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Heat of sublimation and vapour pressure of strontium
Boerboom, Reyn,
A. J, H.
Physica
H. W.
Kistemaker,
30
254-257 J.
1964
HEAT
OF SUBLIMATION AND VAPOUR OF STRONTIUM
by A. J. H. BOERBOOM, F.O.M. - Laboratorium
H. W. REYN
PRESSURE
and J. KISTEMAKER
voor Massascheiding,
Amsterdam,
Nederland
Synopsis By
mass spectrometric
variable
temperature
analysis
of the vapour
the heat of sublimation
kcal/gram
atom
between
kcal/gram given by
atom
at 298 “K.
500 and 65O”C, corresponding The experimental
log&m) in the same temperature
effusing
=
from
a Knudsen
of Sr was determined value
-7,628/T
with
a value
of the vapour
cell of
as 34.9 & 0.15 of 36.1 + 0.15
pressure
of Sr is
+ 7.498
region.
Only Sr atoms were found in the vapour, thus being lower than our detection
the concentrations
limit of 1
of poly-atomic
species
: 106.
Introduction. The heat of evaporation and the vapour pressure of strontium have found little attention. In 1924 Ruff and Hartmannl) measured the loss of weight of a crucible filled with the metal at elevated temperatures in an inert atmosphere, as a function of time and gas pressure. They found from an empirical relation between the heat of evaporation and the boiling point of a substance a figure of 32,500 Cal/gram atom for the heat of evaporation. In 1929 Hartmann and Schneiderz) published much more reliable measurements of the temperature of condensation of the vapour as a function of the pressure of the inert gas. They mention 35,900 for this quantity at the measuring temperature of about 1000°C. Priselkov and Nesmeianovs) determined the amount of Sr vapour effused from a Knudsen cell during a certain time, by measuring the quantity of the condensed vapour. In the temperature region of 400-600°C they find 34.530 kcal/gram atom for the heat of sublimation. No other data have been reported in the literature. Eqberimental. The apparatus, used for the experiments has been described previously4). A molecular beam emerging from a Knudsen cell is ionised by collision with electrons in the ionisation chamber of a 15 cm radius, 60 degree mass spectrometer. After having passed the analyser tube, the -
254
-
HEAT OF SUBLIMATION AND VAPOUR PRESSURE OF STRONTIUM
ions are detected
either
with a Faraday
cage collector
255
or with a particle
multiplier. The Knudsen cell consisted of a molybdenum crucible with a hole of 2 mm diameter. The ratio of the internal surface of the crucible to the effusion hole was 110 : 1. The crucible was heated by radiation of two tungsten filaments. The temperature of the cell was measured within f 1“C with an iron- constantan thermocouple, calibrated at the melting points of lead and antimony. The pressure in the mass spectrometer tube was kept at about 5.1 O-7 mm. The reproducibility of the measurements could be greatly improved by rapid scanning of the temperature region and constantly referring to a base peak at some standard temperature. Individual measurements gave a precision of f0.20 kcal/gram atom, whereas the various runs varied about f 0.30 kcal/gram atom.
5
.s
>
11.0
11.5
12.0
!!
12.5
l/T
Fig. 1. Typical run of &+-ion intensity between 500 and 600°C. The points are numbered in the sequence in which they were measured. Though the connecting lines 1-2, 2-3, etc. etc. are not completely parallel, giving rise to different values for the heat of evaporation, the bisectors of the angles l-2-3, 2-3-4, etc. are much more parallel.
As an average over our scans we found from the log $ versus 1/T plot for the heat of sublimation 34.9 f 0.15 kcal/gram atom over the temperature region of 500-650°C of our measurements. No other species than mono-atomic Sr could be detected above our detection of 1 : 106. Finally an accurately weighed quantity of Sr was quantitatively evaporated under continuous control of the temperature. The duration of the evaporation was checked with the mass spectrometer. From Knudsen’s effusion formula
(1)
256
A.
H. J. BOERBOOM,
H. W.
REYN
AND
J. KISTEMAKER
where
P= g=
K=
r= t= T= M=
vapour pressure (in mm). weight of the evaporated Sr. Clausing’s coefficient, taken to be 0.96. radius of the aperture. time of the evaporation. absolute temperature. atomic weight of Sr.
we found as an average of three measurements p = 4.14 x lo-smmat Together with our heat of evaporation
this leads to
= -7,628/T
logp(m,,
499°C.
