Kinetic Method for Quantitative Determination of Individual Organic Peroxides in Peroxide Mixtures James P. Hawk, Edgar L. McDaniel, Trueman D. Parish, and Kenneth E. Simmons Research Laboratories, Tennessee Eastman Company, Division of Eastman Kodak Company, Kingsport, Tenn. 37662 used methods for determining organic The most frequently peroxides involve reducing them by iodide ion. Such methods were reviewed by Mair and Hall in 1971 (7). By varying the solvent and temperature of reaction, in certain cases one can determine peroxides of one class in the presence of peroxides of other classes (2). In 1958, Horner and Jurgens showed how to determine peroxides quantitatively by class by use of a reducing agent followed by treatment with iodide ion (3). However, no one has shown how to determine iodometrically the individual peroxides in a mixture when more than one peroxide of a particular class is present. This paper reports a kinetic method for the quantitative determination of individual peroxides in a mixture containing five organic peroxides (two ‘peroxycarboxylic acids, two diacyl peroxides, and a
hydroperoxide). Because these five peroxides are all quite reactive toward iodide ion, a basis besides this reactivity is used to distinguish them individually. Sulfides reduce peroxides at rates which are affected by the structures of both the sulfide and the peroxide. We believed that the concentrations of the individual peroxides could be measured by using several sulfides, each having a different reducing power. The method is based on partial reduction of the mixture of peroxides by each sulfide followed by iodometric determination of residual unreduced peroxides. A matrix of five simultaneous linear equations is derived, and the solution of these equation? yields the concentration of each peroxide.
THEORETICAL
To determine five peroxides, at least five different measurements of the peroxide concentrations are required. Each measurement must result from a different relationship among the concentrations of the peroxides. From the measurements, five linear equations with five unknowns resulted. The general forms of the equations are
ttuCi +
CX12C2
+
Oíi 3C3
+ auCí + ai¡C¡
=
7)
Ti
CU21C1
+
(X22C2
+
CX23C3
+
CX24C4
+
CX25C5
=
OiziCl
+
OL32C2
+
CtssCs
+
OÍ34C4
+
OÍ35C5
=
T3
oqiCi +
0L42C2
+
(X43C3
+
OÍ44C4
+
ÜÍ45C5
=
T4
+
CttfCz
+
OÍ53C3
+
OL34C.1
+
«ssCs
=
T¡
titers found for the aliquot portions of the mixture after the various treatments. In matrix notation,
AC C
A"1
T
(1) (2)
23T
The equations for the peroxide concentrations, C, were solved by calculating the inverse matrix, A-1, using a digital computer and a program based on the Morris escalator method. It is required that the matrix of coefficients be square and nonsingular. Linearity of the equations was achieved by reducing the peroxides with sulfides under conditions so that the reductions were kinetically first-order or pseudo-first-order in peroxide concentration. The first-order kinetics result from using an excess of sulfide with respect to peroxide so that the concentration of sulfide does not change significantly during the reductions. For a first-order reaction at constant temperature
C
=
C0e-kt
(3)
At constant reaction time, kt is constant, and C
=
(4)
C0e~k'
Thus, concentration C is a constant fraction of initial concentration C0. This fraction enters each coefficient in the simultaneous equations. Since there are five equations with five unknowns, the solution is mathematically exact. With mixtures which do not contain a particular peroxide, the solution may indicate a negative value of small magnitude for the concentration of this peroxide. When this situation arises, the absolute value of the calculated concentration will be low (about the level of detection). In this case, the concentration of this peroxide is set equal to zero in all the equations, and the concentrations of the remaining four peroxides are found by using five determinations and a least squares analysis. If the concentration of one of these four peroxides is insignificant, its concentration is set equal to zero and the least squares analysis is calculated for three species. In matrix notation,
AT A C ·
CX51C1
=
=
·
=
·
(5)
( · A)-1· · ( · )- ·( · ) C ( · )-1· ·
(AT-A)-1’AT-A'C
=
The unknown quantities, C, are the concentrations of the peroxides in the original sample. The coefficients, a, are the fractions of individual peroxides remaining in the mixture after treatment. (The coefficients, a, must be determined individually for the various peroxides before analysis of a peroxide mixture.) The constant terms, T, are the peroxide (1) R. D. Mair and R. T. Hall, in “Organic Peroxides,” Vol. 2, D. Swern, Ed., Wiley-Interscience, New York, N.Y., 1971, pp 535-635. (2) R. D. Mair and A. J. Graupner, Anal. Chem., 36, 194-204 (1964). (3) L. Horner and E. Jürgens, Angew. Chem., 70, 266 (1958).
