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Solid-liquid phase equilibria in water + ethylene glycol
J. Chem. Thermodynamics1972, 4, 123-126
Solid-liquid phase equilibria in water-Iethylene glycol J. BEVAN OTT, J. REX GOATES, and JOHN D. LAMB
Department of Chemistry, Brigham Young University, Provo, Utah 84601, U.S.A. (Received 25 May 1971; in revised form 10 August 1971) Thermal methods were used to determine with high precision the solid-liquid phase equilibria diagram for water + ethylene glycol. A stable 1-1 solid hydrate forms. Because of supercooling, the hydrate is difficultto obtain; a cooling procedure was devised to initiate its crystallization. The two eutectics are at (224.12~: 0.05) and (230.224-0.05)K and mole fractions 0.288 and 0.541 of ethylene glycol, respectively. We also obtained a metastable phase diagram with a single metastable eutectic at (209.5 :[: 0.5)K and mole fraction 0.335 of ethylene glycol. 1. Introduction In spite of the widespread application of water+ethylene glycol liquid mixtures as antifreeze material, the solid-liquid phase diagram for this system is not well understood. Ewert (1) in 1937 reported three eutectics. Other investigators (2-9) have found only a simple eutectic. It was apparent that further investigation was needed to establish an accurate phase diagram. This paper reports the results of our investigation of this system. 2. Experimental MATERIALS J. T. Baker reagent grade ethylene glycol was further purified by fractional distillation in a 40 cm vacuum-jacketed distillation column. The distillation was performed at 75 °C under reduced pressure. Only the center third cut was used in the measurements. Doubly distilled water was used as the other component. The amount of liquidsoluble solid-insoluble impurity as estimated from the change in melting temperature with fraction melted indicates that the purified ethylene glycol is better than 99.99 moles per cent pure. The melting temperature of the water indicates that it is also better than 99.99 moles per cent pure. APPARATUS AND TEMPERATURE MEASUREMENT The freezing apparatus has been described previously in the literature. (1o) Temperatures were measured with a Leeds and Northrup calibrated strain-free platinum resistance thermometer in combination with a Leeds and Northrup high precision resistance
124
J.B. OTT, J. R. GOATES, AND J. D. LAMB
recorder. The thermometer (No. 1769133) was calibrated by Leeds and Northrup at the oxygen normal boiling temperature, the triple point temperature of water, and the melting temperatures of tin and zinc. The calibration was checked by us at the ice point (273.150K), the freezing temperature of mercury (234.29K), and the sodium sulfate decahydrate transition temperature (305.534K) at the beginning and at the conclusion of the measurements. In all cases, the values obtained agreed with the calibration to within 0.01 K. We estimate our temperatures to be accurate to within _ 0.02 K over the range of the experimental measurements. SAMPLE PREPARATION, MEASUREMENTS, AND ACCURACY Samples were prepared by weighing the components to +-0.1mg on an analytical balance. The water and ethylene glycol were stored in bottles sealed with rubber hypodermic caps. All transfer of materials including the injection of the sample into the freezing apparatus were made with a hypodermic syringe. The phase diagram was constructed principally from warming curves of temperature against time. In the portions of the diagram where supercooling was not a serious proble m, cooling curves were also used. Agreement between the two methods was generally within 0.03 K. The accuracy of the stable melting temperatures and the stable eutectic temperatures where stirring is possible is estimated to be within +-0.1 K. The metastable eutectie temperature was hard to hold at equilibrium. The uncertainty in its value is +-0.5 K. The metastable freezing temperatures were on a very steep portion of the freezing curves. As a result it was sometimes difficult to determine the melting break in the warming curve. Uncertainties in some of these points are +_0.2K. The freezing temperature of the pure ethylene glycol is estimated to be within ___0.05 K. The value obtained of 260.46 K was corrected to zero-impurity.
