THE SOLUBILITY O F THIOUREA IN WATER, METHANOL, AND ETHANOL LOUIS SHNIDMAN
Chemical Laboratory of the University of Rochester, Rochester, New York Received January 88, 1933 INTRODUCTION
Solubility data on thiourea, an analogue of urea, in water is limited to a few isolated temperatures (4, 8, 11, 12). No previous determinations on the solubility of thiourea in water, in methanol, or in ethanol over a temperature range have been made. Such data are of some interest from the standpoint of a study of concentrated solutions-their ideality, non-ideality, and the like. MATERIALS
The thiourea used was obtained from the Eastman Kodak Company and was their best grade. It was purified by recrystallization three times from water. The final product had a melting point of 181.4"C., which was slightly below that recorded in the literature (6). A portion of the above thiourea was further purified by two recrystallizations from C.P. methanol. The final product showed a melting point of 181.4"C., which was the same as that of the sample after the third water recrystallization. The purity of the thiourea was checked by chemical analysis for its sulfur, nitrogen, and thiourea content. It was felt that the data from analysis would be more reliable than the melting point determinations, because of the decomposition of thiourea at its melting point. The first sample showed an average sulfur content of 42.10 per cent for twelve determinations (eight by a sodium peroxide oxidation (3), and four by a nitric acid-bromine oxidation (13, 19)), with maximum deviations from the average of 0.07 per cent. The second sample (methanol recrystallization) showed an average sulfur content of 42.14 f 0.04 per cent for three determinations (3). The nitrogen content of the thiourea samples, determined by the Gunning method (2) showed 36.79 += 0.03 and 30.80 & 0.2 per cent. The thiourea content, determined by the method employed by Kappana (7) as recommended by Reynolds and Werner (14) and modified by Werner (21), showed 99.97 f 0.04 per cent and 99.98 =t 0.02 per cent for the water-recrystallized and methanol-recrystallized samples. 693
694
LOUIS SHNIDMAN
Boiled distilled water was employed in all determinations where water was the solvent. Baker's c . P. absolute methanol was twice refluxed with lime for six hours and then distilled through a six-bulb Le Bel-Henninger column. That portion boiling within 64.63 f 0.01OC. (corr.) was collected and used in the solubility determinations. Pure grain alcohol was twice refluxed with lime for over six hours and distilled as under methanol. The distillate boiling within 78.43 f 0.02"C. (corr.) was collected and used for the solubility determinations. The procedure for ethanol was essentially that of Merriman (9). PROCEDURE AND APPARATUS
The synthetic method of Alexejew (1) was employed in making the solubility determinations. This method consisted in heating weighed quantities of solvent and solute in a sealed tube, rotated in a water bath, and noting the temperature at which the solid phase had nearly disappeared. In recent years other investigators (16, 17, 18, 20) have found this method to be accurate and a reliable means for determining the solubility of solids in various solvents. As pointed out by these investigators, care must be taken in attaining true equilibrium conditions at the solubility temperature; this can ordinarily be obtained through slow heating and using low rates of temperature rise. A temperature rise of O.0loC. per minute was used in some cases, though in many cases thermostating for a period of time was employed. Sunier (17) pointed out that with a rate of heating of 0.01"C. per minute, results well within 0.1OC. of the true solubility temperature were obtained for naphthalene in aliphatic alcohol systems. The author feels that this same degree of accuracy would hold for the systems studied in the present research. The results obtained by the synthetic method are necessarily under different pressures. This question has been given extensive theoretical and practical consideration. Although it is possible that extremely high pressures would exert an influence, statements (15) are found that under ordinary conditions in which the pressure does not exceed ten atmospheres, no noticeable effect on solubility would be produced. No abnormalities in the solubility curves obtained by this method have been attributed to variation in pressure. The apparatus used has been described earlier (17). Other investigators (16, 17, 18, 20) have shown that the size of the crystal is of importance in attaining true equilibrium conditions. The method was that ordinarily employed and consisted in rapidly heating the tube to a temperature where all the solute dissolved, and then cooling rapidly with vigorous shaking. Thin wall Pyrex tubes of 7 mm. internal diameter and approximately 14 cm. long were used. The tubes were cleaned with sulfuric-chromic acid cleaning solution, rinsed with distilled water, and then heated over an open
695
MOLUBILITY OF THIOUREA
Bunsen burner to dull redness, placed in a desiccator, allowed to cool, and weighed. For marking the respective tubes, they were first coated with paraffin, the desired identification marks inscribed with a steel pen, and hydrofluoric acid (48 per cent) added. After five minutes, the paraffin was dissolved off with acetone, and the tube prepared as described above. I n these determinations a thermometer certified by the Bureau of Standards was employed. The thermometer could be read to f 0.01"C. with the aid of a magnifying glass. The temperatures recorded should be accurate to ~ 0 . 0 2 ~ C . Some preliminary data was obtained for the possible decomposition of thiourea in water solutions. The respective samples of these materials TABLE 1 Solubility of thiourea (recrystallized from water) i n water THIOUREA
ROLUTBILITI TZMPERATURB
weight per cent
mol fraction
degrees C .