(6)
=
(7)
1
(8)
=
This calculation is a least squares estimate of the significant peroxide concentrations (4). EXPERIMENTAL Reagents. Peroxyacetic acid was purchased from the Becco Chemicals Division, FMC, as a 40% solution in acetic acid. (A recently purchased bottle was selected; it contained (4) R. Deutsch, “Estimation Theory,” Prentice-Hall, Englewood Cliffs, N.J., 1965, pp 60-61.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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Table I.
Peroxide Analysis Scheme
Sample
vol, ml
Analysis No.
3.0 3.0
1
2
4
3.0 3.0
5
35
3
6
31
Reactant vol, ml
Reactant
None (a) 2,2-Dimethoxypropane (b) 2,2'-Thiodiethanol (c) H2G Phenyl sulfide
Methyl p-tolyl sulfide None Phenyl sulfide
Reaction time, min
5.0
1
1.0
-
1.0 0.1 0.1
0"
0.1
10-
-
Time of reaction with sulfide before addition of sample to Nal-HOAc-isopropyl alcohol mixture for liberation of I2. h 20-ml sample partitioned between 10 ml of isobutyl acetate and 40 ml of H20; ester phase analyzed. 0
Table II.
Analysis of Known Peroxide Mixture Least squares
Error,
0.00418
Found, meq/ml 0.00451
+7.9
solution Found, Error, meq/ml % 0.00444 +6.2
0.00182
0.00178
-2.2
0.00187
0.00310 0.00267 p-Methylbenzy! hydroperoxide 0.00345 0.00333 Peroxy-p-toluic
-13.9
0.00272
-12.2
-3.5
0.00335
-2.9
Known concn,
Peroxide
Diacetyl
meq/ml
Exact solution
peroxide
Di-p-toluoyl peroxide
acid Peroxyacetic acid
0.0000
0.00013
%
+ 2.7
0.0000
1.5 wt % hydrogen peroxide as determined by the method of Greenspan and MacKellar (5). No correction was made for the hydrogen peroxide in the determination of coefficients for peroxyacetic acid.) Diacetyl peroxide was purchased from the Lucidol Division, Wallace and Tiernan, Inc., as a 25% solution in dimethyl phthalate. p-Methylbenzyl hydroperoxide was prepared by the reaction of oxygen with p-xylene in the presence of di-?er/-butyl peroxide initiator (6). Di-ptoluoyl peroxide was prepared in quantities of less than 1 gram by the reaction of p-toluoyl chloride with sodium peroxide (7). Peroxy-p-toluic acid was prepared in less than 0.5-gram quantities in dilute solution in CH3OH by reacting di-p-toluoyl peroxide with sodium methoxide at —5 °C (8, 9). Peroxy-p-toluic acid was then extracted into ethyl acetate solution and stored in a refrigerator as a water-saturated solution. The sulfides used in the peroxide reductions were phenyl sulfide (PS), methyl p-tolyl sulfide (MTS) and 2,2'-thiodiethanol (TDE). The PS and MTS (Eastman reagent-grade materials) were used as obtained. The TDE (Eastman practical-grade material) was distilled twice before use to obtain reproducible rates of peroxide reduction.
(5) F. P. Greenspan and D. G. MacKellar, Anal. Chem., 20, 10611063(1948). (6) E. J. Lorand and E. I. Edwards, J. Amer. Chem. Soc., 77, 40354037(1955). (7) C. C. Price and E. Krebs, Org. Syn., 23, 65-67 (1943). (8) G. Braun, “Organic Syntheses,” Vol. 1,2nd Ed., 1941, pp 431-4. (9) I. M. Kolthoff, T. S. Lee, and M. A. Mairs, J. Polym. Sci., 2, 199-202(1947).