3. Results Table 1 summarizes the freezing results and figure 1 shows the resulting phase diagram. The stable diagram is indicated by the solid lines. Eutectics at (224.12+-0.05)K and (230.22+_0.05) K and mole fractions 0.228 and 0.541 of ethylene glycol, respectively, are evident in the diagram. The two eutectics result from the formation of a 1-1 ethylene glycol hydrate. The melting temperature of this hydrate occurring on the phase diagram at x = 0.5 is (230.55 +_0.05)K. It is not surprising that many workers failed to observe the hydrate. Direct cooling with stirring of the solutions gave the metastable melting temperatures as represented by the dotted line in figure 1 and the metastable eutectie at (209.5 +_0.5)K and mole fraction 0.335 of ethylene glycol. The result is a simple binary phase diagram with a single eutectie. We found a method which would nearly always result in the formation of the hydrate in the portion of the diagram where it is the stable phase. The sample was cooled without stirring to approximately 175 K. At this temperature the sample is very viscous but definitely still liquid. Starting the stirrer at this temperature resulted in the formation of a solid which was almost always the hydrate. We did not obtain the stable euteetie halts at mole fractions less than 0.11 and greater than 0.66 of ethylene glycol, probably because very little liquid is left in the
WATER + ETHYLENE GLYCOL
125
TABLE 1. Solid-liquid phase equilibrium in ethylene glycol + water; x denotes the mole fraction of ethylene glycol and Tm the melting temperature
v
Tm/K
Tm/K
x
stable
metastable
x
stable
metastable
0 0.0509 0.1130 0.1547 0.2094 0.2616 (0.288) 0.3097 0.3122 0.3287 (0.335) 0.3662 0.4276 0.4800
273.15 267.46 258.50 251.72 241.58 230.96 224.12 a 225.28 225.49 226.22 -228.03 229.89 230.50
-----~ -218.0 ° -211.7 c 209.5 -219.26 224.39
0.4934 0.5136 0.5320 (0.541) 0.5636 0.5758 0.5905 0.6612 0.6798 0.7083 0.7639 0.8529 0.9103 1
230.54 230.49 230.37 230.22 b 232.12 233.42 234.62 240.27 241.58 243.72 247.42 252.88 256.04 260.46 a
225.57 227.39 229.35
--
a Melting temperature corrected to zero impurity. b Eutectic temperature. Less accurate value on steep portion of the curve.
280
I
L
I "' I
1
I
I-
I
I
I
I
I
l
260
240
220
2oo I 0
0.2
I
I
I
0.4
0.6
0.8
1
FIGURE 1. Solid-liquid phase diagram for water + ethylene glycol; x denotes the mole fraction of ethylene glycol. ©, Stable melting temperature; O , metastable melting temperature.
126
J.B. OTT, J. R, GOATES, AND J. D. LAMB
sample when the eutectic temperature is reached. Supercooling of the remaining liquid prevents the second component from crystallizing. The absence of eutectic halts cannot result from solid solution formation since the metastable eutectic was obtained over the entire composition range. A comparison of our results with those of previous investigations shows significant differences. Ewert ~) is the only one of the previous investigators who observed more than one eutectic. His three eutectics at - 51.0, - 63.3, and - 49.4 °C are in qualitative agreement with the corresponding values we obtained ( - 49.03, - 63.6, and - 42.93 °C). However, Ewert ~) considered all three to be stable eutectics, resulting from the formation of two solid hydrates. On the water-rich side of the phase diagram, the freezing temperatures of all previous workers are roughly in agreement. Some of the temperatures scatter by as much as 3 K but the average fit is very close to our curve. The agreement is very poor, however, on the ethylene glycol-rich side of the diagram. The results of the few investigators who made measurements in this region are consistently lower than ours, the average difference being about 5 K. Some of Ewert's results (z), are as much as 10K low. His results are very erratic in the region of the hydrate, suggesting that perhaps he was sometimes measuring the melting temperature of the hydrate and at other times the metastable melting temperature of ethylene glycol. The uncertainty of the melting temperatures of the other investigators on the ethylene glycol-rich side of the diagram can probably be attributed to supercooling. Mironov and Deryabina ~9) reported that the precise melting temperature determination was made very difficult as a result of supercooling for mixtures with mole fractions of ethylene glycol greater than 0.2. To overcome this problem, we measured melting temperatures, rather than freezing temperatures. The values we obtained were very definite and reproducible. The authors acknowledge the support given this research project by the Brigham Young University Research Division. The help of Miss Elizabeth Delawarde with the freezing measurements is also appreciated. REFERENCES 1. Ewert, M. Bull. Soc. Chim. Belg. 1937, 46, 90. 2. Olsen, J. C.; Brunjes, A. S.; Olsen, J. W. Ind. Eng. Chem. 1930, 22, 1315. 3. Conrad, F. H.; Hill, E. F.; Ballman, E. A. Ind. Eng. Chem. 1940, 32, 542. 4. Spangler, J. A.; Davies, E. C. H. Ind. Eng. Chem. Anal. Ed. 1943, 15, 96. 5. Clendenning, K. A. Can. J. Res. See. F 1946, 24, 249. 6. Tombaugh, R. M.; Choguill, H. S. Trans. Karts. Acad. Sci. 1951, 54, 411. 7. Ross, H. K. Ind. Eng. Chem. 1954, 46, 601. 8. Dykyi, J. ; Kuska, V.; Seprakova, M. Chem. Zvesti 1956, 10, 193. 9. Mironov, K. E.; Deryabina, L. D. Fiz. Khim. Anal. Tr. Yubileinoi Konf. 1960, 283 (published 1963). Also published in J. AppL Chem. USSR 1962, 35, 1285. 10. Goates, J. R.; Ott, J. B.; Budge, A. H. J. Phys. Chem. 1961, 65, 2•62.
