T H E SOLUBILITY OF AMMONIUM THIOCYANATE 1N WATER, METHANOL, AND ETHANOL LOUIS SHNIDMAN Department of Chemistry, University of Rochester, Rochester, Xe w J'ork Received December 28, 1933 INTRODUCTION
The solubility of ammonium thiocyanate in water a t low temperatures has been determined by several investigators (3, 6, 9, 10). No previous determinations of the solubility of ammonium thiocyanate in methanol or in ethanol have been made. These data are of some interest from the standpoint of a study of concentrated solutions-iheir ideality, nonideality, and the like. MATERIALS
The ammonium thiocyanate used was Baker's C.P. grade. The animonium thiocyanate as received shom-ed a melting point of 149.0°C., but as some solid insoluble materials were present, it was further purified by two recrystallizations from methanol according to Lunge (7), who states that the resulting product is non-hygroscopic, This was found to be the case. The melting point of the ammonium thiocyanate after the above treatment was 149.7"C., which agrees closely with the value 149.6OC. recorded in the literature (5). It was found necessary to employ all glass or porcelain apparatus in every stage of the process and to avoid possible contact with iron in any form. Solid ammonium lhiocyanate or its solution on contact with even an iron spatula gave a pink colored product, no doubt due to theformation of iron thiocyanate, Fe(SCN)3. By using all glass or porcelain apparatus, the resulting recrystallized salt was absolutely white and free from color. It was further observed that when the ammonium thiocyanate was colored with ferric thiocyanate, and the product was heated at 60°C. for a period of hours, the color disappeared. In addition t o the melting point determination, the purity of the ammonium thiocyanate was checked by chemical analysis for iis ammonia and thiocyanate content. The ammonia was determined by distillation (2) and showed an average of 22.40 per cent (three determinations) with a maximum deviation o l 0.06 per cent. The slightly higher ammonia content than the theoretical may be due t o the possible liberation of am901
902
LOUIS SHNIDMAN
monia from the thiocyanate group as mentioned by Pfeiffer (8). The thiocyanate content was determined by Volhard's method (15) and showed an average of 76.29 per cent (three determinations) with a maximum deviation of 0.05 per cent. The water, methanol, and ethanol used in the present research have been described previously (11). PROCEDVRE AND APPARATUS
The synthetic method of Alexejew (1) was employed in making the solubility determinations. The procedure and apparatus used in the present research have been described earlier (11, 3 2, 14). Some preliminary data were obtained for the possible conversion of ammonium thiocyanate in water solutions. Samples were heated with water a t 100°C. for two hours and then analyzed for ammonium thiocyanate. The ammonium thiocyanate solution when analyzed indicated practically 100 per cent ammonium thiocyanate. This indicated that the procedure had not affected the ammonium thiocyanate. In the determinations that follow the solubility temperatures were considerably lower than 100°C. and were always kept for less than two hours at their respective solubility temperatures. This preliminary data would indicate that no change in the nature of the solute had taken place during the solubility temperature determinations. These findings are in accordance with the results of Waddell (16), who showed that ammonium thiocyanate is very slowly if a t all changed a t temperatures below llO"C., whether heated alone or in aqueous solution. It had been pointed out earlier (11) that precautions to insure the presence of small crystals are one of the means of obtaining true equilibrium conditions. An apparent anomaly to this criterion was found in the case of the ammonium thiocyanate-ethanol system. In this system fine crystals could be obtained without difficulty. However, the crystals were so fine as to give a colloidal turbid opalescent solution with the result that, when the ordinary procedure for solubility determinations was followed, the solubility temperatures were overstepped by one or more degrees. To overcome this difficulty, it was found necessary to cool the solution slowly so as to favor the formation of large crystals. Starting with large crystals and thermostating for a period of ten minutes, it was found that the true solubility temperature could be obtained. EXPERIMENTAL RESULTS
The results of the various ammonium thiocyanate solubility determinations are presented in tables 1, 2, and 3. 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 Hilde-
903
SOLUBILITY O F AMMONIUM THIOCYANATE
TABLE 1 Solubility of ammonium thiocyanate in water (Recrystallized from methanol) SOLVENT
NHaSCN
NHaSCN
SOLUBILITY TEXPERATURE
grams
grams
weight per cent
mol fraction
degrees C.
1.7853 1.8507 1.6550 1,9970 1.9798 2.3633 2.1629 2.5037 2.8356 2.2651 2.3848 2.2045 2.5536
1.2119 1.1247 0.8933 1.0319 0.8954 1.0105 0.8058 0.8557 0.9465 0.6909 0,6619 0.5451 0.5710
59.57 62.20 64.95 65.93 68.86 70.05 72.86 74.53 74.94 76.63 78.53 80.18 81.73
0.2588 0.2803 0.3043 0.3138 0.3435 0.3567 0.3885 0.4092 0.4154 0.4376 0.4637 0,4890 0,5145
13.00 18.99 26.33 28.82 36.36 39.44 46.92 51.37 52.50 57.23 62.46 67.21 71.53
NHaSCS
TABLE 2 Solubility of ammonium thiocyanate in methanol NHaSCN
NHaSCPi
SOLVENT
NHaSCN
-
SOLUBILITY TEMPERATURE
grams
grams
&eight per cent
mol fraction
degrees C.
