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The vapour pressure of chlorine
M-1037 J. Chem. Thermodynamics 1979,11, 1089-1094
The vapour
pressure
of chlorine
D. AMBROSE, D. J. HALL, D. A. LEE, G. B. LEWIS, and C. J. MASH Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex TWlI OL W, U.K. (Received 21 March
1979)
The vapour pressure of chlorine was measured in the ranges 206 to 270 K and 334 to 417 K, and the values found were fitted by equations applicable from the triple point to the critical point. The results of the present work are compared with those previously available.
1. Introduction The most recent measurements of the vapour pressure of chlorine prior to the work described here are those by Giauque and Powell from the triple point to atmospheric pressure,(l) published in 1939, and the currently accepted supra-atmospheric values depend on work by Pellaton 25 years earlier. (‘) Recently, Mond Division, Imperial Chemical Industries Ltd. proposed that the vapour pressure of chlorine should be remeasured in this laboratory over as wide a range as possible as a joint project With them. This paper describes the work. It was done under constraint of cost so that there could be no more than simple modification of existing equipment, and as a result, no measurements could be made at temperatures below those attainable by use of solid carbon dioxide, and the supra-atmospheric measurements were made in the apparatus described by Ambrose, Broderick, and Townsend.t3)
2. Experimental For measurements in the range 206 to 270 K the chlorine was contained in a glass bulb of capacity about 10 cm3 which fitted in a hole bored in a brass block. The block was a cuboid of sides about 5 cm, and was also bored to receive a standard platinum resistance thermometer. The temperature of the block was maintained within rf: 20 mK in a commercial cryostat containing ethanol, but during the final 5 to 10 min when the observations reported were made it was possible to maintain constancy to rf:2 mK by manual operation of the temperature control. The vapour-pressure buIb was connected to a system comprising a vacuum pump, the chlorine supply cylinder, oneand two-litre storage flasks, condensing vessels into which the chlorine could be transferred by refrigeration, and a pressure gauge, the flow of the gas being controlled by greaseless stopcocks sealed with PTFE O-rings. The quartz-helix pressure gauge (Texas Instruments) was calibrated against an air-operated pressure baIance (CEC) and the corrected readings were repeatable to 1 Pa. Full-scale reading formed the 0021-9614/79/111089+06
$01.00/O
0 1979 Academic Press Inc. (London)
Ltd.
D. AMBROSE ET AL. 1090 upper bound of this set of measurements. All parts of the measurement system with which chlorine came into contact were made of glass, quartz, stainless steel, or PTFE. As a test of the apparatus, measurements were made of the vapour pressures of diethyl ether at 251 and 269 K, and of carbon dioxide at 210 K. The pressures measured for ether were within 0.1 per cent of those derived from earlier work in this laboratoryc4) which, at the lower temperature, only just exceeds the scatter of the original work. The pressure measured for carbon dioxide differed by less than 0.02 per cent from the literature value.(‘) Measurements in the range 334 K to the critical temperature were made in an apparatus already described in which the sample is confined over mercury.(3) Reaction between chlorine and mercury is instant and rapid, and the mercury was protected from attack by interposing a column of Fluorolube LG-160, the most viscous liquid suitable and available, which filled the limb A of the U-tube in figure 1 of reference 3. When the experimental tube was in place there was a column of Fluorolube more than 30 cm high between the mercury surface at the bottom of limb A and the chlorine, and complete protection was obtained for the duration of the experiments. There was little deterioration in the appearance of the chlorine-to-Fluorolube boundary but the top layer of Fluorolube gradually took on a yellow colour. Diffusion and manipulation of the apparatus could gradually bring Fluorolube in solution to the gas-liquid interface, even though this was separated by a column of chlorine about 35 cm high from the chlorine-to-Fluorolube interface, and ultimately this might have caused the apparent vapour pressure to fall. To check whether there was any effect of this nature three tubes were used in the experiments as follows: with the first tube the vapour pressure from 334 K to the critical temperature was measured (which took about 6 h), and with the other two tubes determinations were made as quickly as possible (within 2 h) of the critical temperature and pressure only. All the values for the critical temperature and pressure were concordant and it was concluded that accuracy had not been seriously impaired by the presence. of the Fluorolube. However, when the critical temperature was determined”) in a sealed tube containing chlorine alone without the Fluorolube, the value obtained was 0.6 K less than that found in the vapour-pressure measurements and given in table 1, and this suggests that the presence of the Fluorolube might have increased the apparent critical temperature. It is however surprising in view of the good visual separation of the two fluids throughout the experiments and the long distance through which the Fluorolube would have had to travel before reaching the experimental zone. The apparatus is not suitable for use at temperatures less than about 35 K above ambient temperature and this therefore was the lower bound of this set of readings. All temperature measurements were made with platinum resistance thermometers used with an automatic a.c. bridge (Automatic Systems) or a Smith’s bridge no. 3. The chlorine was supplied by Mond Division, I.C.I. with an analysis indicating a mass fraction of volatile impurities approaching 250 x 10m6. More than half of the impurity was bromine, the effect of which would be negligible, but more important would be the 80 x 10q6 of hydrogen chloride, or any air that might inadvertently be present, and it was found when samples were condensed from the cylinder that they degassed on warming and that the vapour pressure determined in the range 206 to
THE
VAPOUR
PRESSURE
OF
1091
CHLORINE
270 K fell as part was distilled away. About half of each sample taken in the work, including those for the measurements in the range 334 K to the critical temperature, was therefore distilled away until a constant vapour pressure at a temperature corresponding to about 80 kPa was obtained from the residue.
