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The vapour-pressure and transition points of TcF6
J. lnorg. Nucl. Chem., 1962, Vol. 24, pp. 641 to 644. Pergamon Press Ltd. Printed in Northern Ireland
THE
VAPOUR-PRESSURE OF
AND TRANSITION TcFr*
POINTS
H. SELIG and J. G. MALM Argonne National Laboratory, Argonne, Illinois (Received 12 January 1962) Abstract--The vapour-pressure and transition points of TcF6 have been measured. The vapourpressure is represented by the following equations: Solid I ( - 1 6 ' 3 2 to --5.3 °) log~op = --(3564.8/T) -- 10.787 log~0 T + 41.1252; Solid II ( - 5 . 3 to 37.4 °) log~0p ~ --(2178.0/T) -- 2-295 log~o T -~ 15.33427; Liquid (37.4-51.67 °) log~0p = - - ( 2 4 0 4 . 9 / T ) - 5"80361og10 T + 24.8087. TcF~ undergoes a solid-solid transition at --4"54 ° and melts at 37.4 ~.
THE THIRDtransition group elements from tungsten through platinum form a series of volatile hexafluorides which are remarkable for the regularity they exhibit in their physical properties, such as vapour-pressures and transition points, tl-zl Until recently molybdenum was the only element known to form a volatile hexafluoride in the second transition group. This has recently been augmented by the preparation and identification of additional hexafluorides in this series, those of technetium/ 4) ruthenium, C5~and rhodium. ~' A study of the properties of the hexafluorides of this series has been undertaken at this laboratory. We report here our measurements of the vapour-pressure and transition points of technetium hexafluoride. EXPERIMENTAL Technetium hexafluoride was prepared by the reaction of pure distilled fluorine gas on Tc metal at 400% It was purified by sublimation at reduced pressures as described elsewhereY ~ The purity was checked by inspection of the infra-red spectrum ~s~ and the intercomparison of numerous samples prepared in different batches as well as different cuts from the same batch. A convenient check-point for this purpose is the vapour pressure at 0 °. The absence of extraneous infra-red peaks and the constancy of the ice-point from sample to sample attested to the purity of the TcF~. Vapour-pressure measurements on 1 g samples were made with a quartz differential Bourdon gauge used as a null instrument. A description of this apparatus and its mode of operation is given elsewhereJ 9~ The over-all sensitivity of the gauge was ±0"02 m m Hg. The thermostated bath was regulated to ±0"01 ° as measured with a copper-constantan thermocouple which had been calibrated against a Bureau of Standards platinum resistance thermometer. The triple point and solid-solid transformation temperature of TcF~ were measured by the thermal arrest method. The procedure was similar to that described elsewhere, ~s) except that the glass bulb was replaced by an all-nickel welded bulb. * Based on work performed under the auspices of the U.S. Atomic Energy Commission. ~1~G. H. CADY and G. B. HARGREAVES,J. Chem. Soc. 1563 (1961). ~2, j. G. MALM and H. SELIG, J. lnorg. NueL Chem. 20, 189 (1961). ~sl B. WEINSTOCK, J. G. MALM and E. E. WEAVER, J. Amer. Chem. Soe. 83, 4310 (1961). ~) H. SEEm, C. L. CHERNICK and J. G. MALM, J. Inorg. NueL Chem. 19, 377 (1961). ~'~ H. H. CLAASSEN, H. SELtG, J. G. MALM, C. L. CHERNICK and B. WHNSTOCK, J. Amer. Chem. So¢. 83, 2390 (1961). " C. L. CHERNICK, H. H. CLAASSENand B. WEINSTOCK, J. Amer. Chem. Soc. 83, 3165 (1961). ~:~ B. WEINSTOCK and J. G. MALM, J. Inorg. NucL Chem. 2, 380 (1956). ~s~ H. H. CLAASSEN, H. SEEm and J. G. MALM, J. Chem. Phys. In Press. ~9~ B. WEINSTOCK, E. E. WEAVER and J. G. MALM, J. lnorff. Nu¢l. Chem. 11, 104 (1959). 641