Solid-liquid phase equilibria in water-Iethylene glycol J. BEVAN OTT, J. REX GOATES, and JOHN D. LAMB
Department of Chemistry, Brigham Young University, Provo, Utah 84601, U.S.A. (Received 25 May 1971; in revised form 10 August 1971) Thermal methods were used to determine with high precision the solid-liquid phase equilibria diagram for water + ethylene glycol. A stable 1-1 solid hydrate forms. Because of supercooling, the hydrate is difficultto obtain; a cooling procedure was devised to initiate its crystallization. The two eutectics are at (224.12~: 0.05) and (230.224-0.05)K and mole fractions 0.288 and 0.541 of ethylene glycol, respectively. We also obtained a metastable phase diagram with a single metastable eutectic at (209.5 :[: 0.5)K and mole fraction 0.335 of ethylene glycol. 1. Introduction In spite of the widespread application of water+ethylene glycol liquid mixtures as antifreeze material, the solid-liquid phase diagram for this system is not well understood. Ewert (1) in 1937 reported three eutectics. Other investigators (2-9) have found only a simple eutectic. It was apparent that further investigation was needed to establish an accurate phase diagram. This paper reports the results of our investigation of this system. 2. Experimental MATERIALS J. T. Baker reagent grade ethylene glycol was further purified by fractional distillation in a 40 cm vacuum-jacketed distillation column. The distillation was performed at 75 °C under reduced pressure. Only the center third cut was used in the measurements. Doubly distilled water was used as the other component. The amount of liquidsoluble solid-insoluble impurity as estimated from the change in melting temperature with fraction melted indicates that the purified ethylene glycol is better than 99.99 moles per cent pure. The melting temperature of the water indicates that it is also better than 99.99 moles per cent pure. APPARATUS AND TEMPERATURE MEASUREMENT The freezing apparatus has been described previously in the literature. (1o) Temperatures were measured with a Leeds and Northrup calibrated strain-free platinum resistance thermometer in combination with a Leeds and Northrup high precision resistance
124
J.B. OTT, J. R. GOATES, AND J. D. LAMB
recorder. The thermometer (No. 1769133) was calibrated by Leeds and Northrup at the oxygen normal boiling temperature, the triple point temperature of water, and the melting temperatures of tin and zinc. The calibration was checked by us at the ice point (273.150K), the freezing temperature of mercury (234.29K), and the sodium sulfate decahydrate transition temperature (305.534K) at the beginning and at the conclusion of the measurements. In all cases, the values obtained agreed with the calibration to within 0.01 K. We estimate our temperatures to be accurate to within _ 0.02 K over the range of the experimental measurements. SAMPLE PREPARATION, MEASUREMENTS, AND ACCURACY Samples were prepared by weighing the components to +-0.1mg on an analytical balance. The water and ethylene glycol were stored in bottles sealed with rubber hypodermic caps. All transfer of materials including the injection of the sample into the freezing apparatus were made with a hypodermic syringe. The phase diagram was constructed principally from warming curves of temperature against time. In the portions of the diagram where supercooling was not a serious proble m, cooling curves were also used. Agreement between the two methods was generally within 0.03 K. The accuracy of the stable melting temperatures and the stable eutectic temperatures where stirring is possible is estimated to be within +-0.1 K. The metastable eutectie temperature was hard to hold at equilibrium. The uncertainty in its value is +-0.5 K. The metastable freezing temperatures were on a very steep portion of the freezing curves. As a result it was sometimes difficult to determine the melting break in the warming curve. Uncertainties in some of these points are +_0.2K. The freezing temperature of the pure ethylene glycol is estimated to be within ___0.05 K. The value obtained of 260.46 K was corrected to zero-impurity.
