553
V O L U M E 2 5 , NO. 4, A P R I L 1 9 5 3 positions of the phases and hence the distribution of the s d u t e molecules change a t every stage of its forward movement. Each movement may be restricted to one theoretical plate within the small thickness of which the distribution may be supposed to remain practically constant,. Taken as a whole, two types of distribution of the solute molecules come into play: (1) distribution between the second and the third phases, which is a case of pure partition; and ( 2 ) distribution between the first and the second phases which may he looked upon as an adsorption process. I n general, the former type of distribution is a primary process and is actually responsible for the separation of the constituents of a mixture on a chromatogram, whereas the latter type of distribution is of secondary importance, but i t should not be completely overlooked. I n certain cases ( 4 ) , however, this adsorption may play the primary role. ACKNOWLEDGMENT
The author owes much gratitude to his director D . 11.Bose, for his kind interest and continued encouragement during the progress of the work. The author's sincerest t,hanks are a153 due to S. K. hIukherjee of the University College of Scienre and Technology, Calcutta, for valuable suggestions and active criticism. LITERiTURE CITED
(1) Bate-Smith, E. C., and Westall, R. G., Biochim. et Biophys.
Acta, 4, 427 (1950). (2) Benson, A. -4., Bassham, J. .1.,Calvin, M., Goodale, T. C., Haas, V. -4., and Stepka, If-.,J . -41n. Chem. SOC..72, 1710 (1950).
Bentley, H. R., and Whitehead, J. K., Biochem. J . , 46, 341 (1950). Burma, D. P., Analyst, 77, 382 (1952). Burma, D. P., J . Ind. Chem. Soc., 28, 555 (1951). Ihid., p. 631. Burma, D. P., .Vature, 168, 565 (1951). Burma, D. P., Proc. Indian. Sci. Congr., 39th Congr., Part 111, 64 (1952). Burma, D. P., and Banerjee, B., J . I n d . Chem. SOC.,28, 133 (1951). Burma, D. P.. and Banerjee, B., Science and Cidture (India), 15, 363 (1950). Burstall, F. H., Davies, G. R., and Wells, R. A , Discussions Faraday Soc.. No. 7. 179 (1949.) Chakrabarty, S., and Burma, D. P., Science and Culture (India), 16, 485 (1951). Cohn, E. J., and Edsall. J. T., "Proteins, Amino Acids and Peptides," Sew York, Reinhold Publishing Corp., 1943. Collander, R., Acta Chem. Scund., 4, 1085 (1950). Consden, R., Gordon, H., and Martin, -4.J. P., Biochem. J . , 38 224 (1944). , and Cohn, E. J.. J . A m . Ckem. Soc., 57, 634 (1935). Hanes, C. S., and Isherwood, E. -4.. .Vatwe, 164, 1107 (1949). Jones, S. G. T., Discussions Faraday Soc., X o . 7, 285 (1949). Kowkabany, G. S . ,and Cassidy, H. G., ANAL.CHExr., 24, 643 (1952). Martin, -4,J. P., Ann. Rm. Biochem., 19, 517 (1950). Ott, E., "Cellulose and Cellulose Derivatives," Kew Tork, Interscience Publishers, Inc., 1943. Smith, E. L., Discussions Faraday Sac., T o . 7, 317 (1949). Synge, R. L. AI,, Biochem. Soc. Symposia Cambridge, Engl., S o . 3 , 90 (1950). Tauber, H., J . B i d . Chem., 113, 754 (1936). R E C E I V Eriiigiiit D 13. 1952. Accepted J a n u a r y 26, 1953.
Polarographic Determination of Phthalate Esters in Plastics GERALD C. WHITKACK AND E. ST. CL4IR GANTZ Analytical C h e m i s t r y Branch, Z-. S . Naval Ordnance Test Station, Inyokern, China Lake, Calif.