642
H. SELIGand J. G. MALM
RESULTS The vapour-pressure data for the two solid and the liquid phases of TcF~ are given in Table 1. Temperatures are given in degrees Kelvin and pressures in mm Hg at 0 ° and standard gravity. The following equations which were derived by the method of least squares summarize the results: Solid I (--16.32 to --5"3 °) l°gl° P --
3564.8 T
10.787 log10 T q- 41.1252
(1)
Solid II (--5.3 to -+-37.4°) l°gl° P = --
2178.0 T -- 2.295 log10 T -[- 15.33427
(2)
2404.9 T -- 5.8036 lOglo T + 24.8087
(3)
Liquid (÷37-4-51.67 °) l°gl° P = --
Each point was weighted by its observed vapour-pressure, since the reading of the pressure was considered to be the major uncertainty in the measurement. The calculated solid-solid transition point from simultaneous solution of equations (1) and (2) is --4.54 ° in comparison to the measured value of --5.3 °. The calculated triple point from the simultaneous solution of equations (2) and (3) is 37.36 ° compared to a measured value of 37.4 °. The boiling point of TcF6 calculated using equation (3) is 55.3 ° . Thermodynamic calculations. The heats of sublimation and vaporization of TcF 6 were derived from the vapour-pressure equations using the Clausius-Clapeyron relation. The heat of sublimation at 37.4 ° is 8555 cal mole -1 and the heat of vaporization at this temperature is 7427 cal mole -1. The heat of fusion is 1128 cal mole -1 and the entropy of fusion is 3.63 e.u. The heat of sublimation of solid I at --5.3 ° is 10,577 cal mole-L The heat of sublimation of solid II is 8750 cal mole -1 at this temperature. The heat of transition calculated from the heats of sublimation at the transition point is 1827 cal mole -1 and the entropy of transition is 6.82 e.u. The Trouton constant for liquid TcF 8 is 22.0 cal mole -1 °C-L DISCUSSION Extensive surveys of the vapour-pressures and transition points of the " 5 d " hexafluorides as well as of M o F 6 have been completed recently. ~1.3~ The volatilities of the " 5 d " hexafluorides decrease regularly in going across the series while the solid-solid transition temperatures and triple points increase in a regular fashion. The present work combined with the MoF 6 data as well as some preliminary data for RuF6tS~ and RhF616> confirm that a similar regularity exists in the " 4 d " series. This may possibly be explained qualitatively in the following way. The M-F distances from W F 6 to lrF 6 are practically constant.* By analogy the same situation probably obtains in the " 4 d " series. However, the cell constants of the solid phases in both series decrease regularly in going across the series3 m It is reasonable therefore * Preliminary results by KIMURAet aL~a°> ~10~KaMtmA,WEINSTOCKand SCHOMAKER,Proceedings of the Second International Conference on the Peaceful Use of Atomic Energy, Geneva, 1958, P/942 United Nations (1958). tlt~ D. NORTHROPand S. Sm~EL. Private communication.