3. Results The observed values of pressurep and temperature Tare in table 1. Measured IPTS-68 temperatures T,, were treated throughout this work as thermodynamic temperatures T, and the results were fitted by equations (1) and (2). In equation (1):“’ V/WgloWkPa) E,(x) is the Chebyshev polynomial
x = (2T-(T’a,+
T’iJ)/(L-
(1)
= 42 +s$l@s(x)y in x of degree s, where
GiJ, Tmin= 205 K, T&x = 417 K,
a = 1883.938, a, = 685.571, a2 = -2.344, a3 = 2.346, and a4 = -0.210. In equation (2): In p = In pc + (T,/T)(nlz+ 12&’ + n3r3 4 lt.&, TABLE
1. Vapour
pressure
p of chlorine: obtained
T/K
PlkPa
103A
(1)
205.945 205.970 210.157 210.172 215.005 215.018 219.810 219.824 224.962 224.967 230.097 230.129 234.920 234.933 239.864 239.879
17.90 17.92 23.04 23.06 30.45 30.48 39.62 39.63 51.83 51.82 66.71 66.80 83.63 83.72 104.39 104.46
0.5 0.3 -0.3 -0.4 -0.3 -0.2 -0.1 -0.3 0.4
334.11 345.7 353.30 363.6 373.05 373.78 374.05
1888 2377 2760 3314 3955 4014 4052
“kritical-point specially filled
observations; for determination
from
Alog,,p = loglOpobs-loglOpcalc equations (1) and (2)
log,op
T/K
(2)
plkpa
where
103A
(2) poalo has been log,,p
(1)
(2)
x:: -0.1 -0.1 0.1 0 0
0.6 0.3 -0.4 -0.4 -0.3 -0.2 -0.1 -0.3 0.4 02 0’1 -0:1 -0.1 0.1 0 0
244.942 244.961 250.121 250.143 254.834 254.916 255.117 255.120 256.042 256.063 260.118 260.156 265.119 265.17 270.377 270.380
129.76 129.85 160.22 160.35 192.52 193.12 194.78 194.79 201.59 201.80 234.70 234.92 280.60 281.07 335.87 335.88
0.2 0.1 0 -0.1 -0.2 -0.2 0.2 0.2 -0.1 0 0.1 0 0 0 -0.2 -0.2
0.2 0.2 0 0 -0.1 -0.1 0.3 0.2 -0.1 0 0.1 -0.1 0 -0.1 -0.3 -0.3
7.7 2.6 2.0 -3.1 -0.1 0.7 2.8
7.4 2.5 2.2 -2.6 0.7 1.6 3.6
384.0 391.37 404.96 416.76”
4747 5359 6596 7991
-2.4 -2.3 -5.1 1.7
-1.2 -1.0 -3.9 1.7
416.87b 416.90*
7974 7977
0 0
0 0
%oncluding of critical
observation temperature
of vapour-pressure and pressure.
measurements,
btubes
1092
D. AMBROSE
ET AL.
