LITERATURE CITED
(1) Dyrsscn, D., Svensk Kern. Tidskr. 64, 213 (1952). (2) Gentry, C. H. R., Sherrington, L. G., Analysl 75, 17 (1950). (3) HarriRon, G. C., Jr., Ph.D. thesis, University of Pittsburgh, 1956. (4) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic
balysis,”
Macmillan,
New York,
(10) Welch:?
It‘. J., “Or anic Analytical Reagents, Vol. I, b a n Nostrand, Princeton, N. J., 1947.
1948. (5) Krkhen, A., Ph.D. thesis, University of Pittsburgh, 1958. (6) Lscroix, S., Anal. Chim. Ada 1 , 260 (1947). (7) Luke, C. L., Campbell, M. E., ANAL. CHEM.26, 1778 (1954). (8) SFinbach, J. F., Ph.D. thesis, Uni-
-
RECEIVEDfor review May 3, 19Op. Accepted January 30, 1961. Taken in from thesis submitted by S. J. Janowski in partial fulfillment of the requirements for the Ph.L). degree, University of Pittsburgh, 1059.
yt
versitv of Pittsbureh. -1953. --(9) Umfand, F., Hoffman, W., Anal. Chim. I
Acta 17, 234 (1957).
The Dichromate Oxidation of Glycols C. L. WHITMAN, GEORGE W. ROECKER, and CLAWELL F. McNERNEY Research and Development Department, U. S. Naval Propellant Plant, Indian Head,
b Oxidation with potassium dichromate has been the basis for a method for the determination of concentrations of 1% or less of diethylene or triethylene glycol in aqueous solutions not suitable for the use of refractive index as an assay method. By control of both acidity and reaction time the oxidation can be stopped after consumption of 16 and 20.5 equivalents of dichromate per mole of diethylene glycol and triethylene glycol, respectively. Reproducible results are obtained with recoveries of 99.5%. Data are also presented for the oxidation of propylene glycol.
T
H E potassium dichromate oxidation of glycols was the basis of a n investigation t o establish the conditions for the determination of low concentrations of either diethylene or triethylene glycol in dilute aqueous solutions. The dichromate oxidation of these glycols was considered to have features desirable in a control method because no special apparatus or equipment is required and the analysis can be performed by laboratory technicians. The samples to be analyzed contained 1% or less of a particular glycol. The presence of other componcnts, such as aromatic amines and nitrate esters, would not permit assay by measurement of refractive index and, therefore, preliminary water and subsequent methylene chloride extractions were necessary t o ultimately recover the glycol as a n aqueous solution. Francis (3) used the dichromate oxidation for the determination of small amounts of diethylene and triethylene glycols in monoglycols. He stated that for the diethylene glycol reaction a n actual factor, 0.1381 gram of glycol per gram of dichromate, was used instead of the theoretical factor, 0.1082. Apparently, although i t is not stated, this approximates the consumption of
15.7 equivalents of dichromate per mole of diethylene glycol. Cardone and Compton (I), in studying the effects of sulfuric acid concentration on the oxidation of diethylene glycol, found that with 50% acid by volume at 100” C. the oxidation to carbon dioxide and water waa complete in 30 minutes with the consumption of 20 equivalents of dichromate per mole of the glycol. Also, Werner and Mitchell (4) assumed for the monoalkyl ethers of ethylene glycol t h a t the glycol portion goes t o carbon dioxide and water with consumption of 10 equivalents. Curme and Johnston (9) discussed the potassium dichromate oxidation of glycols and its application to control analyses. They state that this method is particularly useful for dilute aqueous solutions. EXPERIMENTAL
Prepare solutions by transferring an accurately weighed 0.7- to 0.8-gram sample of diethylene glycol (Fisher Reagent) and 1.0- to 1.1-gram sample of triethylene glycol (Fisher Reagent) to separate I-liter volumetric flasks and dilute to volume with water. Determine the amount of water present in the glycol by the Karl Fischer method and correct for this. Prepare standard 0.2N potassium dichromate solution from Bureau of Standards potassium dichromate. This solution is used both aa the oxidant and as a primary standard. Prepare a 0.1N thiosulfate solution and standardize in the customary manner. Dilute 2 volumes of 95% sulfuric acid with 1 volume of water for the sulfuric acid solution. DETERMINATION
OF
GLYCOLS
Transfer a measured portion of the respective samples sufficient to contain 0.01 to 0.02 gram of diethylene glycol, or 0.010 to 0.015 gram of triethylene glycol, to a 500-ml. iodine flask. Using a pipet, add 25 ml. of 0.2N potassium dichromate solution. Add sufficient 2
Md.
to I sulfuric acid to maintain the acidity at 30% for the diethylene glycol sample, or at 28% for the triethylene glycol sample. Stopper the flask loosely and heat on the steam bath for 2 hours. At the end of the heating period, remove the flask and cool it. Dilute with water to approximately 200 ml., add 15 ml. of 15% potassium iodide solution, stopper the flask, and allow to stand for 2 or 3 minutes. Using O.IN sodium thiosulfate, titrate the sample to the usual end point.