+ 7.498
(2)
for the vapour pressures of Sr in our measuring region of 500-650°C. Diswssion. In table I the experimental values of the heats of evaporation respectively sublimation of strontium are tabulated, as they are given by the various authors, together with the value recommended by S tull and Sinke5). All data are reduced to the heat of sublimation at 298”K, with a heat of fusion of 2,200 Cal/gram atom and the specific heat data of Stull and Sinke5). TABLE Heats of evaporation
AH,, T or sublimation
Author Ruff, Hartmann’) Hartmann, Schneider2) Priselkov, Nesmeianov 3) Stull, Sinke 5) and others This work
1
Temp
“K
I
AH,, T, as cited in the literature
1 AH,,T or AHs, T
1244-1410
32,500
1199-1379 673- 873 -
35,900
775-
925
-
1
AH,,am 37,400 40,940
34,500 34,900
35,520 39,100 36,130
The measurements of Ruff and Hartmann generally are considered to be superseded by the subsequent measurements of Hartmann and Schneider and their heat of evaporation is generally accepteds). The experimental procedure of the last authors, however, is not completely satisfactory. They consider the total pressure in their apparatus to be equal to the pressure of the saturated Sr vapour at the place where it condenses. The inert gas, that they use in their experiments, however, will diffuse into the vapour when streaming from the furnace to the condenser and the partial pressure of the vapour pressure of Sr will be less than supposed by Hartmann and Schneider. How this pressure defect will depend on the temperature is uncertain : the diffusion coefficient increases with tempera-
HEAT
ture,
OF SUBLIMATION
but discreases
AND
considerably
VAPOUR
PRESSURE
257
OF STRONTIUM
with the increasing
pressure.
Moreover,
the velocity of the streaming vapour is unknown. Extrapolation of the measurements of Priselkov and Nesmeianov and of the present authors, taking into account the heat of fusion and the specific heats, indeed gives lower vapour pressures than indicated by Hartmann and Schneider. The good agreement between Priselkov and Nesmeianov and the present authors seems to favour a value of about 36 kcal/gram atom for the heat of sublimation at 298°K. The formula (2) for the vapour pressure provides a value of 21.13 Cal/ degree gram atom for the entropy change of the sublimation at 850°K. Stull and Sinke estimate this quantity at the same temperature as 24.63 cal/ degree gram atom. From the data of Priselkov and Nesmeianov a value of 21.11 can be deduced, so Stull and Sinke’s estimation might be too high. Acknowledgements. The authors are indebted to Miss R. Teyema for her skilful help in performing the measurements and to Dr. T. P. J. H. Babeliowsky for suggesting the problem. This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter) and was made possible by financial support from the Nederlandse Organisatie voor Giver Wetenschappelijk Onderzoek (Netherlands Organisation for Pure Scientific Research (Z.W.O.). Received
17-7-63 REFEREKCES
3)
Ruff, 0. and Hartmann, H., 2. anorg. Chem. 138 (1924) 29. Hartmann, H., and Schneider, R., Z. anorg. Chem. 180 (1929) 275. Priselkov, I. A. and Nesmeianov, A. N., Doklady Akad. Nauk. S.S.S.R.
4)
Babeliowsky,
5)
Stull, D. R. and Sinke, 1956, p. 190-191.
6)
See e.g. Kelley, Stull and Sinke
1) 4
T. I’. J. H., Boerboom,
G. C., Thermodynamic
Rossini,
F. D. e.a.,
1952 (Figures
Landolt-BGrnstein,
properties
how
Kelley
Selected
from
derived
values
his proposed
of Chemical
95 (1954)
J., Physica$S
of the elements,
K. K., U.S. Bur. Mines Bull 383, Washington l.c.
It is not clear, however, and Schneider. Further literature: D.C.
A. J. H. and Kistemaker,
1207.
(1962) 1155
Washington
D.C.
D.C. 1935.
figure from the data of Hartmann
Thermodynamic
properties,
\Vashington
Kelley).
Zahlenwerte
(Figures of Stull and Sinke). Honig, R. E., R.C.A. Review
XXIII
und Funktionen,
Berlin
1961, Band
II, Teil 4, pag. 231
(1962) 567 (Figures of Stull and Sinke).