I
X C O S N E C T I O S with studies of various plastics, it was observed that phthalate esters give a characteristic rurrentvoltage curve a t the dropping mercury electrode (12). Furman and Bricker ( 6 ) made a polarographic study of o-phthalic arid and phthalates, but reported no data for the esters of phthalic acid. Although a number of methods of analysis for phthalate esters are known (1-3, 5 , 8-16), none appeared applicable to the rapid analysis of small samples of plastics. Since the polarographic method of analysis is known to he rapid, precise, and convenient for the determination of reducible material in small samples, conditions were investigated and a procedure was developed to the point where it gave satisfactory results. This paper presents and describes in detail a polarographic method of analysis for phthalate esters in plastics. APPARATUS 4 N D MATERIALS
Apparatus. A Sargent Model X X I recording polarograph was used in all analyses. A Fisher Elecdropode was used to obtain values for half-wave potentials (E1/*) versus the saturated calomel electrode (SCE). A Sargent Model I11 manual polarogr aph was found t o be applicable in pilot plant control work. Small borosilicate glass beakers (30 ml.) were used as polarographic cells. A rubber stopper to which was attached the dropping electrode, contact electrode, and a glass tube with a fritted disk, was placed over the top of the small beakers. The currentvoltage curves were obtained in a constant temperature bath a t 30" i 0.2"C.
The apparent pH values of the solutions \\ere obtained with a Beckman Model G pH meter. Materials. The supporting electrolyte used in the polarographic procedure was 0.1 tetramethyl ammonium chloride, containing 10 ml. of a 0.1% alcoholic solution of methyl red per 500 ml. of solution. Eastman Kodak (practical grade) tetramethyl ammonium chloride was used in the preparation of the supporting electrolyte. The salt was not purihed further. The acetone used was purchased from Shell Chemical Co. under the name of synthetic acetone and 95% ethyl alcohol was obtained from General Chemical Co., S e w Tork, S. T. Redistilled mercury, C.P. grade, was used as the anode in all work. The capillary used had the following characteristics: a t a pressure of 92.7 cm. of mercury, the drop time t , on open circuit in 0.1 ill tetramethyl ammonium chloride solution ( i s % ethyl alcohol), n a s 9.0 seconds per drop; the ll-eight n z of me.cury falling per second n as 0.621 mg. and m 2 j 3 t 1 i 6 (6) was therefore 1.048. Eastman Kodak cellulose acetate powdei (E-13), 39.0% acetyl, and viscosity ( h ) 55 to 88 cps., was used in preparing the standard graph for the phthalate esters in cellulose acetate. Ethyl cellulose, DOKChemical Co., Order S o . 88247 11,standard ethoxy and viscosity ( h ) 50 cps., was used in preparing the standard graph for the phthalate esters in ethyl cellulose. The phthalate esters investigated were not repurified. Boiling points melting points, and refractive indices indicated a grade of pui& sufficient for this work. The esters were as follows: Diethyl phthalate, Eastman Kodak Co., n2,0 = 1.5006 and boiling point = 295' C. Dimethyl phthalate, Eastman Kodak Co., ngo = 1.5148 and boiling point = 282' C. Methyl phthalyl ethyl glycolate (Santicizer L1-17), Nonsanto Chemical Co., n:? = 1.5058.
554
ANALYTICAL CHEMISTRY
In connection with studies of various plastic materials it was observed that phthalate esters give reproducible current-voltage curves at the dropping mercury electrode. A polarographic method of analysis was developed for the determination of these esters in plastics. This paper presents and describes the method. Dimethyl, diethyl, dibutyl, diphenyl, and dioctyl phthalates, and methyl phthalyl ethyl glycolate (Santicizer M-17) were investigated. Data indicated that these esters can be determined in plastic with a precision of about 15 parts per thousand. Diffusion currents, diffusion-current constants (id/Crn2/3t1/6), and half-wave potentials ( E l / * ) referred to a standard calomel electrode are reportedfor the esters in 75940 ethyl alcohol and/or acetone solutions containing tetramethyl ammonium chloride as the supporting electrolyte. The method is rapid, convenient, and precise and should find wide application in determining the amount of phthalate ester present in a plastic, explosive, resin, or other material.