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ANALYTICAL CHEMISTRY. VOL. 44, NO. 7, JUNE 1972
Procedure. The peroxide mixtures contained peroxyacetic acid, peroxy-p-toluic acid, diacetyl peroxide, di-p-toluoyl peroxide, and p-methylbenzyl hydroperoxide. The solvent These was acetic acid which contained up to 13% water. mixtures also contained cobalt(III) ion, which oxidizes iodide ion to iodine and, thus, interferes with the peroxide determination. Therefore, the mixtures were treated with oxalic acid and warmed to 40 °C to reduce the cobalt(III) to cobalt(II) and precipitate most of the cobalt(II) before either reduction with sulfide or iodometric determination of peroxides. This treatment with oxalic acid did not affect the concentrations of peroxides in known mixtures. The procedure used for each analysis in the method is shown in Table I. In the first analysis, the total peroxide titer was determined without any sulfide treatment. In the second analysis, the 2,2-dimethoxypropane reacted rapidly with the water to yield methanol and acetone; this removal of water influenced the subsequent reduction, so that more suitable coefficients were obtained for di-p-toluoyl peroxide and pmethylbenzyl hydroperoxide. After reduction of the peroxide mixture by TDE, water was added to destroy unreacted dimethoxypropane and the remaining peroxides were determined iodometrically. The third and fourth analyses involved reductions with 0.1 ml of phenyl sulfide and methyl p-tolyl sulfide, respectively. All the sulfides tested were strong reducing agents for peroxyacetic acid and peroxy-p-toluic acid which resulted in coefficients of about zero in the equations. (This effect makes it difficult to differentiate between these two peroxides.) The problem was solved in the fifth analysis. The sample was partitioned between isobutyl acetate and water, and then the ester layer was washed twice with water. All the peroxyacetic acid and a known fraction of the peroxy-p-toluic acid, along with undetermined fractions of the other peroxides, went into the aqueous phase. Portions of the ester layer, which contained a known fraction of the peroxy-p-toluic acid, were then analyzed iodometrically before and after reduction with phenyl sulfide. This sulfide reduced only the peroxy acid; the difference between the peroxide titers before and after sulfide reduction and a knowledge of the distribution coefficient for peroxy-p-toluic acid in the extraction permitted the amount of peroxy-p-toluic acid present to be estimated. Peroxides were reduced with sulfide at room temperature The reac(approximately 24 °C) in an air-conditioned room. tion times were measured with a second hand on an electric clock (1 min) or an ordinary laboratory timer (10 min). After treatment, residual peroxides were determined iodometrically by Method 1 of Mair and Graupner (2). The rate constant for each peroxide-sulfide combination was determined by treating acetic acid-water solutions containing known amounts of a single peroxide with each of the sulfides and then determining the unreduced peroxide iodo-
metrically. RESULTS AND DISCUSSION
The conditions chosen for the various sulfide reductions control the values of the coefficients for each treatment, and these coefficients affect the accuracy of the estimates of peroxide concentrations. It is not apparent from the coefficient matrix, A, whether accurate estimates of the concentrations, C, will result. Inspection of the inverse matrix, A-1, does permit a judgment. Preliminary estimates of the coefficients were generated and the inverse matrix calculated. Then hypothetical concentrations of peroxides in a mixture were used with the coefficients of A to generate the appropriate values of T. These values of T were adjusted by arbitrary amounts equivalent to the expected experimental error in the peroxide titration and the expected C calculated from Equation 2. Inspection of the errors in C and the terms in A-1, trial adjustments in the coefficients, and recalculation of A-1
indicated which coefficients required adjustment. The treatments were modified as necessary to control the coefficients for accurate estimates of C. Data from the analyses were used to construct the following set of five simultaneous equations for mixtures containing about 6% water: 1.00 Ci
+
1.00 C2
+
1.00 C3
+ 1.00 Ci
0.95
G +
0.38 C2
+
0.72 C3
+ 0.93
C2
+ 0.93
C3
+ 0.60
C2
+ 0.27
C3
+
0.07 c5
+ 0.00
C5
C2
+
0.00 C3
7i
(9)
=
r2
(io)
=
T3
(11)
=
T4
(12)
=
r5
(13)
+ 0.00 Ci + 0.00 C5
0.00 Ci + 0.00
=
+ 0.00 Ci
0.98 Ci
1.00 C5
+ o.oo Ci
0.95 Ci
+
+ 0.00 C4
+
1.53 C6