pE is the critical pressure, IQ&Pa) = 8.98437, T, is the critical temperature, 416.90 IS, z = (T,- T)/T,, nl = - 6.34074, n2 = 1.15037, IZ~ = - 1.40416, and n4 = - 2.23220. The fits of the equations are indicated in table 1 by the residuals A log,,p. Neither equation is markedly superior to the other and between 185 and 360 K the temperatures corresponding to a given pressure calculated from the two equations do not differ by more than 0.05 K, and at standard atmospheric pressure they differ by only 2 mK; from about 375 K the two equations diverge and the two temperatures differ by 0.2 K at 400 K returning to the same value at the critical temperature. Both equations conform to the constraints proposed by Ambrose, Counsell, and Hicks.@’
4. Discussion The results of the present work are compared with that by Giauque and Powell and by Pellaton in figure 1.(l12) In making any comparison with the work published by p/kPa
0.002
”
IA “%O.lKt
2 .i? a
% Y : 0
250
I
300
T/K FIGURE 1. Residuals (A log,op = log,,g,bs-logl~~p,,,, A, Giauque and Powell;C1) V (points at 234 and 252 K are area X, this work. a, Error bands corresponding to 10.05 K; K. The curved lines show the change in loglop corresponding
350
v v VV’JVw I -0.02 400
from equation (1) for measurements: off scale), Pellaton;(z) 0, and hatched b, error bands corresponding to 10.02 to the stated changes in temperature.
Giauque and Powell it is necessary to take account of the difference between the temperature scale in use in Giauque’s laboratory and IPTS-68. Temperatures were published on a scale in which the normal boiling temperature of oxygen was taken as 90.13 K (90.188 K) and that of the ice point as 273.10 K (273.15 K) where the figures in parentheses are the values assigned to these temperatures in IPTS-68. It is therefore necessary to add 0.05 K to Giauque’s values to bring them into line with modern thermometry, and this was done in the calculation of log,,p for plotting in figure 1. The values so obtained were taken to be equivalent to IPTS-68 temperatures because the published values were not on ITS-27, the appropriate international scale at that time, but were reputedly thermodynamic temperatures. The scale was established by calibration in the laboratory of a group of copper-to-constantan thermocouples
THE VAPOUR
PRESSURE OF CHLORINE
1093
against a hydrogen gas-thermometer, and these thermocouples were thought to be more reliable than the resistance thermometers used for temperature measurement in the actual experiments; the resistance thermometers were sufficiently sensitive and repeatable for temperatures to be given to 1 mK, and differences in temperature were much more accurate than were their actual values. In the calibration of the thermocouples, temperatures could be measured to 0.01 K with the gas thermometer but accuracy of only & 0.05 K was claimed for the thermocouple scale, although doubtless the authors hoped it was better. (‘) For example, Giauque and Weibe(‘O) measured the vapour pressure of hydrogen chloride, and found their values were within 0.03 K of those reported by Henning and Stock, cl ‘) which they accepted as being of reference quality. However, a wider limit seems to be indicated by other results: Giauque and Egan(“) found the normal sublimation temperature of carbon dioxide to be 194.72 K (194.674 K, difference 0.046 K), and Giauque, Bufhngton, and Schu.lze(g) found the freezing temperature of mercury to be 234.23 K (234.309 K, difference -0.079 K), where in each case the published value has been increased by 0.05 K and the values in parentheses are again those assigned to these temperatures in IPTS-68. We believe the errors in the present work between 206 and 270 K do not correspond to more than 0.02 K. If we allocate a possible error of + 0.05 K or slightly more to the temperatures published by Giauque and Powell, and lay out the resulting error bands on figure 1, the two sets or results just fail to meet. There is a discrepancy, but for most purposes emphasis on the good agreement between the two investigations is appropriate. Giauque and Powell reported the triple-point temperature of chlorine as 172.12 K; corrected, this becomes 172.17 K but, if their triple-point pressure, 1.392 kPa, and the present measurements are corrected, extrapolation of equation (1) leads to a higher value, 172.22 K. Prior to the work by Giauque and Powell the vapour pressure had been measured in approximately the same range by Trautz and Gerwig(13) and by Harteck.(14) Residuals from equation (1) for the results of these two investigations lie off the scale of figure 1, respectively above and below the zero line by between 5 and 10 per cent of the pressure. In the range above 273.15 K the present results are between 1 and 5 per cent higher than those by Pellaton, which we believe must be in error.{@ Pellaton’s value for the critical temperature “144 “c” (417.2 K) is close to that found in this work (416.90 K) and it seems unlikely that errors in his thermometry would have made the major contribution to the overall error. Pellaton also made a few measurements below 273.15 K using a mercury manometer in which the mercury was protected by a layer of concentrated sulphuric acid: these are of low precision but the residuals from equation (1) are of both signs which, in combination with the good agreement for the critical temperature, suggests that the chlorine was of reasonable purity. In the measurements above 273.15 K Pellaton prevented contact between the chlorine and any material other than glass by using a mercury thermometer as a pressure gauge. This was sealed within the system and kept at a constant temperature in ice; extension of the mercury thread due to compression of the bulb was calibrated in a separate experiment in terms of the pressure applied and was found to be linear. The calibration was done with the aid of apparatus (a Cailletet pump) in which the
1094
D. AMBROSE
I??’ AL.