9.0 12.06 14.60 17.73 20.83 39.05 23.00 27.74 30,B 35.27 44.53 48.61 55.78 59.87
0.0229 0.0314 0.0389 0.0486 0.0586 0.1317 0.0658 0.0833 0.0937 0.1143 0.1598 0.1829 0.2299 0.2610
12.43 19,88 25.11 30.38 35.23 57.05 38.31 44,30 47.34 53.02 62.85 67.43 75.96 81.28
SOLVENT
THIOVREA
grama
grama
0.2968 0.4050 0.5113 0.6369 0.7855 1.2715 0.9010 1.1094 1,2975 1.0700 1.5590 1.8848 1.9197 2.2502
2.9993 2.9543 2.9899 2.9550 2.9862 1.9846 3.0165 2.8897 2,9727 1.9636 1.9423 1.9927 1.5217 1.5084
THIOUREA
-
were heated with water a t 100°C. for two hours and then tested for ammonium thiocyanate. It was found that the thiourea sample showed an absence of ammonium thiocyanate, indicating that under these conditions no conversion had taken place. EXPERIMENTAL RESULTS
The results of the various thiourea solubility determinations are presented in tables 1 and 2. Concentrations have been calculated and tabulated on both the mol fraction and weight per cent basis. The data were plotted on a large scale according to the method of Hildebrand and Jenks (5), as the log N z versus 1000/T. The solubilities a t rounded temperatures were read off and are given in table 3. THE JOVRNAL OF PHYSICAL CHEMISTRY, VOL. XXXVII, NO.
6
696
LOUIS SHNIDMAN
TABLE 2 Solubility of thiourea (recrystallized f r o m water and methanol) SOLUBILITY THIOUREA
SOLVENT
THIOUREA
THIOUREA
TEMPERATURE
Solubility in water grams
I
0.5199 0.4905 0.9347 1.0462 1.9521
grams
weight per cent
mol fraction
degrees C.
2.9260 2.7676 2.6717 2.8386 1.9358
15.09 15.05 25.92 26.93 50.21
0.0404 0.0402 0.0766 0.0800 0.1925
25.90 26.02 42 .OO 43.11 69.26
Solubility in methanol 0.2101 0.3108 0.4064 0.5046
1,5484 1.5879 1.5409 1,5500
11.95 16.37 22.01 24,56
0.0540 0,0760 0.0999 0.1205
25.11 40.80 53.76 62 .OO
0.0864 0.1107 0.0873 0.1426 0.1465 0.2135 0.2457
2.3103 2.2510 1.5303 2,1111 1.8861 2,3031 2.2585
3.61 4.69 5.40 6.33 7.21 8.48 9.81
0.0221 0.0289 0.0334 0.0393 0.0449 0.0531 0.0618
20.25 31.99 37.69 45.14 51.22 58.05 64.77
TABLE 3 Solubility o j thiourea i n water and alcohols at rounded temperatures (Expressed in mol fractions of thiourea) TEMPERATURE
I
WATER
1
METHANOL
1
ETHANOL
degrees C.