concentrations of diacetyl peroxide, di-pwhere, Ci-¡¡ toluoyl peroxide, p-methylbenzyl hydroperoxide, peroxyacetic acid, and peroxy-p-toluic acid, respectively. The coefficients are unity in Equation 9 because total peroxides were determined without prior reduction by sulfide. The coefficients in Equations 10, 11, and 12 represent the fraction of each peroxide remaining after the sulfide reductions. Each coefficient times initial concentration of the individual peroxide represents the contribution of that peroxide to the titer found for this particular analysis. In the fifth analysis, Ts is a direct measure of the peroxy-p-toluic acid which was extracted into the organic phase; therefore, the value of the coefficient in Equation 13 (1.53) is a function of the partition coefficient
and does not involve a rate constant. The coefficient is greater than unity because of a reduction in volume during the extraction. The first-order kinetics requirement for the sulfide reduction was demonstrated experimentally for each peroxide-sulfide combination whose coefficient was less than 0.93 but greater than 0.07. (For coefficients outside the range, either so little or so much of the peroxide has reacted that first-order kinetics is not necessary.) The method was tested with a mixture of known peroxides. The results are shown in Table II. The accuracy of the method was good; the error ranged from 2 to 14% for the exact solution. A low concentration of peroxyacetic acid was calculated, although none was present in the mixture. This level of peroxide (0.0001 meq/ml) was about the level of detection for the method as developed. When the concentration of peroxyacetic acid was set equal to zero and the four remaining peroxide concentrations were calculated by least squares method, the errors ranged from +2.7% to —12.2%. This least squares analysis decreased the relative error for four of the peroxides and increased it very slightly for one.
CONCLUSIONS
=
The wide choice of sulfides and reaction conditions which be used lends flexibility to the presented method of determining quantitatively the concentrations of individual peroxides in a peroxide mixture. The method should be adaptable to analyses of many different peroxide mixtures. To apply the method to other mixtures, one must know which peroxides are present and determine the coefficients by tests on pure samples of each peroxide. can
Received for review September 3, 1971.
Accepted January Paper presented at the 159th National Meeting of the American Chemical Society, Toronto, Ontario, May 1970. 13, 1972.
Determination of Dichloroacetylene in Complex Atmospheres Frederick W. Williams Chemistry Division, Code 6180, Naval Research Laboratory, Washington, D.C. 20390
Several methods have been published on the determination of the extremely toxic compound, dichloroacetylene (DCA), but these methods are limited in effectiveness to concentrations far in excess of the toxicological limit (/) or are very time consuming (2-4). Previously, in enclosed environmental systems where problems have been experienced with DCA (5), the precursor compound 1,1,2-trichloroethene (TCE) was monitored (6). Dichloroacetylene is unstable under certain conditions and becomes spontaneously explosive when present in moderate concentrations in air, but at low concentrations it (1) American Conference of Governmental Industrial Hygienists,
Threshold Limit Values of Airborne Contaminants, Cincinnati, Ohio, 1969. (2) J. Siegel, R. A. Jones, and L. Kurlansik, J. Org. Chem., 35, 3199 (1970). (3) J. H. Wotiz, F. Huba, and R. Vendley, ibid., 26,1626(1961). (4) . E. Umstead and R. A. Saunders, Naval Research Labo-
ratory, Washington, D.C., unpublished data, 1965.
(5) R. J. Defalque, Clin. Pharmacol. Ther., 2,665 (1961). (6) L. S. Young, NASA TMX-62,004, October, 1970.
is quite stable when stabilized by compounds such as ethers, or 1,1,2-trichloroethene (7). In the evaluation of atmospheres of enclosed environmental systems, the analysis for DCA is further complicated by the myriad of other compounds in the atmosphere. One other factor which makes such atmospheres even more complex is the potential interaction of air revitalization equipment with these contaminants (8). Several papers have appeared in the literature reporting the use of the gas chromatograph-microcoulometer combination for the determination of chlorinated hydrocarbons (9, 10). The microcoulometer is particularly attractive as a detector (7) D. W. F. Hardie, Kirk-Othmer Encycl. Chem. Techno!., 2nd ed.,
5,203(1964). (8) R. A. Saunders, Arch. Environ. Health, 14,380(1967). (9) D. M. Coulson and L. A. Cavanagh, Anal. Chem., 32, 1245 (1960). (10) J. A. Stamm, “Lectures on Gas Chromatography—1964,” L. R. Mattick and H. A. Szymanski, Ed., Plenum Press, New
York, N. Y„ 1965,
p 53.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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