pressure was calculated from the compression of nitrogen (pressures up to 2.5 MPa) or hydrogen (higher pressures) confined over mercury in a glass tube. An overall error of 3 per cent in the measurements arising from the uncertainty of the calibration and the relative insensitivity of the instrument does not seem impossible. However, Pellaton’s results are surprisingly self-consistent. The same type of pressure gauge was used by Berthoud, who worked in the same university, in his study of the critical pressures of amines, chloromethane, and chloroethane.(’ 5, Berthoud implied that the thermometer-manometer could be read to 10 kPa and described the method of calibration in greater detail than did Pellaton; in addition to using the gas-compression method, Berthoud generated fixed pressures by maintaining liquid carbon dioxide at a series of constant temperatures and derived the pressures from Amagat’s vapour-pressure measurements. There are no confirmatory values of the critical pressures that Berthoud measured to allow any estimate to be made of his error, but the values he quotes for the vapour pressure of carbon dioxide are below those currently accepted. @) The differences are, however, only about 0.5 per cent. We are unable, therefore, to be sure of the reasons for the systematic difference between Pellaton’s results and those of the present work, but we believe the present work to be more reliable, and in table 2 give selected values of p and dp/dT calculated from equation (1). TABLE
2. Vapour pressures calculated from equation (1) p/kPa
172.17 L1 1.383 176.26 2 239.184 b 101.325 291.83 o 651.76 “Triple point. *Normal boiling %ritical temperature.
dp/dT kPa K-l
T/K
0.128 0.176 4.463 18.55
298.15 386.80 416.90 d
temperature.
777.04 5000 7977
21.13 82.58 116.5
OT = 0.7T, giving w = log,,(7977/651.76)--1=0.088.
We wish to acknowledge the support given to this work by Imperial Industries Ltd., Mond Division.
Chemical
REFERENCES 1. Giauque, W. F.; Powell, T. M. J. Am. Chem. Sot. 1939,51,1970. 2. Pellaton, M. J. Chim. Phys. 1915,13,426. 3. Ambrose, D.; Broderick, B. E.; Townsend, R. J. Chem. Sac. A 1967,633. 4. Ambrose, D. ; Sprake, C.H.S. ; Townsend, R. J. Chem. Thermodynamics 1972,4,247. 5. Ambrose, D. Pure Appl. Chem. 1977,49,1437. 6. Ambrose, D.; Cox, J. D.; Townsend, R. Trans. Far. Sot. 1960,56, 1452. 7. Ambrose, D.; Counsel.& J. F. ; Davenport, A. J. J. Chem. Thermodynamics 1970,2, 283. 8. Ambrose, D.; Counsel& J. F.; Hicks, C. P. J. Chem. Thermodynamics 1978,10, 771. 9. Giauque, W. F.; Buffington, R. M.; Schulze, W. A. J. Am. Chem. Sot. 1927,49,2343. 10. Giauque, W. F.; Wiebe, R. J. Am. Chem. Sot. 1928,50, 101. 11. Henning, F.; Stock, A. Z. Chim. Phys. 1921,4,226. 12. Giauque, W. F. ; Egan, C. J. J. Chem. Phys. 1937,5,45. 13. Trautz, M. ; Gerwig, W. 2. anorg. algem. Chem. 1924,134,417. 14. Harteck, P. Z. physik. Chem. 1928,134,21. 15. Berthoud, A. J. Chim. Phys. 1917,1&l.