20 25 30 35 40 45 50
55 60 65 70 75 80 85
'
0.0314 0.0389 0.0478 0.0583 0.0707 0.0856 0.1023 0.1221 0.1450 0.1700 0.1968 0.2243 0.2533 0.2860
0.0481 0.0539 0.0603 0.0673 0.0748 0.0833 0.0922 0,1025 0.1150 0.1292 0.1452
0.0220 0.0247 0.0277 0.0312 0.0349 0.0392 0,0438 0.0492 0.0554 0.0622 0.0701
......
697
SOLUBILITY OF THIOUREA DISCUSSION O F RESULTS
The results of the solubility determinations in water were compared with those published by earlier workers. Dehn (4), some years ago, made a single qualitative determination of the solubility of thiourea in water a t a temperature of 20°C. to 25°C. This result, as the author states, is not of a high degree of accuracy. Kettner (8) somewhat later determined the solubility of thiourea in water over a temperature range of 0°C. to 145°C. His determinations were made a t 0", 50"' 56.5", 97.2", 120.6", and 145°C. Hence, comparison with the author's data could be made a t two points only, viz., 50°C. and 56.5"C. These determinations show some variance from the ZB
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SOLUBILITY O F THIOUREA I N WATER
results obtained in the present research, especially the value a t 50°C. Oliveri-Mandala (11, 12) more recently determined the solubility of thiourea in water a t temperatures from 10°C. t o 25°C. The results a t 20°C. and 25°C. when compared to the present research show considerable variation, especially the value a t 25°C. The results of the above-mentioned investigators are compared in graphic form with the author's work in figure 1, where the data are plotted according t o the method of Hildebrand and Jenks ( 5 ) . Over a temperature range of 2OOC. to 60"C., it is found that a straight line results; above 60°C. departure from the straight-line function increases and becomes greatest a t the higher temperatures.
1
698
LOUIS SHNIDMAN
Data for a sample of thiourea recrystallized from water and methanol are also presented. Here it is observed that the solubility determinations coincide with those already determined on the sample recrystallized from water only. It is thus apparent that the solubility of thiourea was not affected by the nature of the solvent from which it was recrystallized. A study of tables 1 and 2 shows the deviation obtained relative to the solubility of thiourea on the two samples used, viz., that recrystallized from water and that recrystallized from water and methanol. The fact that the results on the solubility determinations of the two samples of thiourea show such close agreement leads to the conclusion that these determinations are more accurate than those of previous published work. It is felt that the foregoing results are accurate to well within f 0.1"C. of the true solubility temperature. This figure represents the maximum deviation, whereas some of the determinations deviate much less. Between the temperature of 20°C. to 60"C., where the straight-line function exists, the mean deviation in solubility temperature for both samples of thiourea used was 0.07"C. It is believed that the solubility results obtained with both samples are of the same degree of accuracy. For that reason no distinction in the final results is made. The values of the solubility of the respective samples of thiourea were read off from the large plot previously referred to. From this plot the equation of the straight line 4.029. was determined, and found to be loglo N = -1621.6 (1/T) This is valid over the temperature range 20°C. to 60°C. It gives results to within one part per thousand of the values obtained from the plot. The fact that the log N versus 1/T curve is a stright line over the temperature range of 20°C. to 60°C. leads one to inquire whether or not ideal solutions are encountered in this range of temperature. To attempt to extrapolate log N = 0 is difficult, because it is known that above 60°C. the solubility of thiourea departs from a straight-line function, with increasing rapidity as the temperature is elevated. However when Kettner's (8) data at higher temperatures are plotted with those of the author, a smooth curve results and on extrapolation the curve almost intersects a t the absolute melting point, i.e., where log N = 0. The shape of the curve indicates that a reverse S form of curve discussed by Mortimer (10) is not formed. No data seems to be available concerning the latent heat and fusion of thiourea (no doubt because of the molecular transformation a t its melting point). Hence, a comparison of the experimental and ideal shape of the line was not possible. It may be said that when the slope, 1621.6 (assumed to be constant over the entire range, which is not true to the fact) is multiplied by 4.583, a value of approximately 7400 calories is obtained. If the solution were ideal, this value would represent the latent mol heat of fusion of thiourea. To determine further whether or not ideal solutions were formed by
