VAPORPRESSURE AND ENTHALPY OF VAPORIZATION OF MOLTEN MERCURIC CHLORIDE
2985
The Vapor Pressure and Enthalpy of Vaporization of Molten Mercuric Chloride to the Critical Point1
by J. W. Johnson, W. J. Silva, and D. Cubicciotti Stanford Research Institute, Menlo Park, California (Received April 1 1 , 1966)
The vapor pressure of molten mercuric chloride has been measured by an inverted capillary technique from 573°K and 0.96 atm to 968°K and 111.6 atm. The data are fitted by the linear relation log P(atm) = 4.9929 - 2854.8/T°K from 660 to 968°K with an average deviation of 0.9% for the 25 experimental points. Below 660°K) the experimental points fall below the linear relationship. A critical pressure of 113.7 f 1.6 atm is predicted by the linear equation a t a critical temperature of 972 f 2°K. The enthalpy of vaporization of molten mercuric chloride has been calculated from 700 to 970°K.
Introduction An investigation of some of the properties of molten salts above the normal boiling point has been under way for some time in this laboratory. A previous paper2 reported the critical temperature and coexistence curve of mercuric chloride. This report describes the measurement of the vapor pressure of mercuric chloride from the normal boiling point to the critical point.
Experimental Section The semimicro boiling point method used in this study was the same as that employed in the determination of the vapor pressure of bismuth ~ h l o r i d e . ~The apparatus has been described in detail el~ewhere.~ The mercuric chloride was from the same preparation used in the determination of the coexistence curve and critical temperature. The analysis was 73.89% Hg and 26.13% C1 by weight, compared with the theoretical values of 73.88% Hg and 26.12% C1.
Results and Discussion The vapor pressure of molten mercuric chloride was determined at 35 points over a temperature range of 573 to 968°K and a pressure range of 0.96 to 111.6 atm. The experimental data are presented in Table I. The temperatures recorded were corrected for gradients between the position of the thermocouple and the liquid surface. These corrections ranged from 2 to 3" depending on the temperature. The listed pressures, up to 17 atm, were corrected by the addition of 0.02
atm for the static head of molten mercuric chloride. Vapor pressures above 17 atm are recorded to the nearest 0.1 atm and the correction was negligible. Three runs were made using different samples of mercuric chloride as indicated in Table I. It was found that the experimental data could be fitted by the relation log P(atm) = 4.9929 - 2854.8/T°K
(1)
from 660 to 968°K with an average deviation of 0.9% for the 25 experimental points. From 660 to 573°K the experimental points tend to fall below the straight line defined by eq 1. In that region the vapor pressure curve is in transition between the high-temperature linear form and the low-temperature Clausius-Clapeyron equation for low pressures, which includes curvature due to a AC', term. Because there is no established theoretical form for the equation in the transition region, a smooth curve was drawn through the data on a large scale plot. Values taken from the curve are compared with the experimental points in the lower part of Table I in the temperature range 570 to 660°K. Values (1) This work was made possible by the support of the Research Division of the U. S. Atomic Energy Commission under Contract No. AT(043)-106. (2) J. W. Johnson, W. J. Silva, and D. Cubicciotti, J . Phys. Chem., 70, 1169 (1966). (3) J. W.Johnson, W. J. Silva, and D. Cubicciotti, ibid., 69, 3916
(1965). (4) W. J. Silva, J. W. Johnson, and D. Cubicciotti. Rev. Sci. Instr., 36, 1505 (1965).
Volume 70,Number 9 September 1966
2986
J. W. JOHNSON, W. J. SILVA,AND D. CUBICCIOTTI
Table I : Vapor Pressure Data for Molten biercuric Chloride Run no.
-0badTemp,
Pressure, atm
OK
9 7 2 f 2"
-05 09
I
I
I
I
I
I
I
I
I
1
IO
I I
12
13
14
15
16
17
I8
I 9
IOOO/T'K
Figure 1. Vapor pressure of mercuric chloride: 0,run no. 1; e, run no. 2; 0 , run no. 3, this work; Prideaux; 0 , Wiedemann, et al.; A, Johnson. Full line is calculated from eq 1; dashed curve represents smoothed curve through lower temperature data.
+,
of the vapor pressure in that range, taken from the smoothed curve, are given in Table I1 for 10" intervals. Figure 1 presents the experimental data on a log P os. 1/T plot. The circles represent the data points obtained on three different samples. There is no evidence of a systematic deviation between the individual runs. The solid line drawn through the points in the highpressure range w8s calculated from eq 1. The dashed line through the points in the low-pressure range represents the smoothed curve of the lower temperature data. The critical pressure calculated from eq 1 is 113.7 f 1.6 atm using the critical temperature of 972 f 2"K, reported previously,2 for mercuric chloride. The present work overlaps that of previous investigators Prideaux,6 Wiedemann, Stelzner, and Nederschulte,6 and Johnson' over a very short range of pressure and temperature as is shown in Figure 1. The normal boiling point obtained in this work is 575.0"1
