put in a 100-ml. volumetric flask. To this were added 25 ml. of 0.1N sodium hvdroxide. After a hydrolysis time of 3 minutes, the contents were acidified with 10 ml. of 3N hydrochloric acid. The solution was then diluted to 100 ml. with distilled water a t 25' C. .4 portion of this solution was then placed in a Heyrovskj. cell, freed of oxygen, and the dropping mercury electrode polarized from -0.1 volt us. S.C.E. to hydrogen ion discharge. A blank was also run to provide the residual current correction. Table I shows data obtained in a typical ralibration. Preparation of Typical Calibration Curve for Unreacted Diethyl Fumarate. A 1.91-gram sample of highly purified malathion (free of diethyl fumarate) was weighed, transferred to a 200-ml. volumetric flask, and diluted to the mark with 95'% ethyl alcohol (Solution A). Into a 100-ml. volumetric flask, 0.1705 gram of diethyl fumarate was transferred and diluted to the mark with 95% ethyl alcohol (solution B). I n each of four 100-ml. volumetric flasks, 25.0 ml. of Solution A were placed. T o these solutions, 0.0, 2.8, 10.0, and 20.0 ml. of Solution B were added and the solutions diluted to 100 ml. with 95% ethyl alcohol. These solutions were equivalent to approximately 0.25 gram of samples of insecticide containing 0, 2, 6, and 12% free ester. A 15-ml. aliquot of each of these solutions was pipetted into a 50-ml. volumetric flask, 5.0 ml. of 3X hydrochloric acid added, and the solution diluted to the mark a t 25' C. 9portion of the solution was placed in a Heyrovskg cell, freed from oxygen, and the dropping mercury electrode polarized from -0.1 volt us. S.C.E. to hydrogen ion discharge. Table I1 shows data obtained in a typical calibration. Analysis of Technical Grade Malathion. Weigh out 0.2 to 0.25 gram (to the nearest milligram) of sample into a 100-ml. volumetric flask and dilute to the mark with 957, ethyl alcohol. Pipet a 15-ml. aliquot into a 50-ml. volumetric flask, add 5 ml. of 3N hydrochloric acid, and dilute to 50 ml. with water. Deaerate as indicated earlier and olarize from -0.1 volt us. S.C.E. to hydrogen ion discharge. eh!' percentage of diethyl fumarate may be calculated with the aid of the calibration curve.
V O L U M E 27, N O . 4, A P R I L 1 9 5 5
527
Weigh out approximately 0.30 gram (to the nearest milligram) of sample into a 250-ml. volumetric flask and dilute to the mark with 95% ethyl alcohol. Pipet a 25-ml. aliquot of this solution into a 100-ml. volumetric flask and add 25 ml. of 0 . l N sodium hydroxide. After 3 minutes-longer time is harmlessacidify the solution with 10 ml. of 32V hydrochloric acid and dilute to the mark with water. Deaerate a portion of the solution and polarize the dropping mercury electrode from -0.1 volt vs. S.C.E. to hydrogen ion discharge. After subtracting the contribution of the hydrolyzed free ester to the fumaric acid wave, the malathion content may be computed with the aid of calibration data. All waves are corrected for residual currents and typical results obtained on technical grade malathion are tabulated in Table 111. In Figure 1, curve b, is shown a typical polarogram obtained in the determination of the free ester in a technical grade sample. The material responsible for the small wave appearing a t approximately -0.2 volt us. S.C.E. has never been identified. Figure 1, curve a, shows the polarogram obtained after acidification of a hydrolyzed solution of the same sample. DISCLSSION
Early in the polarographic work on this problem, it was noticed that malathion had the capacity of displacing the polarographic waves of many substances towards more negative potentials by as much as 0.1 to 0.3 volt. For example, the waves of maleic acid, fumaric acid, diethyl maleate, and diethyl fumarate (at concentrations of approximately 0.OOOlS.W) were all shifted towards more negative potentials in a medium of 30% ethyl alcohol, 0.3'V hydrochloric acid, containing 0.0022iM malathion (Figure 2). In the presence of malathion, and under the same conditions, the wave representing the electroreduction of oxygen to hydrogen peroxide is shifted to more negative potentials by 0.3 volt; the second wave, representing the reduction of hydrogen peroxide, is
Table 111. Typical Results Obtained by Polarographic Analysis Malathion (Technical Grade)a Malathion, Diethyl Fumarate, % 93.5,90.8 2.46, 2 . 4 2 86 6, 85 9 , 87 1 3.70, 3.66 2.45, 2.38 87.5,88.8,88.Z CS 9 R 91.9, 91.1 2.18, 2 . 2 6 89.4, 8 8 . 7 C S 13R 3.37,3.47 CS 16R 89.6. 89.1 2.70, 2.76 CE-18-493 87.8, 8 8 . 2 4.53,4.57 0 Each analysis represents an analysis of separate sample and not an analysis of separate aliquots of a single master solution of the sample in question.
