May, 1958
FREEZING POINTS IN MULTICOMPONENT SYSTEMS
simple acid buffers, to those obtainable with the Ag ;AgC1 electrode. The author wishes to acknowledge his indebtedness to Drs. E. M. Crook and S. P. Datta for their invaluable advice on many of the problems en-
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countered in this work, and to Mr. J. Trendall for making the glass apparatus. H e also wishes to thank the Central Research Fund of the University of London for a grant to cover part of the cost of the equipment.
A PRECISION METHOD WITH AUTOMATIC RECORDING FOR THE STUDY OF FREEZING POINTS IAT MULTICOMPONENT SYSTEMS BY ROBERT H. DETTRE'AND DONALD H. ANDREWS Department of Chemistry, The Johns Hopkins University, Baltimore,Maryland Received October 11, 1967
An improved method has been developed for measuring the temperature of equilibrium between the crystal and solution phases of multicomponent systems using a constant temperature differential to control rate of cooling and automatic recording of temperature as a function of time. This makes possible the determination of true equilibrium temperature without corrections for undercooling. Where crystallization velocities and rate of attainment of equilibrium are small, this enables one to obtain the true equilibrium temperature from a study of the effect of cooling rate on the temperature displacement of the cooling curve. The method has been applied to the system diphenylmethane-diphenyl ether and compared with other methods.
As pointed out by Guggenheim,2 there has long been a need for more extensive and precise measurements of the equilibrium properties of simple liquid mixtures. This is important not only to improve our theories of mixtures, but also to expand our knowledge of the liquid state itself, for which no adequate theory has yet been developed. One of the most promising approaches lies in the study of the equilibrium between a liquid mixture and the crystalline phase of one of its components, since our knowledge of the nature of crystals has far outdistanced our knowledge of liquids, and in this type of system we have an exchange of molecules between an ordered and a disordered state, both being a t approximately the same density. Ideally one would like to have a sufficiently precise knowledge of the composition and the corresponding equilibrium temperature to permit an evaluation of activity coefficients, and to be of help in calculating potential functions and various statistical properties. Generally speaking this demands a measurement of equilibrium temperature to a t least 0.01'. The early methods of measurement were far from this goal, though later improvements have made it possible to attain this precision, especially in cases where there is a relatively small concentration of the component not in equilibrium with a crystalline phase. Particularly noteworthy work in this field has been done by Skau and Saxby Mair, Glasgow and R o ~ s i n i ,by ~ Witschonke6 and by Stull.'j ( 1 ) Thia article is based on a dissertationsubmitted in partialfulfillment of the requirement for the Ph.D. degree a t The Johns Hopkins University. This research was supported in part by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command, under contract No. A F 18(600)-765. Grateful acknowledgment is made of fellowships from the Procter and Gamble Company (1954-55) and from the Kennecott Copper Corporation (1955-56). (2) E. A. Guggenheim, "iMixtures," Oxford University Press, London, 1952, Preface. (3) E. L. Skau and B. Saxton, THIS JOURNAL, 37, 183 (1933). (4) B. J. Mair, A. R. Glasgow and B. D. Rossini, J . Research Natl. Bur. Standards, 26, 591 (1941). (5) C. R. Witschonke, Anal. Chem., 24, 350 (1952). (6) D. R. Stull, Ind. Eng. Chem., Anal. Ed., 18, 234 (1946).
There still remains a large number of interesting systems, however, where small rates of crystallization in some concentration ranges make the present methods inadequate. For this reason we have undertaken the study reported here in order to develop a method where automation results both in improved precision and a greater ease of operation, which will be beneficial in making an extensive survey of different types of systems. This method uses a temperature differential between sample and shield automatically held a t a constant value, while the temperature of the sample is observed with a recording potentiometer with an absolute accuracy of f0.002'. The advantages of this method will be discussed further in connection with studies we have made to compare it with some of the previously used methods. Apparatus and Test Materials.-The temperature sensitive element is a six-junction copper-Advance wire thermopile. ("Advance" wire, manufactured by the DriverHarris Company has approximately the same proportion of copper and nickel as constantan wire). High accuracy and sensitivity are obtained by maintaining the reference junction of the thermopile within 0.2" of the temperature that is being measured. The resulting output of the thermopile (0 to 50 microvolts) is amplified and recorded using a stabilized d.c. amplifier in combination with a recording potentiometoer, with full scale deflection of the latter corresponding to 0.2 . The reference junction temperature can be kept constant to 0.001' or better at any temperature from -40 to +40°. The working junction and reference-junction assemblies are enclosed in an insulated box which is cooled by circulation of cold dioxide gas produced by the evaporation of "Dry Ice" located in the left side of the box. The box temperature is controlled to f1 using a heater placed in front of a circulating fan and thermoregulator and it is usually kept 6 to 7 " below the temperature of the reference junction. Figure 1 shows the details of the thermopile and its location in the sample holder; six junctions were chosen as the optimum number for the desired sensitivity and space requirements, for minimal heat conduction along the wires and for small heat capacity. The work of White' and Robertson and LaMerEserved as a guide in the construction of the thermopile. The elements consist of No. 32 copper wire and No. 30 advance wire. T.le whole assembly is stationary and all the alloy wires are inside the constant-tempera(7) W. P. White, Phus. Reu., 31, 135 (1910); W. P. White, J . Am. Chem. Soc., 36, 2292 (1914). (8) C. Robertson and V. I
