B. D. BUTLERAND J. C. R. TURNER
3598
Soret Coefficients and Heats of Transport of Some Aqueous Electrolytes at 90
by B. D. Butler and J. C. R. Turner' Department of Chemical Engineering, University of Cambridge, Cambridge, England
(Received M a y 10, 1966)
Soret coefficients of seven 0.01 M aqueous electrolytes have been measured conductometrically at 9.35". The heats of transport calculated from these results are considerably smaller than those a t 25', previously measured. The temperature dependence of the heat of transport appears to be much the same for all of the electrolytes investigated even though the values of the heats of transport cover a wide range.
Introduction
Experimental Arrangements and Results
ciently precise for some conclusions about the temperature dependence of s to be drawn. The concentration of the solutions used was 0.01 M in all cases. The temperature difference applied across the cell was about 9.5", and the mean temperature of the cell in the different runs varied from 9.31 to 9.42'. To convert values of s to give the appropriate molar heat of transport, &*, it is necessary to estimate the ( b In y / b In m)T,in which y is the mean factor 1 ionic activity coefficient. This was done using the information of Robinson and Stokes6 and of Guggenheim and Stokes,' making appropriate adjustment for the different temperature. For BaCl2 the factor was estimated to be 0.878; for all the other solutions the factor lay between 0.954 and 0.958. The results are summarized in Table I. -4n estimate of the reliability of the value of &* is given in each case. This is based on an assessment of the reproducibility, sensitivity, and convectional stability. For KC1 the effect is negative at this temperature. Hence, the concentration differences produced by thermal diffusion act to reduce the density gradient set up by the temperature gradient. This tends to make worse any con-
The ratio cell of Snowdon and Turner2bwas used, in conjunction with the thermostats and bridge network of Price.5 The method of measurement was not different in any important way from that described by Snowdon arid Turner.2b It proved rather more difficult to maintain and control the end-plate temperatures a t approximately 5 and 15" than a t 20 and 30°, as in earlier work. This led i o impairment of the accuracy and reproducibility of the results, as compared with the 25" results obtained earlier, but the results remain suffi-
(1) Department of Chemical Engineering, The University of Texas, Austm, Texas. (2) (a) J. N. Agar and J. C. R. Turner, Proc. R o y . Soc. (London), A255, 307 (1960); (b) P. N. Snowdon and J. C. R. Turner, Trans. Faraday Sac., 56, 1409 (1960). (3) P. N. Snowdon and J. C. R. Turner, ibid., 56, 1812 (1960). (4) A. D. Payton and J. C. R. Turner, ibid., 58, 55 (1962). (5) C. D. Price, Ph.D. Thesis, Cambridge University, 1962. (6) R. -4.Robinson and R. H. Stokes, "Electrolyte Solutions," Butterworth and Co. Ltd., London, 1955. (7) E. -4.Guggenheim and R. H. Stokes, Trans. Faraday Sac., 54, 1646 (1958).
Conductometric methods have recently proved valuable in examining thermal diffusion (the Soret effect) in dilute aqueous electrolytes. The ratio bridge method2 has been applied to a variety of solutions, mostly a t 25", and investigations of the concentration dependence of the Soret coefficients and heats of transport of several salts have also been carried 0 u t . 9 ~ These showed (i) that Soret coefficients, defined by the equation s =
- (d In m/dT)steady state
range from about -2 X to +14 X deg.-l for the electrolytes studied, where m is molality and T is temperature, and (ii) that the variation of s with m is primarily a matter of valence type (at least in dilute solutions) and is independent of the magnitude of 8. Some early measurements indicated that the temperature dependence of s might show the same characteristic. Experiments have therefore been carried out on seven electrolytes at around 9.35" mean temperature.
The Journal of Physical Chemistry
+
SORETCOEFFICIENTS AND HEATSOF AQUEOUS ELECTROLYTES
Table I: Values of the Soret Coefficient, s, and the Molar Heat of Transport, Q*,for 0.01 M Solutions AQ*/AT,
Q* at 9.36‘,
Substance
HC1 BaC12 CSCl KC1
NaC1 NaF
NaOH a
loss, deg.-1
7.9
3.0 0.43 -0.65 0.76 3.5 13.6
Q* a t 2 5 O ,
cal./mole
cal./moleo
2405 f 60 1260 i 30 130 f 15 -200 =k 50 230 i 60 1060 i 60 4120 f 40
3062 2093 827 496 693 1529 4652
cal./mole, deg. 41 54 44 44 29 29
33
From ref. 2b :md 4.
vectional instability2&; the Soret coefficient for KC1 had to be estimated by the “initial slope” method.2a
Discussion In Table I the values of Q* at 25” refer to 25.3°,2b except for the case of BaCL, where the mean temperature was 24.9.1 We can thus make an estimate of AQ*/AT between 9 and 25”, and these estimates are given in Table I. BaClz gives i;hree ions in solution, and, if one multiplies its result (54) by 2/3, the spread of these figures is remarkably small in comparison with the spread in the values of Q*, especially when the experimental errors are considered.
3599
Agar8 has defined a specific heat C,* by the relationship C,* = T(bS*/bT) = bQ*/bT - Q*/T, and he gives some estimates of C,* based on values of Q* from ref. 2a. Some of these are also given by Tyrrell.9 A more extensive list of values is given in a later publication by Agar.1° Usually Q*/T amounts to only a few calories per mole per degree, and so C,* cv bQ*/bT. However, with 0.01 M NaOH Q*/T N 15 cal./mole deg., and it would thus appear that bQ*/bT may be more closely the same for different salts than C,*. Our results also show that bQ*/dT does not depend very much on the temperature. Discussions of the significance of C,* are to be found in ref. 8-10.
Acknowledgments.
The authors thank Professor
R. G. W. Norrish and Dr. J. N. Agar for allowing experimental facilities in the Physical Chemistry Laboratory at Cambridge. B. D. B. also thanks the D.S.I.R. for a maintenance grant for the period within which this work was carried out. (8) J. N. Agar in “The Structure of Electrolytic Solutions,” W. J. Hamer, Ed., John Wiley and Sons, Inc., New York, N. Y., 1959, Chapter 13. (9) H.J. V. Tyrrell, “Diffusion and Heat Flow in Liquids,” Butterworth and Co. Ltd., London, 1961, Chapter 10. (10)3. N. Agar in “Advances in Electrochemistry and Electroohemical Engineering,” Vol. 3, P. Delahay, Ed., Interscience Publishers, Inc., New York, N. Y., 1963, Chapter 2.
Volume 69, Number 10 October 1966
