ON TIJE DETERMINATION O F T R A N S I T I O N TEMPERATURES
BY H. 11, DAWSON AND P. WILLIAMS
*
T h e influence of temperature on the equilibrium of homogeneous systems is such that the variation of the latter can be represented as a continuous function of the temperature. In the case of heterogeneous systems the variation of the equilibrium is also represented as a continuous function of the temperature provided the various phases of the system remain intact. I n many cases, however, such a heterogeneous system undergoes at a definite temperature n complete change whereby one of the phases of the system disappears and is replaced by a new one. Systems which are characterized by the existence of such a transition teniperature at which the two forms are capable of existing together in stable equilibrium, have been given the special designation of u condensed systems” by van ’t Hoff. Such a transition temperature corresponds to a multiple point on the diagram representing the solubility relationships of the system, the number of coexistent phases at this poitit for a system composed of n bodies being n $- 2. Several different methods have been employed for the investigation of such transition temperatures, e. g., the dilatometric and thermometric, depending upon the volume and thermal changes accompanying the transformation, the Method of isothermal crystallization, which is of especial value in determining the qualitative nature of the change, and finally the socalled identity method. By application of this latter method, which .depends upon the identity of the saturated solutions of
the two systems transformable one into the other, at the transition temperature, various properties of the solution have been made use of as a means of determining transition temperatures. T h e properties of the solution which have as yet been investigated with this object are the concentration, the vaporpressure, the solution pressure, and the difference of potential between the solution and a reversible electrode. I n the choice of a property of the solution suitable for determination of transition temperatures according to the identity method, the essential considerations are the exactitude with which the property in question can be measured, the facility with which the determination of the value of the property can be carried out, and in many cases the avoidance of a reduction i n the quantity of saturated solution as a result of the measurement. T h e determination of the density and the electrical conductivity of the saturated solution appeared to offer convenient methods of measuring transition temperatures and the experiments contained in this paper were carried out with a view of ascertaininq the suitability of these methods for such measurements. T h e results obtained from the measurement of the density of the saturated solutions are given first and afterwards those from the electrical conductivity. The density method
To test the applicability of this method of determination, a series of measurements was carried out with Glauber's salt, the transition temperature of which has already been determined by several different methods. From solubility determinations of Mulder, Loewel, and Gay-Lussac the transition temperature is calculated to be 32.6"-33" C ; by measurement of the vaporpressure van 't Hoff and van Deventer found 32.6" C ; by the solution pressure method Verschaffelt obtained 32.74' C and Cohen and Bredig, by determining the temperature coefficient of the electromotive force, obtained 33.2" C. Finally a very accurate determination by Richards and Churchill' by the therZeit. phps Chem.
26,
690 (1898).
372
H. M Dawson and P. Willlains
niometric method, gave 32.379" C (Hydrogen thermometer). T h e differences exhibited by the values obtained by the different methods are not great, but in view of the last-mentioned result, would all appear to be too high. I t seemed necessary, therefore, as a control to determine the solubility simultaneously with the density of the saturated solution, which was possible without any special measurements, since the former only involved the evaporation of the solution pipetted in the latter determination. T h e saturated solutions were prepared by agitating suitable quantities of pure salt and water i n a large thickwalled test-tube, provided with a screw-shaped stirrer set in rapid rotation by a turbine. T h e solubility tube provided with a tightfitting cork, was placed in an Ostwald thermostat, the temperature of which was maintained within 1/20' C of that desired by means of a special form of regulator. T h e mean time of stirringamounted to about four hours. T h e density determinations were made by means of the capillary pipette described by Meyerhoffer and Saunders.I T h e latter had a capacity of about 5 cc, and, before introduction into the solubility tube for the purpose of pipetting, was heated to the temperature of the bath by placing for some time in a second tube immersed in the thermostat. After the pipette had been filled by suction it was again introduced into the second tube and left for some time before the level of the solution in the capillary was read off, whereby errors resulting from any small changes of temperature during the time required for pipetting were entirely avoided. Since the variation of solubility of the hydrated salt with the temperature is very considerable, the latter was maintained constant to O.OI' C for at least an hour previous to pipetting, a Beckmann thermometer being used for this purpose. After the density of the solution had been measured the contents of the pipette were carefully removed and the quantity of sodium sulphate determined. T h e following table gives the results so obtained. T h e thermometer used was a normal one corrected at the Physikalische Reichsanstalt in Charlottenburg. Zeit. phys. Chem. 28, 466 (1899).
'
I
,
Composition of solution I I
Temperature
Saturated solution of 1
I
IOO water
1
( b ) mols
1
100 H 2 0
I
' -
,
Density
I
___---__________L_-
26.03' 28.01 29.93 31.04 31.94 32.93 32.06 33.23 33.93 34.33 36.22 38.31
Glauber's salt
I
1
/ /
'
1
-
I
( 1
i
47.83 52.12 49.76 49.53
I
-
1
' I
Anhydrous Na,SO, 1 I 1
