Estimation of Water in Alcohol with Aid of
Dicvclohexvl J
J
G. ROSS ROBERTSON University of California, Los Angeles, Calif.
critical composition during calibration, no great precision in measurement of volumes is required in subsequent determinations, as shown in Figure 1. Since the main aim of this work is to test alcohol that is nearly absolute, the simple volume ratio of 1 to 2 was chosen for the final graph of Figure 2.
Water content in otherwise pure ethyl alcohol of nearly absolute grade may be determined by measurement of critical solution temperature in the system alcoholwa ter-dicyclohexyl The ‘‘di cy cl ohe x y 1 point”, easily obtained in a few minutes without necessity of standard solutions or special apparatus, is referred to a graph for percentage value.
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Sources of Error The shift of peak of the miscibility curve toward the dicyclohexyl axis with increase of water content calls for increasing care in measurement of volume, and the probable error grows. Since the precise determination of water in alcohol over the range 96 to 99 per cent (by weight) seems to be a n unimportant problem, no attempt is made here to complicate matters by reporting data at other volume ratios than the uniform 1 to 2 value adopted. Determinations below 99 per cent are thus only approximate. Unfortunately, mixtures of dicyclohexyl and alcohol, either of the critical composition or nearly that ratio, markedly display the phenomenon of critical opalescence (8) a t temperatures just above the maximum temperature of genuine turbidity. This cuts down slightly the extreme precision characteristic of the Crismer technique using kerosene. With dicyclohexyl and a good thermometer it is nevertheless easy to distinguish alcohol preparations as close to each other, for example, as 99.90 and 99.91 per cent; by the Crismer method, 99.900 and 99.903 per cent. More important is the problem of purity of the dicyclohexyl. Fortunately, the present industrial product is a synthetic individual of high grade, derived by hydrogenation of diphenyl. Several lots of the hydrocarbon, both “technical” and purified, were tested against a standardized alcohol of 99.9+ per cent grade. No significant difference in critical solution temperature was found in this series; certainly
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H E critical solution temperature of a kerosene-alcohol mixture serves as a remarkably precise index of water content, as reported by Crismer (S), Andrews (2), and the Bureau of Standards ( 5 ) . Each lot of kerosene, however, must be calibrated by a laborious process. The method is, therefore, of little use to the majority of laboratory workers requiring only occasional determinations of water in alcohol. A single substance, similar in physical properties to kerosene but which could be calibrated once for all, would be a desirable substitute. For this purpose a paraffin hydrocarbon with molecular weight of about 170 would be ideal-for example, a dodecane. Unfortunately, pure open-chain paraffins of such high molecular weight are available only as costly academic curiosities. A new solution of this problem has become possible through the commercial appearance of dicyclohexyl (bicyclohexyl, dodecahyclrodiphenyl). The system ethanol-dicyclohexyl has the convenient critical solution temperature of 23.4’ C., with an elevation of 18’ for the first 1per cent of water added to the alcohol so tested. Provided any given alcohol preparation and the dicyclohexyl are mixed in or near the ratio of
FIGURE1. M.cmtutr TEMPERATURES OF TURBIDITY
Solutions of aqueous ethyl alcohol preparations in dicyclohexyl. Abscissa, volume per cent alcohol
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I N D U S T R I A L A N D E N G I N E E R I N;G C H E M I S T-R-Y
Vol. 15, No. 7
results. Particular attention was paid to the most highly dehydrated alcohol which was attainable. Six lots, prepared on different days by the two methods, had critical solution temperature values (with the paraffin oil) varying from 23.27" to 23.37", and densities at 25.00' C. from 0.78506 to 0.78508, there being no consistency nor correlation within those ranges; average 23.31 and 0.78507+. By short extrapolation, 23.2" was taken as the critical solution temperature for 100 per cent alcohol, for which the extremely reliable density value of 0.78506 is known (5). DICYCLOHEXYL. Three lots of industrial origin were investigated: O
Melting Point
c. 1. Eastman Kodak Co., P4641, merchandise pf 1941 2. Dow Chemical Co., technical product of 1943 3. Special preparation made by fractionation of 2 through a 30-plate oolumn, followed by recrystallization
3.5 3.4 3.63
