Determination of Monoalkyl Ethers of
Ethylene Glycol 'HAROLD W. WERNER AND JAMES L. MITCHELL Division of Industrial Hygiene, National Institute of Health, Bethesda, >Id.
W
reactions consumed' 15.7, 13.8, 19.9, and 26 equivalents of oxygen, and they required 15, 15, 240, and 120 minutes, respectively, a t water bath temperature, 90" to 100" C. As is illustrated by the curves, heating beyond these minimal periods did not significantly increase the extent of oxidation. From the number of equivalents consumed in these reactions it appears that, in all cases, the glycol portion of the molecule is completely oxidized to carbon dioxide and water. This oxidation theoretically requires 10 oxygen equivalents. Considering the equivalents used in excess of 10 for each compound, it appears that the methyl radical is oxidized to carbon dioxide and water, the ethyl to water and acetic acid, and the propyl and butyl to carbon dioxide, water, and acetic acid. These oxidations of alkyl radicals are in agreement with the results of Polonovski (6) for propionic and butyric acids, Nicloux (4) for ethyl alcohol, and Chapman and Thorp (1) for the methyl radical in esters. K i t h an oxidizing mixture containing 55 per cent sulfuric acid, reactions requiring 20.4 instead of the theoretical 20 equivalents for the propyl derivative, occur within 60 minutes (fifth curve of Figure 1). Additional heating results in further oxidation, and a t the end of 480 minutes 21.1 equivalents have been consumed. Reactions of this type are less suited to determinative work t h m the ones previously described; nevertheless, there are instances where a saving in time justifies the use of less accurate methods. The theoretical considerations and results already presented suggest that theoretical factors, based on reactions requiring 16, 14, 20, and 26 equivalents, respectively, for the methyl, ethyl, propyl, and butyl derivatives, should be satisfactory for determining these glycol ethers. These factors as well as conditions producing corresponding reactions are shown in Table I. The suitability of these factors and conditions for determining various samples in aqueous solution is indicated by recovery experiments summarized in Table 11.
HEX toxicological work on the monoalkyl ethers of ethylene glycol was undertaken (6) no suitable method for determining these compounds was available. Thus an immediate need in toxicological studies as well as the likelihood of a future need in attempts to evaluate and prevent any possible hazards connected with the use of these solvents in industry led to a study of methods of estimation. Preliminary experiments with the methyl, ethyl, n-propyl, and n-butyl derivatives, using specimens for which constants have been described (6), indicated that these derivatives could be oxidized quantitatively with potassium dichromate. (Methyl Cellosolve, Cellosolve, and Butyl Cellosolve are trade names for the methyl, ethyl, and butyl derivatives.) These experiments indicated, however, that progress of the oxidations was influenced by acid concentration, period of heating, order of adding reagents, and quantities of reagents used. Thus, the first part of the experimental work necessarily is concerned with the definition of conditions yielding precise quantitative reactions, while the second part is an investigation of the suitability of these reactions for estimating various concentrations in water and in air. Experimental Method In general, the method of conducting oxidations and determining amounts of oxygen consumed is similar to a modification of the Nicloux (4)method for ethyl alcohol described by Nuehlberger ( 3 ) . With the glycol ethers, however, the results were found to be most consistent when standard aqueous solutions of glycol derivatives (1,2, or 3 ml.) were added to cooled mixtures of 5 ml. of 0.33 N pot.assium dichromate and an amount of concentrated sulfuric acid equal to the combined volume of potassium dichromate and sample solutions. This 50 per cent proportion of acid was used with all compounds. Other concentrations of sulfuric acid \yere investigated, but with the exception of 55 per cent acid employed in determining the propyld derivative, the other concentrations offer no advantages. The oxidations of the aqueous samples were conducted in 25 X 200 mm. test tubes which were heated in a boiling water bath for definite periods. A t the termination of heating, the- tubes were cooled and the contents were transferred into 500-ml. wide-mouthed Erlenmeyer flasks. Volumes were made up to approximately 300 ml., about 3 grams of potassium iodide were added, and the liberated iodine was titrated with 0.05 N sodium thiosulfate. Starch solution was added, toward the end of the titration, as an indicator.
