XVI: The Vapor Pressure of Hydrated Cupric Sulfate THOMAS S. LOGAN Davidson College, Davidson, North Carolina
T E x m o o ~ sof~ physical chemistry and of general chemistry very often use the system cupric sulfatewater to illustrate the principles of equilibrium which apply to hydrate systems and which are embodied in the phase rule. It will be recalled that the application of the phase rule to such systems shows that the solid material in equilibrium with a constant pressure of water vapor, a t a fixed temperature, must consist of two phases, or two different solids. A single phase can exist in contact with water vapor over a range of pressure. The limits of this range of stability are the two fixed pressures at which the particular hydrate can coexist with either a higher or a lower hydrate. Thus a Suggestions of material suitable for this column are eagerly sought and will be acknowledged. They should be sent with as many details as possible to Karol J . Mysels, Chemistry Department, University of Southern California, Los Angeles 7, Cdifornia. Contributors of discussions in a, form suitable for publication directly will be acknowledged as guest authors. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of the errors discussed will not be cited. The error must occur in at lemt two independent standard books to be presented.
definite pressure of water vapor cannot be ascribed to any single hydrate but only to a pair of solid phases. The amounts of the two phases in equilibrium with the fmed pressure of the vapor may vary. This article is concerned with the numerical values commonly quoted for these fixed pressures. The quantitative data generally quoted are shown in the table. These are appreciably in error as shown by comparison with the most reliable values reported in the literature. The textbook values shown in the first column for the pressure of water vapor in equilibrium with each pair of solids seem to be based on work prior to 1900 for 50°C. and on incorrect experimental data for the monohydrate-anhydrous pair. I t is only for the highest pair (pentahydrate-trihydrate) a t 25'C. that the usual textbook values seem to be correct. There is interesting diversity in the values for the lowest pair. Mellor (1) gives a value of 4.5 mm. a t 50°C., and elsewhere (2) states that the value is below 1 mm. a t 45°C. A value of 0.18 mm. a t 3 5 T . was obtained by the writer (3). Foote and Scholes (4) obtained a value of 0.8 mm. a t 25OC. by an indirect
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method. These workers also give a value of 0.5 mm. at 25'C. which they state is calculated from the relative data of Mueller-Erzbach, and which is also given in a review by Wilson (5). A thermodynamic study of high temperature data indicates that all these values are much too high. Some of the difficulties encountered in trying to get equilibrium at the lower water content are illustrated in three papers by Mueller-Erzbach (6). He reported a value of about 0.01 mm. a t 25°C. for a water content slightly below that represented by the stoichiometry for CuS04.2H20. From this he was led to suggest the existence of a dihydrate which he thought to be confirmed by a few other similar observations at low temperature. The writer had a somewhat similar experience when he first made measnremente in the composition range represented by CuS04.5H10CuS04.3H20. A series of decreasing pressure values was obtained as the water content was lowered. For a while it was thought that the results indicated a solid solution, a point of view which has been previously suggested (I, 7). However a repetition under more carefully controlled conditions gave constant values. This considerable variation among experimental values is due to a bevy of experimental difficulties. Equilibrium is established slowly. Supercooling and superheating may be encountered. Regulation of the water content and the process of water withdrawal require extreme care in order that the solid phases may have a true equilibrium composition. Small amounts of adsorbed or occluded air are difficult to eliminate completely. All these factors become more pronounced a t low pressures and temperatures. Accurate work on the lowest pair is particularly fraught with difficulties, as the writer can testify from personal experience. Because of this uncertainty of experimental results in the case of the lowest pair, extrapolated values based primarily on measurements a t higher temperatures (8, 9) seem to be the most reliable. Randall and Nielson (10) have made an evaluation of the data available up t o about 1930 and summarized them in three standard free energy equations which lead to the values given in hold face in the table.
VOLUME 35, NO. 3, MARCH, 1958
Other values shown in the table are experimental results found in the literature. These, in some cases, are in much better agreement with the calculated values than are the data generally quoted in textbooks. Probably the most realistic picture is given by the values calculated from the thermodynamic equations, particularly in the case of the monohydrate-anhydrous pair. The writer is greatly indebted to Dr. K. J. Mysels for his interest, his extensive aid in revising the original manuscript, and his checking of the early references. Pressurea of Water Vapor (mm. Hg) i n Equilibrium with Pairs of Cupric Sulfate Hydrates
--- -CuSOd.6H-0-
Temp.
laC.)
CuSO+-SHIO
Tezls
Lit.
CUSOI.JHIOCuSO,. I Hz0
Tezla
Lit.
CuSOa. 1HIO-
---
CuSOa
Terla
Lit.
Calculated from exoerimental vsluea at sliehtlr differenttem~erntures.
LITERATURE CITED
J. W., "A Comprehensive Treatise on Inorganic, (1) MELLOR, Land,'Theoretical Chem~stry,"Longmans, Green & Co. Ine., London, 1922, Vol. I, pp. 501-2. (2) Zbzd., Vol. 111, p. 244. (3) LOGAN, T. S., J. Phys. Chem., 36, 1043x1932). ( 4 ) FOOTE,H. W., AND S. R. SCHOLES, J . Am. Chem. Sac., 33, 1324 (1911). (5) W n s o ~ R. , E., J. Am. Chem. Sac., 43, 708 (1921). W., Wwd. Ann. Phys., 26,409 (1885); (6) MITELLER-EEZBACH, 32, 316 (1887); Ber., 19, 2877 (1886). (7) BLACKMAN, P., J. P h p . Chem., 15, 871 (1911) (8) LEBCOUER, H., Ann. chim. el phya., 21, 547 (1 (9) SIGGEL, A,, Z. Elektroehem., 19,340 (1913). (10) "Internstional Critical Tables," McGrilw-Hill Book Co., Inc., New York, 1930, Vol. VII, p. 263. (11) SCHUMB, W. C., J . Am. Chem. Sac., 45, 349 (1923). (12) CARPENTER, C. D., AND E. R. JETTE, J. Am. Chem Sac., 45, 583 (1923). (13) COLLINS, E. M., AND A. W. C. MENZIES, J. Phys. Chem., 40, 386 (1936).