Table IV. Effect of Malathion on the Electrocapillary Curve of a Solution of 0.3N Hydrochloric Acid, 30% Ethyl Alcohol
Eapplied US.
Volt -0.2 -0.3 -0.4 -0.5 -0.6
-0.7 -0.8 -0.9 -1.0 -1.1
30% EtOH, 0 . 3 NHCl 40.4 41.7 41.9 42.0 41.5 41.2 40.7 40.4 39.8 39.4 38.6
also shifted toward more negative potentials by approximately 0.2 volt and appears to be more reversible. Heyrovskjr ( 5 )also observed that when hydrogen peroxide is reduced in the presence of camphor-a strongly capillary-active material-the wave is shifted to more negative potentials and appears to be more reversible.
66
6.5
I
i
0
0
0
0 0
secords
64t
0 0
0
6;
Sample
cs 1 cs 4 cs 5
S.C.E.,
Seconds per 10 Drops 30% EtOH, 0 . 3 N HC1. 0.00015M 30% EtOH, 0 . 3 N diethyl fumarate, HCl, 0.00015M 0.00227M diethyl fumarate malathion 40.4 39.8 41.5 40.2 41.9 40.2 42.0 40.2 41.8 40.0 41.3 40.0 40.9 40.0 40.3 39.5 39.7 39.4 39.0 39.2 38.9 38.1
0
6 2 1
0 @-30%EtOH, 0 3 N HCI
I
00
I I
-0 2
I
I
-04 Volts
Figure 3.
, -04
I
-05
-06
-07 -38 -0'9 Volts vs S C E
1
-110
I
-11
Figure 2. Displacement of diethyl fumarate wave in presence of malathion a. Oxygen-free solution. 0.00015M diethyl fumarate, 0.3,V hydrochloric acid 30% ethyl alcohol b. Oxygen-frek solution. 0.00015M diethyl fumarate 0.3'2. hydrochloric acid, 30% ethyl alcohol, 0.00227M malathion'
, 227mM
"
1
I
-06 vs S C E
Malathion
,
I
I
-08
I I
I
-I
0
Electrocapillary curves showing suppressive effect of malathion
Heyrovskf and coworkers (6) obtained oscillographic potentialtime curves by polarizing the dropping mercury electrode or a streaming mercury electrode with an alternating current. The resulting patterns showed marked time lags when certain electrolytically inactive substances, such as pyridine in alkaline solutions, are present. These same substances also caused a displacement of polarographic waves towards more negative potentials, and it was concluded by Heyrovskf and coworkers that the phenomenon was due to the formation of a film of nonelectrolyte (pyridine) adsorbed a t the electrode surface The electrode processes of several cations and nitrobenzene were found to be hindered by the adsorbed film and proceeded freely only with a breakdown of the film which occurred a t more negative potentials. An exception to these results was found, by the same workers, in the case of the behavior of thallous ion. It was
528
ANALYTICAL CHEMISTRY
found that the reduction process of the thallous ion was unaffected by the presence of the pyridine film. Identical results were obtained by the present author in the electroreduction of the thallous ion in the presence of malathion. The half-wave potential of the thallous ion wave was found to be the same in the presence and absence of malathion. Capillary activity of malathion is further indicated by its suppressive effect on the electrocapillary curve as indicated by the data in Table IV and by Figure 3.
Table V. Effect of Ethyl Alcohol Content on Half-Wave Potentials of .Rlaleic and Fumaric Acids and Their Ethyl Esters” Half-Wave Potential, Volt us. S.C.E. Diethyl Diethyl % Maleic acid Fumaric acid maleate 6 fumarate -0.598 0 -0.605 -0.594 10 -0.678 -0:767 - 0 621 -0.734 -0.611 -0,833 20 - 0 646 -0.633 -0.770 30 -0.884 - 0 674 -0.654 -0.808 40 -0,930 -0.716 -0.667 -0.834 50 ... ... -0.676 -0.855 ... 60 ... -0.672 -0.868 70 0 Solutions were O.5m.W in electroactive component, 0.1.V hydrochloric acid, temperature was 25’ i. 0.1’ C.. and solutions were free of oxygen. Xitrogen was saturated with portion of sample before it was passed through polarographic cell. b 0.005% sodium methyl red present to suppress maximum. 95% Ethyl Alcohol, Volume
...