0.8746 1.0754 1.3375 0.9994 1,3020
1.4824 I.6099 1.6649 1.0279 1,0850
37. I1 40.05 44.70 49 30 54 55
0.1989 0.2194 0.2538 0.2904 0.3356
24.58 32.94 44.80 54.76 64.55
TBBLE 3 Solubility of ammonium thiocyanate in ethanol NHaSCN
SOLVENT
PiHaSCS
NH4SCPi
SOLUBILIIY TEMPERATURE
grams
grams
u e i g h t per cent
mol fraction
degrees 6.
0.3867 0.6283 0.6418 0,4571 0.6044 0,5958
1 6416 2.2884 2 2545 1.4915 1.6572 1.4855
19.07 21.54 22.16 23.46 26.72 28 63
0.1247 0.1421 0.1469 0,1565 0.1807 0.1953
18.45 33.25 36.93 43.36 57.62 64.20
904
LOUIS SHNIDMAN
brand and Jenks (4), as the log Nz vs. 1000/T. The solubilit~iesat rounded temperatures were read off and are given in table 4. DISCUSSION O F RESULTS
The results of the solubility determinations of ammonium thiocyanate in water were compared with those published by Rudorff (9, lo), Smitts and Kettner (13), Iiettner (6), and Foote (3). This comparison is shown in graphic form in figure 1, where the data are plotted according to the method of Hildebrand and Jenks (4). It is felt that the foregoing results are accurate to well within fO.lO°C. of the true solubility temperature, As shown in figure 1, the curve of the author's data is not a straight line function. If an attempt is made t o extrapolate the curve to the point TABLE 4 Solubility of ammonium thiocyanate in water and alcohols at rounded temperatuws (Expressed in mol fractions of ammonium thiocyanate) ROLUBILITY OF AMMONIUM THIOCYANATE I N TEJiPERATURE
Water
Methanol
Ethanol
0,2831 0.2999 0.3183 0,3381 0.3583 0,3803 0.4031 0.4266 0.4512 0.4764 0.5053
0.1886 0,1999 0.2119 0.2249 0,2388 0.2545 0.2719 0.2914 0.3136 0.3373 0.3631
0.1264 0.1322 0.1380 0.1443 0.1512 0.1591 0,1670 0.1757 0.1859 0.1972 0.2095
degrees C .
20 25 30 35 40 45 50 55 60 65 70
when the log N = 0, a value below the absolute melting point is obtained, indicating the ammonium thiocyanate does not form an ideal solution in water. Calculations from the vapor pressure data of Foote (3) also indicated that ammonium thiocyanate does not form an ideal solution in water. The solubility results of ammonium thiocyanate in methanol and ethanol are found in tables 2 and 3. These results are also presented in gra,phic form in figure 2. It is observed that in the temperature range studied, 20 to 7OoC., ammonium thiocyanate in water, methanol, and ethanol does not form straight line functions when plotted as in figure 2 . Furthermore, it is seen that ammonium thiocyanate is highly soluble in water, moderately soluble in methanol, and least soluble in ethanol. When carrying out the solubility determinations of ammonium hhiocy-
SOLUBILITY O F AMMONIUM THIOCYANATE
905
-
-
JQ
JI
-40
-30 -25
-20
LDb MOL FRACTIOPJ AMMON/UM ?Z'~OCYANATE
FIG.2. SOLUBILITY OF AMMONIUM THIOCYANATE IS WATER,IX METHANOL, ASD ETHE~OL
IN
906
LOUIS SHNIDMAN
anate in ethanol, it was observed that the tubes as prepared at first were perfectly clear and colorless, but on standing and on exposure to light, in the heating period they turned yellow, and the yellow coloration increased with time. This observation would indicate that some change or reaction between the ammonium thiocyanate and ethyl alcohol was taking place. SUMMARY
1. A sample of ammonium thiocyanate was carefully purified and analyzed. 2. Some thirteen determinations of the solubility of ammonium thiocyanate in water by the synthetic method in the temperature interval 33 to 72°C. have been made. The precision of measurement in these runs is higher than any previously published. 3. The solubility of ammonium thiocyanate in methanol and ethanol has been determined by the synthetic method from 20 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) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
ALEXEJEW:Wied. Ann. 28, 305 (1886). Assoc. Official Agr. Chemists: Methods of Analysis, p. 8 (1925). FOOTE:J. Am. Chem. Soc. 43, 1031 (1921). HILDEBRAND AND JENKS: J. Am. Chem. SOC.42,2180 (1920). International Critical Tables, Vol. I, p. 177. McGraw-Hill Book Co., New York (1926). KETTNER:Dissertation, Amsterdam, 1919. LUNGE:Coal Tar and Ammonia, 5th edition, Vol. 111, p. 1310. D. Van Nostrand Co., New York (1916). PFEIFFER: Lunge-Berl, Chemisch-technische Untersuchungsmethoden, 6th edition, Vol. 111, p. 373. Julius Springer, Berlin (1911). RUDORFF:Ber. 2, 70 (1869). RUDORFF: Pogg. Ann. 146, 608 (1872). SHNIDMAN: J. Phys. Chem. 37,693 (1933). SHNIDMAN: Proc. Am. Gas Assoc. 1932, p. 950. SMITSAND KETTNER:Proc. Acad. Sei. Amsterdam 16,685 (1912). SUNIER:J. Phys. Chem. 34,2582 (1930). VOLHARD: Ann. 190, l(1878). WADDELL:J. Phys. Chem. 2 , 5 2 5 (1898).