Dibutyl phthalate, Fisher Scientific Co., nao = 1.4933. Diphenyl phthalate, Monsanto Chemical Co., melting point = 73.5 to 74.0 C. Dioctyl phthalate, Ohio -4pex Co., n2,0 = 1.4871. PROCEDURE
Preparation of Sample. The plastic is first put through a Wiley Mill using a 20-mesh screen. A 50- to 100-mg. sample is then placed in a 25-mI. volumetric flask. Cellulose acetate plastic is dissolved in acetone and made up to volume. Ethylcellulose plastic is dissolved in warm 95% ethyl alcohol, cooled to room temperature, and made up to volunie in a 25-ml. volumetric flask. Polarographic Analysis of Ester. .4n aliquot of the ethyl alcohol or acetone solution of the plastic is added to the polarographic cell (30 ml. beaker). The final volume of solution in the cell is 20 ml. When analyzing cellulose acetate samples, the total volume of acetone in the cell is 10 ml. Thus, if a 5-ml. aliquot of the acetone solution of the plastic is used, 5 ml. of acetone is added to the cell. The remaining 10 ml. of solution is made up of 5 ml. of 95% ethyl alcohol and 5 ml. of the 0.1 M tetramethyl ammonium chloride solution (supporting electrolyte). When ethylcellulose samples are analyzed, the total volume of ethyl alcohol in the cell is 15 ml. Thus, if a 5-ml. aliquot of the ethyl alcohol solution of plastic is used, 10 ml. of ethyl alcohol is added t o the cell. The remaining 5 nil. is 0.1 M tetramethyl ammonium chloride. The apparent p H of this solution is about 5.0. The total volume of 20 ml. in the polarographic cell is stirred well. a small amount of C.P. mercury is added, and the cell is then placed under the dropping mercury electrode in the constant temperature bath. The stopper containing the dropping mercury electrode assembly is put in place and the glass tube for introduction of nitrogen is lowered into the solution. After nitrogen is flushed through the solution for 5 minutes, the glass tube is raised and nitrogen is allowed to flow over the solution while the current-voltage curve for the phthalate ester is obtained. Current-voltage curves are obtained in duplicate from - 1.50 to about -2.25 volts. The average wave height in millimeters is referred to a standard graph for the ester. Preparation of Standard Graph. An amount of the plastic equivalent to that expected in unknown samples is dissolved in either acetone or ethyl alcohol. Cellulose acetate is dissolved in acetone and ethylcellulose is dissolved in ethyl alcohol. The phthalate ester is then added to the solution so that the concentration of ester will be in the range expected in unknown samples. For example, with cellulose acetate containing about 8 t o 10% by weight of diethyl phthalate, a convenient standard graph can be prepared by dissolving 100 mg. of cellulose acetate (same viscosity and acetyl content as in samples to be analyzed) in acetone, adding about 10 mg. of diethyl phthalate, and then diluting to volume with acetone in a 25-rn1. volumetric flask. Aliquots of this stock solution are added t o the supporting electrolyte and 95% ethyl alcohol so that 20 ml. is the final volume of solution in the polarographic cell. The total amount of acetone is 10 ml.,
the total amount of 95% ethyl alcohol is 5 ml., and the total amount of supporting electrolyte is 5 ml. for each point determined in a standard graph. The data should produce a linear graph. Calculation. The per cent of phthalate ester is calculated from the value obtained by referring the current-voltage wave height in milliliters to a standard graph (plot of wave height us. concentration in milligrams). Thus, Per cent phthalate ester = mg. (per total volume of solution) wt. of sample
x 100
DATA
In establishing the precision of the polarographic procedure for phthalate esters in plastic, samples of cellulose acetate (Eastman Kodak E-13) and ethylcellulose (Dow Order KO.88247 AI) nere dissolved in acetone and warm ethyl alcohol, respectively. A knov n amount of a phthalate ester was added to these solutions. The results in Table I indicate that the analysis for an ester in cellulose acetate has a precision, shown by a standard deviation obtained from five values, of about 12 parts per thousand and that the analysis for an ester in ethylcellulose has a precision, shown by a standard deviation obtained from five values, of about 15 parts per thousand. Several samples of cellulose acetate plastic were analyzed for phthalate esters by the method described. The results in Table I1 indicate the analysis has a precision, shown by a standard deviation obtained from seven values, of about 12 parts per thousand. Comparable data were obtained for the analysis of phthalate esters in ethylcellulose. On the basis of these results, a confidence range of 99.9 =t1.10% may be expected for total phthalate ester recovery on samples of plastic containing up to 30% by weight of phthalate ester.
Table 1.
Recovery of Diethyl Phthalate in Cellulose Acetate and Ethylcellulose
Diethyl Phthalate Present, Mg.