T h e v a p o u r - p r e s s u r e a n d transition points o f TcFn
643
TABLE I.--VAPOUR-PRESSURE OF T c F 6 Solid I
T(K) 256"83 257"95 258"97 259"74 260.77 261"85 262.03 263.00 263' 19 264"06 264.19 265-17 265-68 266.24 266'29 266-69 267.68
Solid II
Observed pressure (ram Hg)
Pob8 -- Peate ( r a m Hg)
17'90 19"60 21 "24 22-64 24.62 26'62 27"24 29.39 29.76 31'76 32.11 34" 54 35.94 37.40 37.75 38.97 41.73
0'02 0"00 --0"05 --0"01 0.03 --0'15 0'08 0.10 0'03 --0.03 0.00 -- 0"07 --0"03 --0"12 0"08 0"16 --0.06
Solid 11 268-32 268.71 269"88 269'98 272.00 272.2(I 273.15 274-08 274.54 275.12 275'69 276"12 277"31 278-52 279"25 280'08 281' 32 282.17 284'16 286" 12 286"49 288.12 289-12 290.10 291.05 293.06
43.96 44.95 48-35 48"62 54-85 55.70 58.79 62"17 63"42 65'93 68.50 69.79 75"13 80"50 83.20 87.34 93'45 97.89 109-24 121.21 123.81 134-75 142.13 149.29 157-05 173.88
that the intermolecular
0"03 -0.10 - 0-01 --0"03 --0.05 0.14 0"03 0.12 --0.32 0.01 0'37 --0.05 0.37 0.44 --0.22 --0.04 - 0-15 --0-20 --0-07 --0.22 --0.03 --0.21 --0.07 --0.33 --0"07 --0.18
eobs -- Pealc (ram Hg)
294"59 296"05 297'53 297"79 298'19 299.22 299.96 300.51 301-98 302.60 302.64 302.71 302.72 303.44 304-10 304-19 304"28 304.96 305-90 305.93 306.92 308.16 309-16 31003
188"19 202"6 217"8 220'3 224'2 236"8 244.2 251.8 270. l 276-9 277.8 278-4 278.6 289.3 297.4 298.0 299"9 310'4 323.1 323.3 340.6 358.9 374.1 390.6
0"22 0"4 0"4 0"1 - 0.3 0.4 - 0.4 0.6 0 7 0.5 0. l 0.4 0-3 0-8 - 0-2 - 0-8 - 0"2 0'7 -0-4 0.6 1.6 0.2 12 0.4
Liquid 31 I-11 312.05 313'15 313'94 314.07 315.33 316.11 317.17 318"04 319.19 319.19 320.08 321.20 322.19 322.26 323.16 323'81 324.82
507.9 323.5 440"5 454"6 457"4 478-1 494-3 511.9 530-4 550.9 551.6 571.1 591.9 614.2 615.1 635'8 651 "4 673.2
-0.2 0.4 --0.7 --0" 1 0"6 - 08 1-3 --0"8 I" 1 - 1-0 -~ 0.2 l '3 - 1-2 - 0-1 - 07 0.1 I '2 --0.3
d i s t a n c e s d e c r e a s e , a n d t h e r e f o r e tile V a n d e r W a a l s f o r c e s i n
the crystals increase in going across both pressures and transition
TCK)
Observed pressure (ram Hg)
series.
The observed
changes
in vapour
points agree with this view.
The "4d" hexafluorides
are considerably
T h e r e a s o n f o r t h i s is n o t u n d e r s t o o d
less volatile than their "5d" counterparts.
at present.
644
H. SELIG and J. G. MALM
Finally, it should be pointed out that the vapour-pressure data and infra-red spectra of TcF6 did not show any evidence for the existence of TcF 7. Discrepancies in similar data on the volatile fluorination products of Re metal led to the first identification of ReF 7.~12) ~12, j. G.
MALM,H. SELIG and S. FRIED,J. Amer. Chem. Soc. 82, 1510 (1960).
THE
VAPOUR-PRESSURE OF
AND TRANSITION TcFr*
POINTS
H. SELIG and J. G. MALM Argonne National Laboratory, Argonne, Illinois (Received 12 January 1962) Abstract--The vapour-pressure and transition points of TcF6 have been measured. The vapourpressure is represented by the following equations: Solid I ( - 1 6 ' 3 2 to --5.3 °) log~op = --(3564.8/T) -- 10.787 log~0 T + 41.1252; Solid II ( - 5 . 3 to 37.4 °) log~0p ~ --(2178.0/T) -- 2-295 log~o T -~ 15.33427; Liquid (37.4-51.67 °) log~0p = - - ( 2 4 0 4 . 9 / T ) - 5"80361og10 T + 24.8087. TcF~ undergoes a solid-solid transition at --4"54 ° and melts at 37.4 ~.