The melting-point value for KO.3 refers to the constant equilibrium temperature of a mush of the hydrocarbon alternately freezing and melting slowly in a bath varied from 3 " to 4", there being from to z/3 of solid present. A National Bureau of Standards certified Beckman thermometer was used, with corrections for setting, certificate, and stem emergence, and the figure 3.63' signifies merely the elevation above the ice point determined just before and after the main experiment. Product 1 was recrystallized without use of solvent, yielding product 4, a part of which was washed with concentrated sul100%Alc. 99 90 furic acid, yielding 5. Both 4 and 5 agreed exactly with 3 in melting point. In view of the constancy of the temperature, it OF ETHYL ALCOHOL AND CORRESPOND- was judged that the theoreticalo melting point (disappearanc: FIGURE 2. PERCEXTAGES ING DICYCLOHEXYL POINTS of last crystal) is not over 3.65 , and that this value *0.03 1 t o 2 alcohol-dicyclohexyl ratio is the melting point of pure dicyclohexyl. Apparently the figures of 4" (4) and "above 4"" ( 7 ) reported for this compound in the literature were not determined with special precision. none equivalent to the difference between 99.90 and 99.91 per cent alcohol. Water content in the dicyclohexyl is not a serious problem. Simple filtration of the clear liquid product through dry filter paper removes all stray suspended globules of water, and perhaps even some of the extremely small content of dissolved water. The data used in preparing Figure 2 refer to clear samples of the hydrocarbon which have been exposed freely to ordinary atmospheric conditions. When such material was thoroughly dried, no significant change in melting point could be detected with a Beckman thermometer. A series of purified alcohol preparations (98 to 99.99 per cent), the densities of which at 25.00' were determined with a graduated pycnometer ( 6 ) , !vas first used to standardize a special paraffin oil by the Crismer method. The oil consisted of highly refined kerosene to which had been added enough colorless paraffin oil to raise the critical solution temperature values out of ranges requiring an ice bath, and to a region close to the critical solution temperature values for dicyclohexyl. Once this paraffin oil standard was established, cross comparisons of an evtended series of alcohol samples, ,prepared in random concentrations, were easily made, each with the two hydrocarbon preparations, oil and dicyclohexyl. The miscibility-temperature determinations were made in a short 20-mm. test tube n-ith siphon drain from the bottom. This device was fitted with thermometer, micromechanical stirrer, burets. and inlet for desiccated air. The alcohol supply was admitted from a distillation receiver without intermediate exposure to the outer atmosphere.
Materials Used PARAFFIN OIL. "Elaine" kerosene (Standard Oil Co. of Calif.), 7 volumes. Standard White Oil No. 7 (Standard Oil Co. of Calif.), 1 volume. Such a mixture yields a critical solution temperature, with 100 per cent alcohol, in the vicinity of 23" C. ETHYLALCOHOL. Commercial 95 per cent fermented spirit was given three reflux treatments over quicklime, each followed by distillation with substantial rejections of foreruns and tailings t o eliminate acetaldehyde and higher alcohols. In one case the rapid and extremely convenient Adickes ( 1 ) method, employing ethyl formate, was used in the final operation with concordant
Presumably the impurities in any of the products so far encountered are not only of small amount, but of physical nature not entirely foreign to dicyclohexyl itself. Accordingly, the critical solution temperature values obtained with the five preparations are even more closely concordant than the melting points. Thus Nos. 1 and 2 are satisfactory for ordinary accuracy in estimation of water in alcohol.
Dicyclohexyl Point PROCEDURE. To 2.0 cc. of the alcohol being tested, in a dry 15-mm. test tube, add 4.0 cc. of dicyclohexyl and stir with a dry thermometer. Heat until the mixture becomes a clear solution, and then allow t o cool slomly, with continued stirring. As the critical solution temperature is approached, the liquid becomes opalescent, suggestive of very dilute soap solution. It is still clear enough so that the mercury thread in the immersed section of the thermometer is readily distinguished. Suddenly (within 0.2 temperature range) the liquid becomes completely turbid, and the mercury thread is no longer discernible even through as little as 5-mm. thickness of the liquid. The temperature at this stage (approximately the critical solution temperature) is recorded as the dicyclohexyl point, and the corresponding percentage of alcohol is read from the graph of Figure 2.
Acknowledgment The author wishes to thank the Dow Chemical Company of Midland, Mich., and Ralph P. Perkins of that organization, for special materials and technical service furnished during this study.
Literature Cited Adickes, F., Ber., 63, 2753 (1930). Andrews, L. W., J. Am. Chem. SOC.,30, 353 (1908). Crismer, L., Bull. SOC. chim. belg., 18, 18 (1904). Hell, C., and Schaal, O., Ber., 40, 4164 (1907). Osborne, N. S., McKelvy, E. C., and Bearce, H. W., Bur. Standards, Bull. 9, 344 (1913). Robertson, G. R., IND. ENQ.CHEM., ANAL.ED.,11, 464 (1939) Sabatier, P., and Murat, M., Ann. chim., 191 4, 301 (1915). Smoluchowski,M. v., Ann. Physik, [ I V ] 25, 219 (1908).