Diethyl Phthalate Found,
%
7.6 5.0 8.5 2.5 7.7 2.0 7.3 7.6
101.3 100.0 100.0 100.0 102.7 100.0 97.3 101.3 PA 7 . 98.7
7 4
5.g
6.0b a
b
50 mg. of cellulose acetate dissolved in acetone, 50 mg. of ethylcellulose dissolved in alcohol.
Cellulose acetate Average 5 = 100.8% Standard deviation, 8 1.21Yo
Ethylcellulose Average f = 99.2% Standarh deviation, s = 1.51%
-
Table 11.
Recovery of Phthalate Esters in Cellulose Acetate Plastic
Phthalate Ester Presenta,
Phthalate Ester Found,
Phthalate Ester Recovery,
?4.5b 16.0g 20.7 23.10 21.O d 28.64 23.2
24.3 16.1 20.9 23.1 21.3 28.1 23.0
99.0 100.6 101.1 100.0 101.4 98.2 99.1
%
Q
b C
d 0
Diethyl Phthalate Recovery,
11g.
%
%
Determined b gravimetric method ( I d ) . Dimethyl phtxalate. Diethyl phthalate. Methyl phthalyl ethyl glycolate. Dibutyl phthalate.
-
Average f = 99.9% Standard deviation. 8 1.19% Standard deviation of meam (of 7), sm = 0.45% Confidence range (fiducial limits), d f tam = 99.9% =k 1.10
555
V O L U M E 25, NO. 4, A P R I L 1 9 5 3 Table 111. Reduction of Phthalate Esters in 0.1 M Tetramethyl Ammonium Chloride Solution (75% Ethyl Alcohol) > ~ ~ Ei/r i (Referred ~ ~ ~t o SCE) l ~ ~ idha.) id/Cma/atl/S Phthalate Estera Weight 1st wave 2nd wave 1st wave 2nd wave 1st wave 2nd wave 4.78 2.89 3.03 -2.17 5.01 -1.83 Dimethyl phthalate 194 4.64 3.26 3.42 4.86 -2.17 -1.87 222 Diethyl phthalate 4.06 2.23 2.34 4.26 -2.17 -1.89 278 Dibutyl phthalate 4.47 3.19 3.34 4.69 -2.08 -1.65 318 Diphenyl phthalate 2.04 3.45 2.14 3.62 -2.18 390 -1.93 Diocty! phthalate 2.68 3.75 2.81 -2.15 3.93 -1.79 266 Methyl phthalyl ethyl glycolate =All esters were 1.0 mMsolutions. ~ ~ Cagillarl- ixharacteristics: head, 92.7 CLII.;m = 0.621 nig.,'sec.; t = 9 seconds/drop: m / ~ l =' 1.048.
__-
Table 1V. Effect of Cellulose Acetate and Ethylcellulose on Diffusion Current of a Phthalate Ester Diethyl Phthalate, hl g
idra.)
Ei/z, Observed
2.98 2.98 2.77 2.77
5.91 5.84 4.19 4 32
-1.91 -1.91 -1.90 -1.94
Cellulose Acetate, 3Ig.
2.25 53,OO e . . .
...
EthjI Cellulose, h l R.
...
...
1.16 40.00
Table I11 gives the E , 2 values referred to the saturated calomel electrode (SCE), diffusion current values, and the diffusion C U I rent constants for several phthalate esters. Table 11-shows the effect of cellulose acetate and ethylcellulose on the diffusion current of a phthalate ester. The data indicate practically no change in the El/*values of the esters with increased plastic concentration and a small change in the diffusion current with greatly increased plastic concentration. The indexes of precision used in the statistical analysis of the data presented in Tables I and I1 are as follows: Standard deviation (estimate) s = 4 2 (z - &)*/(n- 1) Standard deviation of mean of n, srn= s / 4 ; Confidence range (fiducial limits) = 2 i. ts, I n the above 3: = mean of n observations of x, t = Student's t ( 4 ) for the significance level desired and 71 - 1 degrees of freedom.