The vapour
pressure
of chlorine
D. AMBROSE, D. J. HALL, D. A. LEE, G. B. LEWIS, and C. J. MASH Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex TWlI OL W, U.K. (Received 21 March
1979)
The vapour pressure of chlorine was measured in the ranges 206 to 270 K and 334 to 417 K, and the values found were fitted by equations applicable from the triple point to the critical point. The results of the present work are compared with those previously available.
1. Introduction The most recent measurements of the vapour pressure of chlorine prior to the work described here are those by Giauque and Powell from the triple point to atmospheric pressure,(l) published in 1939, and the currently accepted supra-atmospheric values depend on work by Pellaton 25 years earlier. (‘) Recently, Mond Division, Imperial Chemical Industries Ltd. proposed that the vapour pressure of chlorine should be remeasured in this laboratory over as wide a range as possible as a joint project With them. This paper describes the work. It was done under constraint of cost so that there could be no more than simple modification of existing equipment, and as a result, no measurements could be made at temperatures below those attainable by use of solid carbon dioxide, and the supra-atmospheric measurements were made in the apparatus described by Ambrose, Broderick, and Townsend.t3)
2. Experimental For measurements in the range 206 to 270 K the chlorine was contained in a glass bulb of capacity about 10 cm3 which fitted in a hole bored in a brass block. The block was a cuboid of sides about 5 cm, and was also bored to receive a standard platinum resistance thermometer. The temperature of the block was maintained within rf: 20 mK in a commercial cryostat containing ethanol, but during the final 5 to 10 min when the observations reported were made it was possible to maintain constancy to rf:2 mK by manual operation of the temperature control. The vapour-pressure buIb was connected to a system comprising a vacuum pump, the chlorine supply cylinder, oneand two-litre storage flasks, condensing vessels into which the chlorine could be transferred by refrigeration, and a pressure gauge, the flow of the gas being controlled by greaseless stopcocks sealed with PTFE O-rings. The quartz-helix pressure gauge (Texas Instruments) was calibrated against an air-operated pressure baIance (CEC) and the corrected readings were repeatable to 1 Pa. Full-scale reading formed the 0021-9614/79/111089+06
$01.00/O
0 1979 Academic Press Inc. (London)
Ltd.
D. AMBROSE ET AL. 1090 upper bound of this set of measurements. All parts of the measurement system with which chlorine came into contact were made of glass, quartz, stainless steel, or PTFE. As a test of the apparatus, measurements were made of the vapour pressures of diethyl ether at 251 and 269 K, and of carbon dioxide at 210 K. The pressures measured for ether were within 0.1 per cent of those derived from earlier work in this laboratoryc4) which, at the lower temperature, only just exceeds the scatter of the original work. The pressure measured for carbon dioxide differed by less than 0.02 per cent from the literature value.(‘) Measurements in the range 334 K to the critical temperature were made in an apparatus already described in which the sample is confined over mercury.(3) Reaction between chlorine and mercury is instant and rapid, and the mercury was protected from attack by interposing a column of Fluorolube LG-160, the most viscous liquid suitable and available, which filled the limb A of the U-tube in figure 1 of reference 3. When the experimental tube was in place there was a column of Fluorolube more than 30 cm high between the mercury surface at the bottom of limb A and the chlorine, and complete protection was obtained for the duration of the experiments. There was little deterioration in the appearance of the chlorine-to-Fluorolube boundary but the top layer of Fluorolube gradually took on a yellow colour. Diffusion and manipulation of the apparatus could gradually bring Fluorolube in solution to the gas-liquid interface, even though this was separated by a column of chlorine about 35 cm high from the chlorine-to-Fluorolube interface, and ultimately this might have caused the apparent vapour pressure to fall. To check whether there was any effect of this nature three tubes were used in the experiments as follows: with the first tube the vapour pressure from 334 K to the critical temperature was measured (which took about 6 h), and with the other two tubes determinations were made as quickly as possible (within 2 h) of the critical temperature and pressure only. All the values for the critical temperature and pressure were concordant and it was concluded that accuracy had not been seriously impaired by the presence. of the Fluorolube. However, when the critical temperature was determined”) in a sealed tube containing chlorine alone without the Fluorolube, the value obtained was 0.6 K less than that found in the vapour-pressure measurements and given in table 1, and this suggests that the presence of the Fluorolube might have increased the apparent critical temperature. It is however surprising in view of the good visual separation of the two fluids throughout the experiments and the long distance through which the Fluorolube would have had to travel before reaching the experimental zone. The apparatus is not suitable for use at temperatures less than about 35 K above ambient temperature and this therefore was the lower bound of this set of readings. All temperature measurements were made with platinum resistance thermometers used with an automatic a.c. bridge (Automatic Systems) or a Smith’s bridge no. 3. The chlorine was supplied by Mond Division, I.C.I. with an analysis indicating a mass fraction of volatile impurities approaching 250 x 10m6. More than half of the impurity was bromine, the effect of which would be negligible, but more important would be the 80 x 10q6 of hydrogen chloride, or any air that might inadvertently be present, and it was found when samples were condensed from the cylinder that they degassed on warming and that the vapour pressure determined in the range 206 to
THE
VAPOUR
PRESSURE
OF
1091
CHLORINE
270 K fell as part was distilled away. About half of each sample taken in the work, including those for the measurements in the range 334 K to the critical temperature, was therefore distilled away until a constant vapour pressure at a temperature corresponding to about 80 kPa was obtained from the residue.