THE THIRDtransition group elements from tungsten through platinum form a series of volatile hexafluorides which are remarkable for the regularity they exhibit in their physical properties, such as vapour-pressures and transition points, tl-zl Until recently molybdenum was the only element known to form a volatile hexafluoride in the second transition group. This has recently been augmented by the preparation and identification of additional hexafluorides in this series, those of technetium/ 4) ruthenium, C5~and rhodium. ~' A study of the properties of the hexafluorides of this series has been undertaken at this laboratory. We report here our measurements of the vapour-pressure and transition points of technetium hexafluoride. EXPERIMENTAL Technetium hexafluoride was prepared by the reaction of pure distilled fluorine gas on Tc metal at 400% It was purified by sublimation at reduced pressures as described elsewhereY ~ The purity was checked by inspection of the infra-red spectrum ~s~ and the intercomparison of numerous samples prepared in different batches as well as different cuts from the same batch. A convenient check-point for this purpose is the vapour pressure at 0 °. The absence of extraneous infra-red peaks and the constancy of the ice-point from sample to sample attested to the purity of the TcF~. Vapour-pressure measurements on 1 g samples were made with a quartz differential Bourdon gauge used as a null instrument. A description of this apparatus and its mode of operation is given elsewhereJ 9~ The over-all sensitivity of the gauge was ±0"02 m m Hg. The thermostated bath was regulated to ±0"01 ° as measured with a copper-constantan thermocouple which had been calibrated against a Bureau of Standards platinum resistance thermometer. The triple point and solid-solid transformation temperature of TcF~ were measured by the thermal arrest method. The procedure was similar to that described elsewhere, ~s) except that the glass bulb was replaced by an all-nickel welded bulb. * Based on work performed under the auspices of the U.S. Atomic Energy Commission. ~1~G. H. CADY and G. B. HARGREAVES,J. Chem. Soc. 1563 (1961). ~2, j. G. MALM and H. SELIG, J. lnorg. NueL Chem. 20, 189 (1961). ~sl B. WEINSTOCK, J. G. MALM and E. E. WEAVER, J. Amer. Chem. Soe. 83, 4310 (1961). ~) H. SEEm, C. L. CHERNICK and J. G. MALM, J. Inorg. NueL Chem. 19, 377 (1961). ~'~ H. H. CLAASSEN, H. SELtG, J. G. MALM, C. L. CHERNICK and B. WHNSTOCK, J. Amer. Chem. So¢. 83, 2390 (1961). " C. L. CHERNICK, H. H. CLAASSENand B. WEINSTOCK, J. Amer. Chem. Soc. 83, 3165 (1961). ~:~ B. WEINSTOCK and J. G. MALM, J. Inorg. NucL Chem. 2, 380 (1956). ~s~ H. H. CLAASSEN, H. SEEm and J. G. MALM, J. Chem. Phys. In Press. ~9~ B. WEINSTOCK, E. E. WEAVER and J. G. MALM, J. lnorff. Nu¢l. Chem. 11, 104 (1959). 641
642
H. SELIGand J. G. MALM
RESULTS The vapour-pressure data for the two solid and the liquid phases of TcF~ are given in Table 1. Temperatures are given in degrees Kelvin and pressures in mm Hg at 0 ° and standard gravity. The following equations which were derived by the method of least squares summarize the results: Solid I (--16.32 to --5"3 °) l°gl° P --
3564.8 T
10.787 log10 T q- 41.1252
(1)
Solid II (--5.3 to -+-37.4°) l°gl° P = --
2178.0 T -- 2.295 log10 T -[- 15.33427
(2)
2404.9 T -- 5.8036 lOglo T + 24.8087
(3)
Liquid (÷37-4-51.67 °) l°gl° P = --
Each point was weighted by its observed vapour-pressure, since the reading of the pressure was considered to be the major uncertainty in the measurement. The calculated solid-solid transition point from simultaneous solution of equations (1) and (2) is --4.54 ° in comparison to the measured value of --5.3 °. The calculated triple point from the simultaneous solution of equations (2) and (3) is 37.36 ° compared to a measured value of 37.4 °. The boiling point of TcF6 calculated using equation (3) is 55.3 ° . Thermodynamic calculations. The heats of sublimation and vaporization of TcF 6 were derived from the vapour-pressure equations using the Clausius-Clapeyron relation. The heat of sublimation at 37.4 ° is 8555 cal mole -1 and the heat of vaporization at this temperature is 7427 cal mole -1. The heat of fusion is 1128 cal mole -1 and the entropy of fusion is 3.63 e.u. The heat of sublimation of solid I at --5.3 ° is 10,577 cal mole-L The heat of sublimation of solid II is 8750 cal mole -1 at this temperature. The heat of transition calculated from the heats of sublimation at the transition point is 1827 cal mole -1 and the entropy of transition is 6.82 e.u. The Trouton constant for liquid TcF 8 is 22.0 cal mole -1 °C-L DISCUSSION Extensive surveys of the vapour-pressures and transition points of the " 5 d " hexafluorides as well as of M o F 6 have been completed recently. ~1.3~ The volatilities of the " 5 d " hexafluorides decrease regularly in going across the series while the solid-solid transition temperatures and triple points increase in a regular fashion. The present work combined with the MoF 6 data as well as some preliminary data for RuF6tS~ and RhF616> confirm that a similar regularity exists in the " 4 d " series. This may possibly be explained qualitatively in the following way. The M-F distances from W F 6 to lrF 6 are practically constant.* By analogy the same situation probably obtains in the " 4 d " series. However, the cell constants of the solid phases in both series decrease regularly in going across the series3 m It is reasonable therefore * Preliminary results by KIMURAet aL~a°> ~10~KaMtmA,WEINSTOCKand SCHOMAKER,Proceedings of the Second International Conference on the Peaceful Use of Atomic Energy, Geneva, 1958, P/942 United Nations (1958). tlt~ D. NORTHROPand S. Sm~EL. Private communication.
