Indirect Spectrophotometric Determination of Nanomole Quantities of Oxiranes H. Edward Mishmash’ and Clifton E. Meloan Department of Chemistry, Kansas State University, Manhattan, Kan. 66502
OXIRANE COMPOUNDS,which are also known as alpha-epoxides and 1,Zepoxides, have been investigated quite extensively because of their academic and industrial importance. They have applications as reagents in the manufacture of such things as dyes and surfactants, as intermediates in biological systems, and also as end products, such as glycidyl ethers. The strained three-member cyclic structure makes the oxirane far more reactive than other ether linkages. A review of the methods available for the determination of a-epoxides, covering the literature up to 1952, was published by Jungnickel and coworkers ( I ) . Burge and Geyer (2) have reviewed the analysis of epoxy resins. The present methods are either for large quantities of oxiranes or are limited to certain types of epoxides which are free from many interfering substances. The reactions used in this method involve first the cleavage of the a-epoxide in the presence of a mineral acid to form the glycol 45 Minutes 45 o c
R’
R”
R’ R ”
OH
0
OH
The glycol is then cleaved with excess periodate R‘ R ”
R’
I I
~
R-C-C-R’”
/
OH
I
~
I
1
R” 0
~
R-C
+
/
C-R”’
\ \ /
+ HI08
0 0
OH
Left in solution then, is both iodate and periodate. Knowing the amount of periodate used would give the amount of epoxide present in a given sample. In the titrimetric method, Duke and Bulgrin (3) used the following reactions:
+ 104 + 71- 412 + 4Hz0 6H+ + 1 0 3 - + 51- * + 3Hz0 8H+
+
312
in which the difference between an original and an observed titer is used to calculate the amount consumed by the glycol. This same idea was reduced to a micro scale and the difference was measured spectrophotometrically by forming the deep blue, starch tri-iodide complex. A calibration curve of concentration us. absorbance was linear and the concentration Present address, 3M Company, St. Paul, Minn. (1) J. L. Jungnickel, E. D. Peters, A. Polgar, and F. T. Weiss, “Organic Analysis,” Vol. I, Interscience, New York, N.Y., 1952, p 127. (2) R. E. Burge, Jr., and B. P. Geyer, in “Analytical Chemistry of Polymers,” G. M. Kline, Ed., Part I, Interscience, New York, N.Y., 1959, p 123. (3) F. R. Duke and V. Bulgrin, J . Amer. Chem. Soc., 76, 3803 (1954).
range could be varied by changing conditions. The most used concentrations were from essentially 0 to 2 micromoles. EXPERIMENTAL
Chemicals. The 750-,~Msolutions of seven epoxides were prepared by dissolving the appropriate amount in 1 :1 (v/v) glyme-water. The epoxides used in this study were: propylene oxide; 1,2-butylene oxide; (epoxyethy1)benzene (styrene oxide); 1,2-epoxy-5,6-rrans-9,1O-cis-cyclododecadiene; 16,17-epoxydesoxycorticosteroneacetate; epoxybutyl stearate; and dieldrin. Cadmium Iodide-Linear Reagent. Eleven grams of CdI, was dissolved in 400 ml of distilled-deionized water and gently boiled for 15 minutes to expel any iodine. This was diluted to 800 ml and 15.0 grams of Superlose HAA-11-HV were added. The boiling and stirring were continued for 15 minutes. The reagent was then filtered and diluted to 1 liter with distilled-deionized water. Procedure. The procedure used in this method of determining a-epoxides is similar to the Critchfield and Johnson method (4) in that it involves the formation of the glycol and consequent cleavage with periodate to the carbonyls. The method of detection and solvent, however, is what makes it unique and more useful. The following procedure was used for all of the oxirane systems investigated with the exception of a few modifications for epoxylbutyl stearate. A range of concentrations, varying from 0 to 2.25 pmoles, of the oxirane was used to make a calibration curve. The samples were placed in 100-ml volumetric flasks. Additional amounts of the glyme-water (1 :1) were added. This was followed by 5 ml of the standard periodate. The flasks were then sealed with “poly” stoppers and placed in a water bath at 45 “C for 30-40 minutes. The flasks were then withdrawn and allowed to cool. Two milliliters of 1N NaOH and 10 ml of buffer (pH 4.5) were added. The flask was then filled to about 90 ml with distilleddeionized water. One milliliter of the cadmium iodide-starch was added and the flask was filled to the line with deionized water. Allow 20 minutes for the color to develop. The wavelength of maximum absorbance for all but epoxybutyl stearate was 590 nm. Because of the greater insolubility of the epoxybutyl stearate, more glyme was needed and the wavelength of maximum absorbance shifted to 560 nm. RESULTS AND DISCUSSION