3. Results The observed values of pressurep and temperature Tare in table 1. Measured IPTS-68 temperatures T,, were treated throughout this work as thermodynamic temperatures T, and the results were fitted by equations (1) and (2). In equation (1):“’ V/WgloWkPa) E,(x) is the Chebyshev polynomial
x = (2T-(T’a,+
T’iJ)/(L-
(1)
= 42 +s$l@s(x)y in x of degree s, where
GiJ, Tmin= 205 K, T&x = 417 K,
a = 1883.938, a, = 685.571, a2 = -2.344, a3 = 2.346, and a4 = -0.210. In equation (2): In p = In pc + (T,/T)(nlz+ 12&’ + n3r3 4 lt.&, TABLE
1. Vapour
pressure
p of chlorine: obtained
T/K
PlkPa
103A
(1)
205.945 205.970 210.157 210.172 215.005 215.018 219.810 219.824 224.962 224.967 230.097 230.129 234.920 234.933 239.864 239.879
17.90 17.92 23.04 23.06 30.45 30.48 39.62 39.63 51.83 51.82 66.71 66.80 83.63 83.72 104.39 104.46
0.5 0.3 -0.3 -0.4 -0.3 -0.2 -0.1 -0.3 0.4
334.11 345.7 353.30 363.6 373.05 373.78 374.05
1888 2377 2760 3314 3955 4014 4052
“kritical-point specially filled
observations; for determination
from
Alog,,p = loglOpobs-loglOpcalc equations (1) and (2)
log,op
T/K
(2)
plkpa
where
103A
(2) poalo has been log,,p
(1)
(2)
x:: -0.1 -0.1 0.1 0 0
0.6 0.3 -0.4 -0.4 -0.3 -0.2 -0.1 -0.3 0.4 02 0’1 -0:1 -0.1 0.1 0 0
244.942 244.961 250.121 250.143 254.834 254.916 255.117 255.120 256.042 256.063 260.118 260.156 265.119 265.17 270.377 270.380
129.76 129.85 160.22 160.35 192.52 193.12 194.78 194.79 201.59 201.80 234.70 234.92 280.60 281.07 335.87 335.88
0.2 0.1 0 -0.1 -0.2 -0.2 0.2 0.2 -0.1 0 0.1 0 0 0 -0.2 -0.2
0.2 0.2 0 0 -0.1 -0.1 0.3 0.2 -0.1 0 0.1 -0.1 0 -0.1 -0.3 -0.3
7.7 2.6 2.0 -3.1 -0.1 0.7 2.8
7.4 2.5 2.2 -2.6 0.7 1.6 3.6
384.0 391.37 404.96 416.76”
4747 5359 6596 7991
-2.4 -2.3 -5.1 1.7
-1.2 -1.0 -3.9 1.7
416.87b 416.90*
7974 7977
0 0
0 0
%oncluding of critical
observation temperature
of vapour-pressure and pressure.
measurements,
btubes
1092
D. AMBROSE
ET AL.