T h e v a p o u r - p r e s s u r e a n d transition points o f TcFn
643
TABLE I.--VAPOUR-PRESSURE OF T c F 6 Solid I
T(K) 256"83 257"95 258"97 259"74 260.77 261"85 262.03 263.00 263' 19 264"06 264.19 265-17 265-68 266.24 266'29 266-69 267.68
Solid II
Observed pressure (ram Hg)
Pob8 -- Peate ( r a m Hg)
17'90 19"60 21 "24 22-64 24.62 26'62 27"24 29.39 29.76 31'76 32.11 34" 54 35.94 37.40 37.75 38.97 41.73
0'02 0"00 --0"05 --0"01 0.03 --0'15 0'08 0.10 0'03 --0.03 0.00 -- 0"07 --0"03 --0"12 0"08 0"16 --0.06
Solid 11 268-32 268.71 269"88 269'98 272.00 272.2(I 273.15 274-08 274.54 275.12 275'69 276"12 277"31 278-52 279"25 280'08 281' 32 282.17 284'16 286" 12 286"49 288.12 289-12 290.10 291.05 293.06
43.96 44.95 48-35 48"62 54-85 55.70 58.79 62"17 63"42 65'93 68.50 69.79 75"13 80"50 83.20 87.34 93'45 97.89 109-24 121.21 123.81 134-75 142.13 149.29 157-05 173.88
that the intermolecular
0"03 -0.10 - 0-01 --0"03 --0.05 0.14 0"03 0.12 --0.32 0.01 0'37 --0.05 0.37 0.44 --0.22 --0.04 - 0-15 --0-20 --0-07 --0.22 --0.03 --0.21 --0.07 --0.33 --0"07 --0.18
eobs -- Pealc (ram Hg)
294"59 296"05 297'53 297"79 298'19 299.22 299.96 300.51 301-98 302.60 302.64 302.71 302.72 303.44 304-10 304-19 304"28 304.96 305-90 305.93 306.92 308.16 309-16 31003
188"19 202"6 217"8 220'3 224'2 236"8 244.2 251.8 270. l 276-9 277.8 278-4 278.6 289.3 297.4 298.0 299"9 310'4 323.1 323.3 340.6 358.9 374.1 390.6
0"22 0"4 0"4 0"1 - 0.3 0.4 - 0.4 0.6 0 7 0.5 0. l 0.4 0-3 0-8 - 0-2 - 0-8 - 0"2 0'7 -0-4 0.6 1.6 0.2 12 0.4
Liquid 31 I-11 312.05 313'15 313'94 314.07 315.33 316.11 317.17 318"04 319.19 319.19 320.08 321.20 322.19 322.26 323.16 323'81 324.82
507.9 323.5 440"5 454"6 457"4 478-1 494-3 511.9 530-4 550.9 551.6 571.1 591.9 614.2 615.1 635'8 651 "4 673.2
-0.2 0.4 --0.7 --0" 1 0"6 - 08 1-3 --0"8 I" 1 - 1-0 -~ 0.2 l '3 - 1-2 - 0-1 - 07 0.1 I '2 --0.3
d i s t a n c e s d e c r e a s e , a n d t h e r e f o r e tile V a n d e r W a a l s f o r c e s i n
the crystals increase in going across both pressures and transition
TCK)
Observed pressure (ram Hg)
series.
The observed
changes
in vapour
points agree with this view.
The "4d" hexafluorides
are considerably
T h e r e a s o n f o r t h i s is n o t u n d e r s t o o d
less volatile than their "5d" counterparts.
at present.
644
H. SELIG and J. G. MALM
Finally, it should be pointed out that the vapour-pressure data and infra-red spectra of TcF6 did not show any evidence for the existence of TcF 7. Discrepancies in similar data on the volatile fluorination products of Re metal led to the first identification of ReF 7.~12) ~12, j. G.
MALM,H. SELIG and S. FRIED,J. Amer. Chem. Soc. 82, 1510 (1960).