+
699
SOLUBILITY OF THIOUREA
thiourea in water, a search for vapor pressure data was made. Apparently no data on the vapor pressure of thiourea in water solutions is available. ,Hence no comparison or calculation can be made to determine whether or not deviation from Raoult's law exists. The solubility results of thiourea in methanol and in ethanol are found in table 2. These results are also presented in graphic form in figure 2. It is observed that between the temperature range of 20°C. to 70°C., thiourea in methanol and ethanol does not form a straight line when plotted according to the method of Hildebrand and Jenks ( 5 ) . One would expect from the nature and chemical constitution of thiourea that it would be readily soluble in water, less soluble in methanol, and least soluble in
LOG MOL FRACVON 7HlO-UffEA
FIU.2. SOLUBILITY OF THIOUREA I N WATIR,
IN
METHANOL, AND
IN
ETHANOL
ethanol. However, this is not the case. An unexpected phenomenon occurs, as shown in figure 2. The curves of the solubility of thiourea in water and methanol cross at about 43"C., i.e., a t temperatures below 43°C. thiourea is more soluble in methanol than in water, and at temperat,ures above that point is more soluble in water than in methanol, when expressed on the mol fraction basis. The exact point where thiourea in methanol crosses the thiourea and water curve is where the mol fraction is equal to 0.08017, which is equivalent to a temperature of 43.24"C. The solubility of thiourea in ethanol is as would be expected, viz., less than in water or methanol. No previous determinations on the solubility of thiourea in water, in methanol, or in ethanol over the temperature range studied have been made.
700
LOUIS SHNIDMAN SUMMARY
1. Two samples of thiourea have been carefully purified and analyzed. 2. Nineteen determinations of the solubility of thiourea in water have been made, using the synthetic method in the temperature interval 15°C. to 80°C.; the precision in these runs is much higher than any previously
published. The data may be accurately represented by the equation log,, N
-1621.6 (l/T)
+ 4.029
in the temperature interval 20°C. to 60°C. 3. The solubility of thiourea in methanol and ethanol has been det,ermined by the synthetic method from 20°C. to 70°C. The author wishes to express his gratitude to Prof. A. A. Sunier for his interest and advice during the progress of this work, and to the Rochester Gas and Electric Corporation for the use of their equipment. REFERENCES (1) ALEXEJEW:Wied. Ann. 28, 305 (1886). (2) Assoc. Official Agr. Chem., Methods of Analysis, p. 8 (1925). (3) Assoc. Official Agr. Chem., Methods of Analysis (Modification of Total Sulfur Method), p. 68 (1925). (4) DEHN: J. Am. Chem. SOC.39,1400 (1917). (5) HILDEBRAND A N D JENKS: J. Am. Chem. Soc. 42,2180 (1920). (6) International Critical Tables, Vol. I, p. 177. McGraw-Hill Book Co., New York (1926). (7) LUNGE:Coal Tar and Ammonia, Vol. 111, p. 1310, 5th ed. D. Van Nostrand Co., New York (1916). (8) KETTNER:Dissertation, Amsterdam, 1919. (9) MERRIMAN: J. Chem. SOC. 103,629 (1913). (10) MORTIMER:J. Am. Chem. SOC.44, 1416 (1922); 46, 633 (1923). E.: Gam. chim. ital. 66, 889 (1926). (11) OLIVERI-MANDALA, (12) OLIVERI-MANDALA, E.: Gam. chim. ital. 60, 872 (1930). (13) PFEIFFER: In Lunge and Berl’s Techn. Chem. Unt. Meth. 3,373, 6th ed. (14) REYNOLDS A N D WERNER:J. Chem. SOC.83, 1 (1903). (15) SEIDELL:Solubilities of Inorganic and Organic Substances. D. Van Nostrand Co., New York (1919). (16) SHNIDMAN AND SUNIER:J. Phys. Chem. 36, 1232 (1932). (17) SUNIER:J. Phys. Chem. 34,2582 (1930). (18) SUNIERAND ROSENBLUM: J. Phys. Chem. 32, 1049 (1928). (19) TREADWELL-HALL: Analytical Chemistry, Vol. 11, p. 340, 5th ed. (20) WARD:J. Phys. Chem. 30, 1316 (1926). (21) WERNER:J. Chem. SOC.99,2166 (1912).