pE is the critical pressure, IQ&Pa) = 8.98437, T, is the critical temperature, 416.90 IS, z = (T,- T)/T,, nl = - 6.34074, n2 = 1.15037, IZ~ = - 1.40416, and n4 = - 2.23220. The fits of the equations are indicated in table 1 by the residuals A log,,p. Neither equation is markedly superior to the other and between 185 and 360 K the temperatures corresponding to a given pressure calculated from the two equations do not differ by more than 0.05 K, and at standard atmospheric pressure they differ by only 2 mK; from about 375 K the two equations diverge and the two temperatures differ by 0.2 K at 400 K returning to the same value at the critical temperature. Both equations conform to the constraints proposed by Ambrose, Counsell, and Hicks.@’
4. Discussion The results of the present work are compared with that by Giauque and Powell and by Pellaton in figure 1.(l12) In making any comparison with the work published by p/kPa
0.002
”
IA “%O.lKt
2 .i? a
% Y : 0
250
I
300
T/K FIGURE 1. Residuals (A log,op = log,,g,bs-logl~~p,,,, A, Giauque and Powell;C1) V (points at 234 and 252 K are area X, this work. a, Error bands corresponding to 10.05 K; K. The curved lines show the change in loglop corresponding
350
v v VV’JVw I -0.02 400
from equation (1) for measurements: off scale), Pellaton;(z) 0, and hatched b, error bands corresponding to 10.02 to the stated changes in temperature.
Giauque and Powell it is necessary to take account of the difference between the temperature scale in use in Giauque’s laboratory and IPTS-68. Temperatures were published on a scale in which the normal boiling temperature of oxygen was taken as 90.13 K (90.188 K) and that of the ice point as 273.10 K (273.15 K) where the figures in parentheses are the values assigned to these temperatures in IPTS-68. It is therefore necessary to add 0.05 K to Giauque’s values to bring them into line with modern thermometry, and this was done in the calculation of log,,p for plotting in figure 1. The values so obtained were taken to be equivalent to IPTS-68 temperatures because the published values were not on ITS-27, the appropriate international scale at that time, but were reputedly thermodynamic temperatures. The scale was established by calibration in the laboratory of a group of copper-to-constantan thermocouples
THE VAPOUR
PRESSURE OF CHLORINE
1093
against a hydrogen gas-thermometer, and these thermocouples were thought to be more reliable than the resistance thermometers used for temperature measurement in the actual experiments; the resistance thermometers were sufficiently sensitive and repeatable for temperatures to be given to 1 mK, and differences in temperature were much more accurate than were their actual values. In the calibration of the thermocouples, temperatures could be measured to 0.01 K with the gas thermometer but accuracy of only & 0.05 K was claimed for the thermocouple scale, although doubtless the authors hoped it was better. (‘) For example, Giauque and Weibe(‘O) measured the vapour pressure of hydrogen chloride, and found their values were within 0.03 K of those reported by Henning and Stock, cl ‘) which they accepted as being of reference quality. However, a wider limit seems to be indicated by other results: Giauque and Egan(“) found the normal sublimation temperature of carbon dioxide to be 194.72 K (194.674 K, difference 0.046 K), and Giauque, Bufhngton, and Schu.lze(g) found the freezing temperature of mercury to be 234.23 K (234.309 K, difference -0.079 K), where in each case the published value has been increased by 0.05 K and the values in parentheses are again those assigned to these temperatures in IPTS-68. We believe the errors in the present work between 206 and 270 K do not correspond to more than 0.02 K. If we allocate a possible error of + 0.05 K or slightly more to the temperatures published by Giauque and Powell, and lay out the resulting error bands on figure 1, the two sets or results just fail to meet. There is a discrepancy, but for most purposes emphasis on the good agreement between the two investigations is appropriate. Giauque and Powell reported the triple-point temperature of chlorine as 172.12 K; corrected, this becomes 172.17 K but, if their triple-point pressure, 1.392 kPa, and the present measurements are corrected, extrapolation of equation (1) leads to a higher value, 172.22 K. Prior to the work by Giauque and Powell the vapour pressure had been measured in approximately the same range by Trautz and Gerwig(13) and by Harteck.(14) Residuals from equation (1) for the results of these two investigations lie off the scale of figure 1, respectively above and below the zero line by between 5 and 10 per cent of the pressure. In the range above 273.15 K the present results are between 1 and 5 per cent higher than those by Pellaton, which we believe must be in error.{@ Pellaton’s value for the critical temperature “144 “c” (417.2 K) is close to that found in this work (416.90 K) and it seems unlikely that errors in his thermometry would have made the major contribution to the overall error. Pellaton also made a few measurements below 273.15 K using a mercury manometer in which the mercury was protected by a layer of concentrated sulphuric acid: these are of low precision but the residuals from equation (1) are of both signs which, in combination with the good agreement for the critical temperature, suggests that the chlorine was of reasonable purity. In the measurements above 273.15 K Pellaton prevented contact between the chlorine and any material other than glass by using a mercury thermometer as a pressure gauge. This was sealed within the system and kept at a constant temperature in ice; extension of the mercury thread due to compression of the bulb was calibrated in a separate experiment in terms of the pressure applied and was found to be linear. The calibration was done with the aid of apparatus (a Cailletet pump) in which the
1094
D. AMBROSE
I??’ AL.