...
disulfide are wit,hout effect on either t,he free ester and malathion determinations. It was thought that the wave appearing at -0.2 volt us. S.C.E. in unhydrolvzed solutions (Figure 1, curve b ) was due to the disulfide, but the half-wave potentials were not the same. Khile the effect of malathion on the half-wave potentials of maleic and fumaric acids and their ethyl esters n-as studied, it was observed that a separation of t,he maleic and fumaric acid waves could be effected in acid solutions simply by increasing the alcohol content to approximately 40 to 50%. Table V shows the halfwave potentials obt,ained for 0.0005Jl concentrations of these acids and their ethyl esters for different concentrations of et,hanol in 0 . 1 s hydrochloric acid (ZR drop negligible in all cases). While an explanation for the separation of the acid waves is beyond the scope of the present paper, the data are offered, inasmuch as they may prove useful where it is not feasible t o work in the ammoniaammonium chloride buffer ( p H 8.2) s!-steni which is usually employed for the differentiation of the maleic and fumaric acids ( 3 , 4). ACKh-OWLEDGMENT
The helpful discussions rrith Emil F. Williams of these laboratoriee are gratefully acknowledged. LITERATURE CITED
Averell, P. R., Norris, 31. V.,and Tail, .1.,J . Agr. Food Chem., 2, 570 (1954).
A number of compounds were examined to ascertain their effect on the diethyl fumarate and malathion determinations. No interferences were found. I n the presence of malathion, diethyl maleate is reduced at a more negative potential than diethyl fumarate. If present, it could readily be distinguished from, and determined along with, the diethyl fumarate. Only in very special experiments have both esters been found. The presence of maleic and fumaric acids is not expected in view of the conditions under which the preparation of malathion is carried out. Unreacted 0,O-dimethyl ester of phosphorodithioic acid and the corresponding
Cassaday, J. T., U. S. Patent 2,578,652 (1952). Elving, P. J., Nandel, J., and Warshowsky, B., A N ~ LCHEM., . 19, 161 (1947).
Elring, P. J., and Teitelbaum, C., J . Ani. Chem. Soc., 71, 3916 (1949).
Heyrovsk?, J., “Polarographie,” p .
294, J.
Springer, Vienna,
1941.
Heyrovskl, J., &or,,
F., and Forejt, J., Collection Czechosloa. Chem. Communs., 12, 11 (1947). Williams, E. F., I n d . Eng. Chem.. 43, 950 (1951).
RECEIVEDfor review June 11, 1954. Accepted December 10, 1954. Presented before the Division of Analytical Chemistry a t the 126th Meeting of t h e . 4 Y E R I C A N CHEMICAL SOCIETY, S e w I’ork, September 1934.
Identification of Curing Agents in Rubber Products Ultraviolet Absorptiometric Analysis of Selective Solvent Extracts K. E. KRESS end F. G. STEVENS MEES Firestone Tire and Rubber Co., Akron, Ohio
A method has been devised for identification of organic compounds that accelerate the vulcanization of rubber products. Identification of these active trace materials aids product improvement and quality control. Identification is made through ultraviolet spectrophotometric absorbance curves over the 220 to 380 m,u region on the aqueous alkali or acid extracts, or on liquid-liquid ethyl ether extracts of the aqueous solutions. Thiazole, thiuram, thiocarbamate, amine, and guanidine classes of commercial accelerators are regularly identified in 2 grams of uncured or cured rubber products with less than 4 hours’ elapsed time. Interference of common softeners and antioxidants is usually negligible. The method is more rapid than chromatographicprocedures and more sensitive and specific than spot test methods. Quantitative results may be obtained if calibration work is undertaken.
P
REVIOUS published methods used for accelerator identifica-
tion in compounded rubber products have fallen into four classes: chemical spot tests or color reactions, precipitation and melting point tests, chromatography of colored accelerator products, and spectrophotometric identification of chromatographed fractions. Spot tests or colorimetric methods have been reported for 2mercaptobenzothiazole and other accelerators (1, 3, 13, 18-20). Most of these methods were applied to the acetone extracts of large amounts of the rubber compound, and the formula seldom was as complex in softener and antioxidant content as today’s commercial products. The precipitation of guanidine picrates and their identification by melting point have been well developed ( 8 ,21). Precipitation of thiazoles as the cadmium salts has also been reported (16). It has been established in this laboratory that the ultraviolet spectrophotometric method is a t least 10 times more sensitive