3. Results Table 1 summarizes the freezing results and figure 1 shows the resulting phase diagram. The stable diagram is indicated by the solid lines. Eutectics at (224.12+-0.05)K and (230.22+_0.05) K and mole fractions 0.228 and 0.541 of ethylene glycol, respectively, are evident in the diagram. The two eutectics result from the formation of a 1-1 ethylene glycol hydrate. The melting temperature of this hydrate occurring on the phase diagram at x = 0.5 is (230.55 +_0.05)K. It is not surprising that many workers failed to observe the hydrate. Direct cooling with stirring of the solutions gave the metastable melting temperatures as represented by the dotted line in figure 1 and the metastable eutectie at (209.5 +_0.5)K and mole fraction 0.335 of ethylene glycol. The result is a simple binary phase diagram with a single eutectie. We found a method which would nearly always result in the formation of the hydrate in the portion of the diagram where it is the stable phase. The sample was cooled without stirring to approximately 175 K. At this temperature the sample is very viscous but definitely still liquid. Starting the stirrer at this temperature resulted in the formation of a solid which was almost always the hydrate. We did not obtain the stable euteetie halts at mole fractions less than 0.11 and greater than 0.66 of ethylene glycol, probably because very little liquid is left in the
WATER + ETHYLENE GLYCOL
125
TABLE 1. Solid-liquid phase equilibrium in ethylene glycol + water; x denotes the mole fraction of ethylene glycol and Tm the melting temperature
v
Tm/K
Tm/K
x
stable
metastable
x
stable
metastable
0 0.0509 0.1130 0.1547 0.2094 0.2616 (0.288) 0.3097 0.3122 0.3287 (0.335) 0.3662 0.4276 0.4800
273.15 267.46 258.50 251.72 241.58 230.96 224.12 a 225.28 225.49 226.22 -228.03 229.89 230.50
-----~ -218.0 ° -211.7 c 209.5 -219.26 224.39
0.4934 0.5136 0.5320 (0.541) 0.5636 0.5758 0.5905 0.6612 0.6798 0.7083 0.7639 0.8529 0.9103 1
230.54 230.49 230.37 230.22 b 232.12 233.42 234.62 240.27 241.58 243.72 247.42 252.88 256.04 260.46 a
225.57 227.39 229.35
--
a Melting temperature corrected to zero impurity. b Eutectic temperature. Less accurate value on steep portion of the curve.
280
I
L
I "' I
1
I
I-
I
I
I
I
I
l
260
240
220
2oo I 0
0.2
I
I
I
0.4
0.6
0.8
1
FIGURE 1. Solid-liquid phase diagram for water + ethylene glycol; x denotes the mole fraction of ethylene glycol. ©, Stable melting temperature; O , metastable melting temperature.
126
J.B. OTT, J. R, GOATES, AND J. D. LAMB
sample when the eutectic temperature is reached. Supercooling of the remaining liquid prevents the second component from crystallizing. The absence of eutectic halts cannot result from solid solution formation since the metastable eutectic was obtained over the entire composition range. A comparison of our results with those of previous investigations shows significant differences. Ewert ~) is the only one of the previous investigators who observed more than one eutectic. His three eutectics at - 51.0, - 63.3, and - 49.4 °C are in qualitative agreement with the corresponding values we obtained ( - 49.03, - 63.6, and - 42.93 °C). However, Ewert ~) considered all three to be stable eutectics, resulting from the formation of two solid hydrates. On the water-rich side of the phase diagram, the freezing temperatures of all previous workers are roughly in agreement. Some of the temperatures scatter by as much as 3 K but the average fit is very close to our curve. The agreement is very poor, however, on the ethylene glycol-rich side of the diagram. The results of the few investigators who made measurements in this region are consistently lower than ours, the average difference being about 5 K. Some of Ewert's results (z), are as much as 10K low. His results are very erratic in the region of the hydrate, suggesting that perhaps he was sometimes measuring the melting temperature of the hydrate and at other times the metastable melting temperature of ethylene glycol. The uncertainty of the melting temperatures of the other investigators on the ethylene glycol-rich side of the diagram can probably be attributed to supercooling. Mironov and Deryabina ~9) reported that the precise melting temperature determination was made very difficult as a result of supercooling for mixtures with mole fractions of ethylene glycol greater than 0.2. To overcome this problem, we measured melting temperatures, rather than freezing temperatures. The values we obtained were very definite and reproducible. The authors acknowledge the support given this research project by the Brigham Young University Research Division. The help of Miss Elizabeth Delawarde with the freezing measurements is also appreciated. REFERENCES 1. Ewert, M. Bull. Soc. Chim. Belg. 1937, 46, 90. 2. Olsen, J. C.; Brunjes, A. S.; Olsen, J. W. Ind. Eng. Chem. 1930, 22, 1315. 3. Conrad, F. H.; Hill, E. F.; Ballman, E. A. Ind. Eng. Chem. 1940, 32, 542. 4. Spangler, J. A.; Davies, E. C. H. Ind. Eng. Chem. Anal. Ed. 1943, 15, 96. 5. Clendenning, K. A. Can. J. Res. See. F 1946, 24, 249. 6. Tombaugh, R. M.; Choguill, H. S. Trans. Karts. Acad. Sci. 1951, 54, 411. 7. Ross, H. K. Ind. Eng. Chem. 1954, 46, 601. 8. Dykyi, J. ; Kuska, V.; Seprakova, M. Chem. Zvesti 1956, 10, 193. 9. Mironov, K. E.; Deryabina, L. D. Fiz. Khim. Anal. Tr. Yubileinoi Konf. 1960, 283 (published 1963). Also published in J. AppL Chem. USSR 1962, 35, 1285. 10. Goates, J. R.; Ott, J. B.; Budge, A. H. J. Phys. Chem. 1961, 65, 2•62.