A. J, H.
Physica
H. W.
Kistemaker,
30
254-257 J.
1964
HEAT
OF SUBLIMATION AND VAPOUR OF STRONTIUM
by A. J. H. BOERBOOM, F.O.M. - Laboratorium
H. W. REYN
PRESSURE
and J. KISTEMAKER
voor Massascheiding,
Amsterdam,
Nederland
Synopsis By
mass spectrometric
variable
temperature
analysis
of the vapour
the heat of sublimation
kcal/gram
atom
between
kcal/gram given by
atom
at 298 “K.
500 and 65O”C, corresponding The experimental
log&m) in the same temperature
effusing
=
from
a Knudsen
of Sr was determined value
-7,628/T
with
a value
of the vapour
cell of
as 34.9 & 0.15 of 36.1 + 0.15
pressure
of Sr is
+ 7.498
region.
Only Sr atoms were found in the vapour, thus being lower than our detection
the concentrations
limit of 1
of poly-atomic
species
: 106.
Introduction. The heat of evaporation and the vapour pressure of strontium have found little attention. In 1924 Ruff and Hartmannl) measured the loss of weight of a crucible filled with the metal at elevated temperatures in an inert atmosphere, as a function of time and gas pressure. They found from an empirical relation between the heat of evaporation and the boiling point of a substance a figure of 32,500 Cal/gram atom for the heat of evaporation. In 1929 Hartmann and Schneiderz) published much more reliable measurements of the temperature of condensation of the vapour as a function of the pressure of the inert gas. They mention 35,900 for this quantity at the measuring temperature of about 1000°C. Priselkov and Nesmeianovs) determined the amount of Sr vapour effused from a Knudsen cell during a certain time, by measuring the quantity of the condensed vapour. In the temperature region of 400-600°C they find 34.530 kcal/gram atom for the heat of sublimation. No other data have been reported in the literature. Eqberimental. The apparatus, used for the experiments has been described previously4). A molecular beam emerging from a Knudsen cell is ionised by collision with electrons in the ionisation chamber of a 15 cm radius, 60 degree mass spectrometer. After having passed the analyser tube, the -
254
-
HEAT OF SUBLIMATION AND VAPOUR PRESSURE OF STRONTIUM
ions are detected
either
with a Faraday
cage collector
255
or with a particle
multiplier. The Knudsen cell consisted of a molybdenum crucible with a hole of 2 mm diameter. The ratio of the internal surface of the crucible to the effusion hole was 110 : 1. The crucible was heated by radiation of two tungsten filaments. The temperature of the cell was measured within f 1“C with an iron- constantan thermocouple, calibrated at the melting points of lead and antimony. The pressure in the mass spectrometer tube was kept at about 5.1 O-7 mm. The reproducibility of the measurements could be greatly improved by rapid scanning of the temperature region and constantly referring to a base peak at some standard temperature. Individual measurements gave a precision of f0.20 kcal/gram atom, whereas the various runs varied about f 0.30 kcal/gram atom.
5
.s
>
11.0
11.5
12.0
!!
12.5
l/T
Fig. 1. Typical run of &+-ion intensity between 500 and 600°C. The points are numbered in the sequence in which they were measured. Though the connecting lines 1-2, 2-3, etc. etc. are not completely parallel, giving rise to different values for the heat of evaporation, the bisectors of the angles l-2-3, 2-3-4, etc. are much more parallel.
As an average over our scans we found from the log $ versus 1/T plot for the heat of sublimation 34.9 f 0.15 kcal/gram atom over the temperature region of 500-650°C of our measurements. No other species than mono-atomic Sr could be detected above our detection of 1 : 106. Finally an accurately weighed quantity of Sr was quantitatively evaporated under continuous control of the temperature. The duration of the evaporation was checked with the mass spectrometer. From Knudsen’s effusion formula
(1)
256
A.
H. J. BOERBOOM,
H. W.
REYN
AND
J. KISTEMAKER
where
P= g=
K=
r= t= T= M=
vapour pressure (in mm). weight of the evaporated Sr. Clausing’s coefficient, taken to be 0.96. radius of the aperture. time of the evaporation. absolute temperature. atomic weight of Sr.
we found as an average of three measurements p = 4.14 x lo-smmat Together with our heat of evaporation
this leads to
= -7,628/T
logp(m,,
499°C.