Figure 1. Current-Voltage Curves of Two Phthalate
For the 1 in 20 significance level and means of 7 (6 degrees of freedom) t = 2.45. DISCUSSION
Since the reduction of the phthalate esters takes place a t high negative potentials, it was found necessary to use a supporting electrolyte t h a t would allow the formation of a well-defined current-voltage cui've for the ester and have as small a residual current as possible. The quat,ernary ammonium salts are excellent supporting electrolytes a t high negative potentials ( 7 ) . The particular tetramethyl ammonium chloride used in this investigation did not have to be purified further; however, if interference from the tetramethyl ammonium chloride is observed, the salt should be recrystallized several times from ethyl alcohol-water solution before using ( 7 ) . Although no difficulty was experienced with either the 95% ethyl alcohol or acetone used in this work, it may be necessary to purify these solvents before t,hey are used. During the analysis of cellulose acetate samples a scrubber containing 50% acetone, 25% ethyl alcohol, and 25y0 water should be placed between the polarographic cell and the nitrogen source to avoid loss of solvent and Concentration of the ester by evaporation during the analysis. For ethylcellulose samples, the scrubber should contain a 757, ethyl alcohol-25% xyster solution. It was found that a capillary Tvith a drop rate of 6 to 9 seconds per drop could be used indefinitely without contamiliation if certain precautions were taken. The capillary was rinsed with 95% ethyl alcohol or acetone, then xith distilled water, and immersed into clean mercury before stopping the flow of mercury through the orifice. Before analyziiig the sample, the capillary was opened while in the mercury and then allowed to drop mercury for several minutes in the supporting electrolyte used for the analysis. The phthalate esters give two well-defined current-voltage curves in the supporting electrolyte used for this work. Although both waves appear to be well defined, the first wave was chosen for quantitative study because of the lower reduction potential and the ease of measurement. Figure 1 illustrates the method of measurement used in this work and compares the current-voltage curves of two of the phthalate esters investigated. It was found that if the currenbvoltage curve n a s measured in the manner illustrated, no correction for the residual current was necessary and currentvoltage curves were reproducible within i- 1.0%. The wave height appears to be a function of the molecular weight of the ester. It can be suggested that the reduction of the ester takes place i n the carboxyl portion of the molecule. It is recognized that the El/* values for the esters were obtained in a nonbuffered solution and therefore may not have exact significance; however, the data are reported for a relative comparison. Further work on the polarographic behavior of phthalate esters is in progress and such fundamental knowledge as diffusion coefficients, mechanisms of reduction, and the effect of p H on diffusion currents and El:* values of the esters will be published. The work presented in this paper should find wide application where a convenient, rapid, and precise method is desired for the determination of Esters
556
ANALYTICAL CHEMISTRY
phthalate esters. It should be of special interest to those concerned with the analysis of the phthalate esters in plastics, explosives, resins, and other materials. ACKSOWLEDGMENT
This paper is published x i t h the permission of F. TV. Brown, technical director of the C . S. Saval Ordnance Test Station.
Fisher, R. A,, “Statistical Nethods for Research Tvorkers,” 10th ed., Edinburgh, Oliver and Boyd, 1946. Furman, S. H., and Bricker, C . E., J . Am. Chem. Soc., 64,6608 (1942). Kolthoff, I. A I . , and Lingane, J. J., “Polarography,” rev. ed., pp. 60-4, New York, Interscience Publishers, 1946. Ibid.,p. 300. Rvan. J. D.. and Watkins. G. B.. ISD. EXG.CHEM.AXAL.ED.. 5,191-2 (1933).
Shreve, 0. D., and Heether, ;\I R., -4h-i~. CHEM.,23, 441-5 (1951).
LITERATURE CITED
(1) Barnes, R. B., Liddel, Urner, and Williams, V. Z., IND.ENG. CHEM.,ANAL.ED.,15,691-2 (1943). (2) Biggs, B. S., and Erickson, R. H., I b i d . , 16, 93-4 (1944). (3) Butts, P. G., Prine, G. B., Kouba, D. L., and Becker, W.W., Ibid., 20, 1066-7 (1948).
Swann, M. H., Ibid., 21,1448 (1949). Thames, F. C., IND.ENG.CHEM.,ANAL.ED.,8, 418-19 (1936). Whitnack, G. C., and Gante, E. St. Clair, . 2 ~ . 4 ~CHEM., . 24, 1060-1 (1952).