A blank, in which the sample is replaced by an equal amount of water, is prepared and run for each series of samples. The amount of the particular glycol can be calculated by using 0.006633 gram, the milliequivalent weight for diethylenc glycol, or 0.007325 gram, the milliequivalent weight for triethylene glycol. The standard solutions of diethylene and triethylene glycol were used to check yields. A convrnicnt and ticcurateJy measured aliquot of thc respective glycol solution was oxidized, and the recovc.ry was calculatvd as per cent of diethylene or triethylcne glycol. The recovery should fall in the range of 99.0 to 100%. RESULTS A N D DISCUSSION
The number of equivalents for the complcte oxidation of diethylene glycol to carbon dioxide and water is 20. Figure 1 shows the effcct of acid concentration on the extent of oxidation of diethylrne glycol for a 2-hour oxidation period. Howevrr, by suitable control 30% sulfuric of the conditions-i.e., acid for a 2-hour period of oxidationthe rcaction can he stopped at an empirical 16 equivalents. On the basis of 16 equivalents of potassium dichromate per mole of diethylcne glycol in a series of determinations, values of 99.8, 99.5, 99.5, 99.9, 99.7, 99.6 (average 99.7% f 0.1) were obtained. For the complcte oxidation of triVOL. 33, NO. 6, M A Y 1961
781
ethylene glycol to carbon dioxide and water, the theoretical number of equivalents is 30. The effect of acid concentration on the extent of the oxidation of triethylene glycol for a Zhour reaction time is shown in Figure I. With proper control, using 28y0 sulfuric acid concentration and an oxidation period of 2 hours, the oxidation can be stopped at an rmpirical 20.5 equivalents of potassium dichromate pcr mole of triethylene glycol. In a wries of determinations made in accordance with these conditions, recoveries that averaged 100.02% + 0.6 mere obtained. As noted, all the data collected were for an oxidation period of 2 hours. The period of oxidation of triethylene glycol was increased to 5 hourfi, and the acid concentration was varied from 26 t o 30%. The iesults (Figure 1) indicate that there are cxtreme variations in the consumption of equivalents of potassium dichromate per mole of the glycol as compared to the slight variations accompanying a similar range of acid concentration for the >hour oxidation period. The indications that the 5 hour curve would parallel the 2-hour curve for lower acid concentrations were not followed because no advantage would be exhibited and, over-all, these data show that the oxidation time must be controlled.
Table
I.
Orig. Wt.,
Aliquot
yo DEG
Grama
Grams
Ansbrysis
W-1 W-2 W-3 W-2 W-3 11-1 D-2 D-3 D-4 D-1 D-3 D-4
7.1187 9.7435 16.4834 9.7435 16.4834 2.7196 3.2476 12.7731 6.0214 2.7196 12.7731 6.6214
0.71187 0,97435 4.1208 0.4872 4.1208 0.27196 0,32476 3.1933 1.5058 0,0699 3.1933 1.5058
0.096 0.104 0.115
Wt.,
% DEG
Wt,., 0 erator 0 erator No. Grams KO. 1 Ro. 2 1-a 5.0031 1.38 1.34 1.38 a 3.3428 1-b 3.5409 0.22 0.25 2-a 2-b 5.0584 0.19 0.21 3.9575 0.22 0.21 3 0.137 0 4-a 5.0000 0.127 LI 4.9981 4-b 5.3813 0.76 0.76 5 11.1423 0.016 n 6-R 13.5634 0.021 6 h a Duplicate waa not run.
782
PwntuEuyca
2 HQR axmm TK
0 -
t
+
A
Synthetic samples were prepared b y adding convenient amounts of diethylene ~ l y c o lto methylene chloride solutions of an aromatic amine or a nitrate ester in order to simulate the methylene chloride extraction stage
ANALYTICAL CHEMISTRY
Added
&
DEG, Gram Expected Recovered covery
0.261 0.00870
0.0266 0.0131
0.0262 0.0128
98.5 98.1
0.0261 0.00870 0.00870
0.0263 0.0187 0.0140
0.0268 0.0189 0.0138
98.1 101.1 98.6
0.340 0.310 0.294 0.396
Table II. Examples of DEG Analytical Results Used in Control Analyses
Sample
JJ
Comparison of Analytical Results on Typical Materials
Sample Identification
Sample
---- -
12-
subsequent to the water extraction of a sample of a production mixture. The diethylene glycol was recovered from these mixtures by water extractions, and aliquots were oxidized with potassium dichromate as described to measure the extent of the recovery. The values for the recovery from the aromatic amine were 99.72, 99.11, 100.15, 99.40, 98.97, averaging 99.41y0 f 0.42, and those for the recovery from the nitrate ester weie 99.36, 99.20, 100.53, 99.51, or an average of 99.63% =k 0.43. Two samples of material obtained during pilot plant production operations were submitted, and the diethylene glycol content was determined by the
proposed method in accordance with the described conditions. The two materials identified as W, having B slight residual moisture, and D, ovendried, were initially analyzed on an asreceived basis; subsequent determinations were made on these materiala with samples to which known amounts of diethylene glycol had been added. These results are shown in Table I. The proposed method was adopted t o control pilot plant production operations. Examples of the analytical results obtained over a period of several weeks are listed in Table 11. The data shown in Figure 1 for propylene glycol are the results of preliminary work on the oxidation of this material. The theoretical number of equivalents for complete oxidation of propylene glycol is 16. The maximum number obtained was about 8, indicating that complete oxidation was not being attained. The procedures presented are relatively simple and rapid. With proper control of oxidation time and acid concentration of '30% for diet.hylene glycol, and 28% for triethylene glycol, these procedures can be readily adapted to routine use, LITERATURE CITED
(1) Cardone, M. J., Compton, J. W., ANAL.CHEM.25, 1869 (1953);, (2) Curme, G. O., Johnston, F. Glycols," pp. 340-2, Reinhold, New gork, 1952. (3) Francis, C. V., ANAL.CHEM.21, 1238 (1949). (4) Werner, H. W., Mitchell, J. L., Zbid., 15, 375 (1943).
RECEIVEDfor review June 10, 1959. h u b m i t t e d October 28, 1960. Accepted February 14, 1961.