+ 7.498
(2)
for the vapour pressures of Sr in our measuring region of 500-650°C. Diswssion. In table I the experimental values of the heats of evaporation respectively sublimation of strontium are tabulated, as they are given by the various authors, together with the value recommended by S tull and Sinke5). All data are reduced to the heat of sublimation at 298”K, with a heat of fusion of 2,200 Cal/gram atom and the specific heat data of Stull and Sinke5). TABLE Heats of evaporation
AH,, T or sublimation
Author Ruff, Hartmann’) Hartmann, Schneider2) Priselkov, Nesmeianov 3) Stull, Sinke 5) and others This work
1
Temp
“K
I
AH,, T, as cited in the literature
1 AH,,T or AHs, T
1244-1410
32,500
1199-1379 673- 873 -
35,900
775-
925
-
1
AH,,am 37,400 40,940
34,500 34,900
35,520 39,100 36,130
The measurements of Ruff and Hartmann generally are considered to be superseded by the subsequent measurements of Hartmann and Schneider and their heat of evaporation is generally accepteds). The experimental procedure of the last authors, however, is not completely satisfactory. They consider the total pressure in their apparatus to be equal to the pressure of the saturated Sr vapour at the place where it condenses. The inert gas, that they use in their experiments, however, will diffuse into the vapour when streaming from the furnace to the condenser and the partial pressure of the vapour pressure of Sr will be less than supposed by Hartmann and Schneider. How this pressure defect will depend on the temperature is uncertain : the diffusion coefficient increases with tempera-
HEAT
ture,
OF SUBLIMATION
but discreases
AND
considerably
VAPOUR
PRESSURE
257
OF STRONTIUM
with the increasing
pressure.
Moreover,
the velocity of the streaming vapour is unknown. Extrapolation of the measurements of Priselkov and Nesmeianov and of the present authors, taking into account the heat of fusion and the specific heats, indeed gives lower vapour pressures than indicated by Hartmann and Schneider. The good agreement between Priselkov and Nesmeianov and the present authors seems to favour a value of about 36 kcal/gram atom for the heat of sublimation at 298°K. The formula (2) for the vapour pressure provides a value of 21.13 Cal/ degree gram atom for the entropy change of the sublimation at 850°K. Stull and Sinke estimate this quantity at the same temperature as 24.63 cal/ degree gram atom. From the data of Priselkov and Nesmeianov a value of 21.11 can be deduced, so Stull and Sinke’s estimation might be too high. Acknowledgements. The authors are indebted to Miss R. Teyema for her skilful help in performing the measurements and to Dr. T. P. J. H. Babeliowsky for suggesting the problem. This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter) and was made possible by financial support from the Nederlandse Organisatie voor Giver Wetenschappelijk Onderzoek (Netherlands Organisation for Pure Scientific Research (Z.W.O.). Received
17-7-63 REFEREKCES
3)
Ruff, 0. and Hartmann, H., 2. anorg. Chem. 138 (1924) 29. Hartmann, H., and Schneider, R., Z. anorg. Chem. 180 (1929) 275. Priselkov, I. A. and Nesmeianov, A. N., Doklady Akad. Nauk. S.S.S.R.
4)
Babeliowsky,
5)
Stull, D. R. and Sinke, 1956, p. 190-191.
6)
See e.g. Kelley, Stull and Sinke
1) 4
T. I’. J. H., Boerboom,
G. C., Thermodynamic
Rossini,
F. D. e.a.,
1952 (Figures
Landolt-BGrnstein,
properties
how
Kelley
Selected
from
derived
values
his proposed
of Chemical
95 (1954)
J., Physica$S
of the elements,
K. K., U.S. Bur. Mines Bull 383, Washington l.c.
It is not clear, however, and Schneider. Further literature: D.C.
A. J. H. and Kistemaker,
1207.
(1962) 1155
Washington
D.C.
D.C. 1935.
figure from the data of Hartmann
Thermodynamic
properties,
\Vashington
Kelley).
Zahlenwerte
(Figures of Stull and Sinke). Honig, R. E., R.C.A. Review
XXIII
und Funktionen,
Berlin
1961, Band
II, Teil 4, pag. 231
(1962) 567 (Figures of Stull and Sinke).