RECEIVED for review October 14,1962. Accepted January 1 6 , 1 9 5 3
Polarographic Studies of Sulfur Compounds in Petroleum Fractions MAYNARD E. HALL’ Humble Oil & Refining Co., Baytown, Tex. Because the effect of sulfur compounds on refinery process and product quality varies widely with the type of sulfur compound, it has become important to have analytical methods to detect and determine the sulfur compound types most common to the petroleum industry. Elemental sulfur, aliphatic and aromatic disulfides, tert-butyl trisulfide, and tert-butyl polysulfide give characteristic reduction waves with the dropping mercury electrode. Analytical methods, including electrolytes, solvents, and interferences, are discussed for each type when
I
S T H E petroleum industry it has become necessary to resort to
the processing of high sulfur content crudes in order tomeet the increased world denland for petroleum products. The processing of such crudes results in increased refinery costs and tends t o lower product quality. The effect of sulfur compounds on refinery process and product quality varies widely with the type of sulfur compound, and for this reason it has become important to have analytical methods that can detect and determine the various sulfur compounds. The polarograph is an analytical tool that has shown promise as a means of analyzing for elemental sulfur (g, S, 5 ) and organic disulfides (1, 2, 4)in both biological materials and petroleum products. I n order to develop polarographic analytical methods for sulfur compounds, a background of knowledge of the behavior of sulfur compounds a t the dropping mercury electrode is necessary. This report deals mainly with polarographic studies of various sulfur compounds in organic solvents for the purpose of developing future analytical procedures for these compounds. However, examples are given of some applications that have been made of the polarographic technique to the analysis of elemental sulfur in crude oils and aliphatic disulfides in motor gasoline fractions. APPARATUS AND REAGENTS
A Sargent Model XXI recording polarograph mas used in all experiments. An H-type electrolysis cell was employed and a saturated calomel electrode was used as the reference electrode. A mercury pool anode was not used in any experiment because of the tendency of the mercury to react with sulfur. Also, mercaptans (thiols) change the potential of the mercury pool 1
Present address, The Chemstrand Corp., Decatur, Ala
they exist in various mixtures. Using an electrolyte solvent consisting of 40% isopropyl alcohol, 40% methanol, 20%0 water, and 0.025 M tetra-n-butyl ammonium hydroxide, tert-butyl polysulfide gives three waves, one of which is unaccounted for at present. In this same medium aliphatic sulfides and diphenyl sulfide were not found reducible up to -2.6 volts. The most useful application of the polarograph has been the study of sweetening processes, where mercaptans are converted to disulfides and other types of sulfur compounds. anode considerably. 911 experimental measurements were made in an air-conditioned room with the temperature held constant a t 24.5” zt 0.5’ C. The Beckman automatic Titrimeter.was used as a pH meter to measure the degree of acidity or basicity of the electrolyte solutions used and the results are expressed as p H values even though they are not actual pH values as defined for aqueous solutions. Baker’s C.P. methanol, benzene, glacial acetic acid, and sodium acetate trihydrate were used as solvents and buffer for the studies on elemental sulfur. Weighed amounts of powdered monoclinic sulfur, of better than 99% purity, dissolved in n-heptane, were used for standard solutions. The electrolyte solvent for elemental sulfur is prepared by mixing equal volumes of methanol and benzene and making the solution 0.1 M in both sodium acetate and acetic acid. This gives a buffered solution with a pH of 6.3. The electrolyte solvent used for disulfides and polysulfides is prepared by mixing 400 ml. of methanol, 400 ml. of isopropyl alcohol, and 200 ml. of distilled water. This solution is then made 0.025 JI in tetra-n-butyl ammonium hydroxide. The final electrolyte solvent has a p H reading of 12.3. Tetra-n-butyl ammonium hydroxide as 1 M aqueous solutions was purchased from Southwestern Analytical Chemicals, Austin, Tex. Standard solutions of the disulfides and polysulfides were made by dissolving weighed amounts of the compounds in n-heptane. The sulfur compounds studied and their sources ore as follows: Compound Ethyl
n-Propyl disulfide
n-Butyl disulfide Isobutyl disulfide tert-Butyl disulfide n-Amyl disulfide Isoamyl disulfide %-Hexyl disulfidea
Source Eastman Kodak Co. Eastman Kodak Co. Eastman Kodak Co. Eastman Kodak Co. Phillips Petroleum Co. Eastman Kodak Co. Eastman Kodak Co.