RESULTS AND DISCUSSIO~V. Curves illustrating the effect of the period of heating on the extent of oxidations are shown in Figure 1. Each point on these curves is based on four or more determinations, and in each determination the sample (2 ml. of standard solution) was of such size that approximately a 100 per cent excess of potassium dichromate, based on complete reactions, was present. From the four curves for oxidation with 50 per cent acid mixtures it appears that stoichiometrical reactions occur. I n the order methyl, ethyl, propyl, and butyl, these
55
PER
i t
CENT
N-PROPYL
I
I
I
60
120
240
I I 30
M I N U T E S
A C I D
*W
480
H E A T I N G
FIGURE 1. EFFECT OF PERIOD OF HEATING IN
A WATERBATHON COMPLETION OF REACTIONS Complete reactions are theoretical ones requiring 16 14 20 and 26 oxygen equivalents, respectively, for the methyl, ethyl, n-proGyl, And'n-butyl derivatives.
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Vol. 15, No. 6
Elkins and co-workers (2) have recently described a method TABLE I. FACTORS AND CONDITIONS USEDIN DETERMINING for the estimation of Methyl Cellosolve (ethylene glycol CELLOSOLVES
Derivative
Reaction Mixture 70 acid
Heating Period Min.
Methyl Ethyl n-Propyl n-Propyl n-Butyl
50 50 50 55 50
30 30 240 60 120
TABLE
Derivative
11.
DETERMINAT1ON OF
Ethyl n-Propyl n-Butyl
1.584 2.141 1.736 1.736 1.514
CELLosoLrEs I N
SOLUTION Sample
No. of Determinations
Mg.
Methyl
Factor, 0.33 N KzCrz07
2.013 4.026 6.039 2.513 5.026 7.539 2 ,005 4.010 6.015 1,000 2,000 4,000 6.000
0 . 3 3 .\K&rzO; Used ’li1.
50 Per Cent Sulfuric Acid 4 1.263 4 2.491 4 3.732 4 1.181 4 2.309 4 3,492 4 1,152 4 2.292 4 3.338 6 0.660 6 1.318 6 2.642 6 3.879
Averase Deviation
Recoi7w
5%
7c
0.2 0.1 0 2 0.8 0 3 0 5 0 0 .. 73 0.5 0.7 0.5 1.0
90 4 98.2 98.0 100.7 98.4 99.3 100.0 99.3 96.3 100.0 99.8 100.0 97.9
0.7 0.1 0.3
104.6 101.8 97.4
0.8
monomethyl ether) in the presence of certain other solvents. As a result of several determinations using 0.5 N potassium dichromate and 33 per cent sulfuric acid and gently refluxing for 4 hours, they obtained a n oxygen equivalent of 15.5 for Methyl Cellosolve. I n a single similar determination of Cellosolve (ethylene glycol monoethyl ether) they obtained a value of 13.2. The first equivalent is 3.1 per cent and the second is 5.7 per cent below theoretical. Thus, it appears that when these solvents alone are being determined, stronger acid solutions than those employed by Elkins can be used advantageously. With 50 per cent acid, reactions for the methyl and ethyl glycol ethers are complete within 15 minutes at boiling water bath temperature, lvhile similar reactions require over 4 hours’ refluxing with 33 per cent sulfuric acid media.
TABLE 111. DETERMINATIOK O F CELLOSOLVE Derivatives Xlethyl
55 Per Cent Sulfuric Acid
n-Propyl
2,005 4,010 6.015
4 4 4
1.211 2.351 3.374
Ethyl
In general, with 50 per cent acid mixtures, recoveries are not significantly affected by the size of the sample used, although there is a n indication that large samples and minimal heating periods give Ion- recoveries in determinations of the propyl derivative. With the 55 per cent acid mixtures, recoveries of propyl glycol ether show considerable spread. They are 2.6 per cent low from large and 4.6 per cent high from small samples.