pressure was calculated from the compression of nitrogen (pressures up to 2.5 MPa) or hydrogen (higher pressures) confined over mercury in a glass tube. An overall error of 3 per cent in the measurements arising from the uncertainty of the calibration and the relative insensitivity of the instrument does not seem impossible. However, Pellaton’s results are surprisingly self-consistent. The same type of pressure gauge was used by Berthoud, who worked in the same university, in his study of the critical pressures of amines, chloromethane, and chloroethane.(’ 5, Berthoud implied that the thermometer-manometer could be read to 10 kPa and described the method of calibration in greater detail than did Pellaton; in addition to using the gas-compression method, Berthoud generated fixed pressures by maintaining liquid carbon dioxide at a series of constant temperatures and derived the pressures from Amagat’s vapour-pressure measurements. There are no confirmatory values of the critical pressures that Berthoud measured to allow any estimate to be made of his error, but the values he quotes for the vapour pressure of carbon dioxide are below those currently accepted. @) The differences are, however, only about 0.5 per cent. We are unable, therefore, to be sure of the reasons for the systematic difference between Pellaton’s results and those of the present work, but we believe the present work to be more reliable, and in table 2 give selected values of p and dp/dT calculated from equation (1). TABLE
2. Vapour pressures calculated from equation (1) p/kPa
172.17 L1 1.383 176.26 2 239.184 b 101.325 291.83 o 651.76 “Triple point. *Normal boiling %ritical temperature.
dp/dT kPa K-l
T/K
0.128 0.176 4.463 18.55
298.15 386.80 416.90 d
temperature.
777.04 5000 7977
21.13 82.58 116.5
OT = 0.7T, giving w = log,,(7977/651.76)--1=0.088.
We wish to acknowledge the support given to this work by Imperial Industries Ltd., Mond Division.
Chemical
REFERENCES 1. Giauque, W. F.; Powell, T. M. J. Am. Chem. Sot. 1939,51,1970. 2. Pellaton, M. J. Chim. Phys. 1915,13,426. 3. Ambrose, D.; Broderick, B. E.; Townsend, R. J. Chem. Sac. A 1967,633. 4. Ambrose, D. ; Sprake, C.H.S. ; Townsend, R. J. Chem. Thermodynamics 1972,4,247. 5. Ambrose, D. Pure Appl. Chem. 1977,49,1437. 6. Ambrose, D.; Cox, J. D.; Townsend, R. Trans. Far. Sot. 1960,56, 1452. 7. Ambrose, D.; Counsel.& J. F. ; Davenport, A. J. J. Chem. Thermodynamics 1970,2, 283. 8. Ambrose, D.; Counsel& J. F.; Hicks, C. P. J. Chem. Thermodynamics 1978,10, 771. 9. Giauque, W. F.; Buffington, R. M.; Schulze, W. A. J. Am. Chem. Sot. 1927,49,2343. 10. Giauque, W. F.; Wiebe, R. J. Am. Chem. Sot. 1928,50, 101. 11. Henning, F.; Stock, A. Z. Chim. Phys. 1921,4,226. 12. Giauque, W. F. ; Egan, C. J. J. Chem. Phys. 1937,5,45. 13. Trautz, M. ; Gerwig, W. 2. anorg. algem. Chem. 1924,134,417. 14. Harteck, P. Z. physik. Chem. 1928,134,21. 15. Berthoud, A. J. Chim. Phys. 1917,1&l.