Cryoscor>ic Studies L J
CONCENTRATED SOLUTIONS OF HYDROXY COMPOUNDS HALKEY K. ROSS Faint and Chemical Laboratory, Aberdeen Proving Ground, M d .
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RYOSCOPIC studies have generally been made in connection with molecular weight determinations. Such studies were invariably done with very dilute solutions, in which deviations from the ideal solution of nonionizable solutes are too small to merit consideration. In most cases an approximate equation, such as AT, = 1.86W/M, where W is the weight of solute in grams in 1000 grams of water and M is the gram molecular weight of the solute, was used for measuring the freezing point depression of water. Mathematical analysis of concentrated solutions showed considerable deviation from the ideal and these deviations have not been accounted for in most instances. In the present study the actual freezing point of solutions of high concentration is the factor of prime interest. The equation for the depression of the freezing points of solutions developed from the ClapeyronClausius equation may be stated in general terms as follows:
where To is the freezing point of the solvent in OK., T is the freezing point of the solution in OK., R is the gas constant, 1.99 calories per degree per mole, A H , is the molar heat of fusion for the solvent in calories, and z1 is the mole fraction of solvent. Except for the assumption that AH! is constant over the range of temperature involved, the equation is an exact solution for the depression of the freezing point, and should give accurate results if there are no deviations from the ideal solution. Ethylene glycol is of prime importance in the field of antifreeze. I t was therefore felt that it merited more detailed scrutiny, particularly because its behavior, in spite of many studies, did not appear to be a settled question. Previous work (1-3, 7-10) on the freezing point of ethylene glycol solutions was not complete and in most instances agreement between observers was only fair. Discrepancy between observers was probably caused in large measure by the supercooling tendencies of glycol solutions, and in order to minimize these errors a specially designed cryoscopic tube, which reduced supercooling from a normal of around 20" to 25' C. to maximum supercooling of around 6' C., was used in this investigation. In view of the fact that other hydroxy compounds are being made available for use as freezing point depressants for water, an investigation of their behavior was undertaken. This study only concerns itself with concentrations below the eutectic mixture, as they alone would be of practical interest.
APPARATUS AND PROCEDURE
The cryoscopic tube used in this investigation was a niodification of a tube developed by researchers a t the D u Pont Experimental Station. It consists of a jacketed tube with a sealed capillary attached to the inner tube and extending beyond the jacket a t the bottom, as shown in Figure 1. The jacket has a side arm which may be connected to a vacuum pump and evacuated to any degree desired for controlling the cooling rate. The cooling bath consisted of a strip-silvered or clear Dewar flask having an ethanol-dry ice mixture for freezing points above -50' C. and liquid air for freezing points below -50' C. Temperatures were measured by liquid in glass-type thermometers that had been calibrated against a platinum resistance thermometer. Mechanical agitation was used. The agitator consisted of a stainless steel wire approximately inch in diameter, twisted into a coil approximately 1 inch in diameter. Prior to freezing point determination of a given mixture, an approximate freezing point was determined, so that the cooline rate could be slowed down near the expected freezing point. The maximum cooling rate permitted near the freezing point was 1' C. drop per 7 minutes. Freezing points below -30" C. were determined with cooling \ rates of less than 1' C. drop per 10 minutes. The temperature at which the first crystal appeared was taken as ,--the freezing point, if no measurable rise in temperature occurred after the appearance of the crystal. When a rise in temperature was noted, the highest temperature attained prior to a resumption of a drop in temperature was taken as the freezing point. This procedure yielded results that were reproducible to within 0.5' C. by three different ohservprs.
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Figure 1. Cryoscopic Tube
In order to avoid errors in composition of the mixtures due to the hygroscopicity of the hydroxy compounds, the moisture content of mixtures wa8 verified by the method of Jordan and Hatch (6). RESULTS OF INVESTIGATION
For ethylene glycol the average of all readings taken, to the nearest 0.1' C., is plotted in Figure 2. No freezing points could be obtained with concentrations from about 61 to 80% ethylene glycol. The limits of this range approximately coincide with molar ratios of 2 moles of water to 1 mole of glycol and 1 mole of water to 1 mole of glycol. The plots of observed freezing points for the other materials studied are shown in Figures 3 to 12 along with the theoretical curves, where no hydration and complete hydration of each hydroxyl group occur. I n the case of l-propanol, 2-propano1, and hexylene glycol (2-methyl-2,4-pentanediol) in the higher concentrations the freezing points are higher than normal and these materials probably exist in polymeric form.
MATERIALS USED
ETHYLENE GLYCOL.The ethylene glycol used was Eastman Kodak Co.'s No. 133. I t had a freezing point of -12.5' C. and a boiling point range of 195' to 197" C. The materials used in this investigation were C.P. grade in the case of methanol, ethanol, 1-propanol, 2-propanol, and glycerol. The other materials are the best available grade. Prior to use in this investigation they were fractionated, and center cuts only were taken. WATER. Distilled water used was from a laboratory Barnstead stJilland had a conductivity of 3.8 X 10-6 reciprocal ohm per cm.
DISCUSSION
Figure 2 shows a plot of the observed freezing points of ethylene glycol solutions and the theoretical freezing points of an ideal
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vent (uncombined Tvater), a ~ the i nuriiber of moles ol' ethylene glycol dihydrate in any given quantity of mixture. Assunling that the equilibrium governing the hydration follom-s the law of mass action and is in accordancc with the reaction CZHI(OH)? 2 H 2 0f CzH,(OH),.2H20, equilibrium constants for the reaction at the various freezing points were computed using the equation ( M a- X * ) 4 ( & 1 %- 231*)2 Ki = (2) JI,
+
% ETHYLEM GLYCOL BY WEIGHT
Figure 2.
Freezing Points of Ethylene Glycol-Water RIixtures
I . Observed 11. Theoretical, assuming all glycol or water i n CzHa(OH)z.H?O form 111. Theoretical, assuming all glycol or water i n C2Ha(OH)z.ZNzO form IV. Theoretical, n o deviations from ideal solution
solution using Equation 1. The values used for 7'0 and AH, for water were 273" K. and 1435.8 calories for the low ranges of ethylene glycol concentration, \There water is the solvent and the solid phase consists of hexagonal ice stals. I n the higher ranges of ethylene glycol concentration, the solid phase is crystalline ethylene glycol in needle form and ethylene glycol is the solvent. The values for TO and A H j used in this latt.er range were 260.5" IIETHAXOL F R E E Z I S G POIN'TS AKD ;\fOLES O F >IETH.iXol, tion 17 t o all the materials involved in this study XOXOHYDRAT>E followed the same process as in the case of X u 41, S1, 1101es' of IIoles of methanol, which is therefore discuseed here in L,Iethanol, T, iiI?le CHaOH CHaOR*H20 Fraction of % Freezing Pt., detail. Table V shows the observed freezing pointe by Weight InT Uncomb. Hs0 in 100 G . in 100 G . M a - .li in O I