With the exception of Dieldrin, all of the compounds were successfully determined by this method. The following abbreviations will be used for the epoxides throughout the rest of the report: epoxybutyl stearate, EBS; 16,17-Epoxydesoxycorticosterone, 16,17-€DC; and 1,2-epoxy-5,6-trans9,lO-cis-cyclododecadiene, 1,2-ECDD. The problem of peroxides forming in the glyme solvent, causing an error, was eliminated by passing the glyme through a column of activated alumina. The purified glyme was then stored over alumina and the interference eliminated. (4) F. E. Critchfield and J. B. Johnson, ANAL.CHEM., 29,797 (1957). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
835
Table I. Precision at Various Concentrations Trial I Trial I1 0.515 0.510 0.515 0.495
0.460 0.470 0.450 0 470
0.509 f 0.008
0.462 f 0.008
I
Trial I11
Trial IV
0.312 0.322 0.305 0.316
0.255 0.265 0.265 0.258
0.314 f 0.006
0.261 f 0.004
Trial V 0
2
~
400
"
"
'
~
'
"
'
'
'
700
X nm
Figure 1. Spectra A. Plot of starch-triiodide complex in water B. Plot of starch-triiodide complex in glyme-water C. Plot of iodine in glyme-water
Figure 1.4 and 1B shows the spectra of the starch tri-iodide complex in water and water-glyme, respectively. The solvent effect expected should be just the opposite. The observed shift must therefore not be due to solvent polarity. A reasonable explanation is that some of the iodine formed must be dissolving in the glyme rather than forming the complex. Support of this statement comes from Figure lC, which is the spectrum of iodine in glyme, and from the fact that in the case of EBS when even more glyme is used, the wavelength of maximum absorbance shifts to a still lower wavelength of 560 nm. The slope of the absorbance os. concentration curves of the simple epoxides are, within experimental error, all the same. A range of 0 to 5 pmoles can be determined to 1 2 0 pmoles. All of the compounds tested gave quantitative results except Dieldrin. When a highly chlorinated compound, such as Dieldrin, is treated with fairly concentrated acid at elevated temperatures there is a likelihood of some dechlorination. The chloride formed could reduce the periodic and iodic acids to free iodine which would give results which would not be reproducible. A compromise between pH and periodate had to be made. As pH increases, color decreases and as periodate increases, color increases. Keeping periodate constant, as the pH increased, the slope steepened slightly; but then more perodate had to be added to get the same blank absorbance reading
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
0.091 0.096 0.095 0.093 0.094 =k 0.002
and this cancelled out the slope change. A compromise of pH 4.5 and periodate concentration of 5 ml of 100 ppm were the parameters of choice. Reproducibility of the method was checked and this is tabulated in Table I. Interferences include alpha-dicarbonyl, alpha-hydroxycarbonyl, and alpha amino alcohols. Also, any of the interferences which are normally involved in iodometric methods ( i e . , that oxidized the iodide to iodine). Unless the interference is part of the sample compound itself, the interferences can be corrected by using a blank of the same sample size and not reacting the oxirane. RECEIVED for review April 9, 1970. Accepted October 19, 1971. The authors thank N.I.H. for financial support of this program.
Correction Sodium Analysis with a Tunable Dye Laser Because of a regrettable typographical error, the authors and page numbe1 for this paper were listed incorrectly in the Table of Contents, p 6A, February, col. 2, second item from the bottom. The authors are Jiirgen Kuhl, Gerd Marowsky, and Reimund Torge. The correct page number is 375.