Average Concentration Calculated , Found .Mg./l. Mg./l. 0.93 0.88 2.88 2.92 4.03 4.12 5.19 5.15 6.59 6.54 6.74 6.19 7.03 7.16 10.50 9.40 20.36 21.08
V.4PORS
Difference
% -5,4 4-1.4 +2.2 -0.8 -0.7 -8.1 +1.8 -10.4 -3.4
1.36 4.15 5.82 6.40 6.74 8.14 10.30
1.34 4.32 5.95 6.64 7.14 7.92 11.27
-1.5 +4.1 +2.2 +3.8 +5.9 -2.7 +9.4
n-Propyl
1.82 5.54 7.04 9.94
2.00 5.81 7.15 10.11
+9.9 +4.9 +1.6 t1.7
n-Butyl
1.87 2.71 3.22 3.72
1.94 2.66 3.28 3.43 4.47
+3.6 -1.8 f1.9 -7.8 +0.2
4.46
Vapors The results presented indicated that oxidation with potassium dichromate is satisfactory for determining the glycol ethers in water solution; therefore, the suitability of the method for vapors was investigated. Standard vapor-air mixtures were prepared by a method described by the writers (6). Samples were obtained in calibrated, round-bottomed flasks. Sizes of these flasks were selected for individual determinations, so that the samples collected would be equivalent to not more than 50 per cent of the potassium dichromate present. The selected samplers were charged with 5 ml. of 0.33 N potassium dichromate and 5 ml. (6 ml. in determining the propyl derivative) of concentrated sulfuric acid. They were evacuated, vapor samples m-ere obtained, and the stoppered flasks were heated in a boiling water bath for periods listed in Table I. The flasks were cooled, and the contents transferred into 500-ml. wide-mouthed Erlenmeyer flasks for titration. The potassium dichromate consumed was determined as for aqueous samples, and the amounts of glycol ethers viere found by the use of factors listed in Table I.
RESULTSAND DISCUSSIOX.The agreement between average concentrations found in different ihemical determinations made a t intervals and concentrations calculated from lTeight loss, during several hours, in the saturating bubblers and total air flow used is shown in Table 111. I n view of the wellknown difficulties in preparing and sampling standard vaporair mixtures, the agreement shomn in Table 111 is as good be exl)ected. Apparently t’he reactions employed are about as suitable for determining gaseous as aqueous samples.
Summary Conditions for the oxidimetric determination of four monoalkyl glycol ethers have been investigated, and stoichiometrical reactions are described. I n general, these reactions are but slightly influenced by heating longer than the necessary minimum in 50 per cent acid media. The long heating period necessary for oxidation of the propyl derivative is greatly reduced by a small increase in acid concentration; however, the more rapid oxidation yields less accurate recoveries.
Acknowledgment The writers wish to acknowledge helpful suggestions of IT. F. yon Oettingen of the Division of Industrial Hygiene, made in connection with this work.
Literature Cited (1) Chapman, E. T.1 a n d Thorp, Tv.3 J . Chem. SOc.3 19, 477 (1866). (2) E l k i n s , H. B., Storlaazi, E. D., a n d H a m m o n d , J. IT.,J . Ind. Hug. Toxicol., 24, 229 (1942). (3) p. 648, Chicago, I n d u s t r i a l ~. , , Mc?\Tallv. IT, D.. “Toxicology”, Medicine, 1937. (4) Nicloux, >I., Compt. rend. SOC. bid., 48, 841 (1896). (5) Polonorski, M.,Compt. rend., 178, 576 (1924). (6) W e r n e r , H. W,, Sfitchell, J. L., >filler, J. IT., and \-on O e t t i n g e n , TT’ F.. J . I n d . H u g . Tozicol., 25, 157 (1943). ~
