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Mass spectrometric studies on the sublimation of rubidium perrhenate
329
Journal qf the Less-Common Metals, 31 (1973) 329-335 lF\ Elsevier Sequoia S.A., Lausanne Printed in The Netherlands
MASS SPECTROMETRIC STUDIES ON THE SUBLIMATION RUBIDIUM PERRHENATE
KRZYSZTOF
SKUDLARSKI
Institute ,for Inorganic (Poland) (Received
November
and WOJCIECH
Chemistry and Metallurgy
OF
LUKAS
of Rare Elements, Technical University, Wroclaw
29, 1972)
SUMMARY
The equilibrium sublimation of RbReO, over the temperature range 71&863 K has been studied. RbReO, was evaporated from single and double silica Knudsen cells and the effusing vapours were examined by means of a MI-1305 mass spectrometer specially adapted for high temperature studies. Molecules of RbReO, monomer and those of the dimer (RbReO,), were found in the vapours. The partial pressures of monomer and dimer, their sublimation heats and the heat of thermal dissociation of the dimer were determined.
INTRODUCTION
Studies on the composition and partial pressures of the gaseous phase in equilibrium with heated inorganic compounds are now usually carried out by means of suitably adapted mass spectrometers. Studies on the compositions of vapours evolved from inorganic oxy-acid salts have been developed only during the last ten years. Potassium perrhenate, KReO,, was found by Vorlander and Dalichau’ to boil at 1650 K without visible decomposition. Neuman and Costeanu’ determined the vapour pressure over KReO, at temperatures from 780 to 855 K on the assumption that only monomeric molecules exist in the vapours. Spiridonov, Hodchenkov and Akishin3 investigated KReO, vapour by electron diffraction, and Spoliti, Ward and Stafford4 examined CsReO, vapour by infrared spectroscopy. Investigations on the composition of the gaseous phase over perrhenates have been carried out for NaReO, and KReO, by Skudlarski, Drowart, Exteen and Vauder Auwera Mahieu’, by Skudlarski6 for CsReO, and by Semonov7 for TlReO,. The gaseous phase formed over each of these salts contains monomeric and dimeric molecules. In addition to the above perrhenates lithium, rubidium, silver and copper perrhenates also evaporate with the salt molecules retained in the gaseous phase*. The purpose of the present work was to investigate the composition of the gaseous phase over solid RbReO,, to determine the partial pressures of the individual components of the vapour and to calculate the enthalpies of sublimation and dimerization reactions.
330
K. SKUDLARSKI,
W. LUKAS
EXPERIMENTAL
~~surements were carried out in a Soviet made MI-1305 mass spectrometer with a 60” deflection angle and 200 mm radius of curvature, specially adapted for thermochemical work9*“. The Knudsen cells were made of silica glass. A detailed discussion of the measuring procedure has been given previously’. RbReO, was obtained by the neutralization of rubidium hydroxide with perrhenic acid, RbOH was obtained from RbCl by reaction with Ag,O, and HReO, from NH4Re0, by ion exchange. RESULTS
AND DISCUSSION
The mass spectrum of rubidium perrhenate contains the ions: Rbf, RbO+, RbRe+, RbReO+, RbReOT, RbReOz, RbReO:, RbzReOa, Re+, ReO+, ReO:, ReO:, ReO:. The Rb” ions exhibit the highest intensities; the intensities of other ions are much lower. By analogy with the evaporation of alkali metal borates”, halides13, 14, hydroxide$-i7, and cyanidesIs, it may be assumed that, as with NaReO,, KRe045 and CsRe046, all the above mentioned ions result from the monomeric RbReO, and dimeric (RbReO,), molecules. This conclusion is also supported by the fact that as in the case of the ions formed from the ionization of caesium perrhenate vapours6, all other ions, apart from RbReOf, possess a certain excess kinetic energy. The excess kinetic energy was also previously found in the Na’ and K+ ions formed during the electron ionization of sodium and potassium perrhenate vapours5. The mass spectrometer used did not permit the precise determination of the appearance potentials of the ions observed. It was, however, possible to observe that, as in the case of CsReO,, all the ions, apart from Rb+, have appearance potentials higher than that of RbReO:. These potentials decrease in the sequence: RbRe’ > RbReO+ > RbReO: > RbReO: > Rb,ReO: > RbReO:. The above mentioned ions therefore originate from larger molecules which are disintegrated by electrons, most probably from the monomer RbReO, and the dimer, (RbReO,),. Figure 1 illustrates a temperature relationship of the 1. T product which is proportional to the partial pressure of those molecules from which the ions in question are formed. P = IT,/crys
(1)
where P is the pressure, I the intensity.of the ion stream, T the temperature, c the ionization cross section, y the sensitivity factor of the ion coilector, and s the sensitivity constant of the mass spectrometer, This figure shows that the line corresponding to the Rb,ReO: ions has significantly the greatest slope. The other lines have slopes close to that of the RbReOf line though somewhat higher. This is further evidence that the majority of all ions, other than Rb,ReO: originate from the same primary molecule as RbReO:, most probably from the RbReO, monomer. On the other hand, the Rb,ReOf ions come from another primary molecule, probably from the (RbReO,), dimer.
MASS SPECTROMETRIC
STUDIES
Rb'
0 + Rho'
OF RbReO,
SUBLIMATION
331
\b
Fig. 1. Dependence of log IT upon l/T. I is the relative of the gas from which the ions are formed.
intensity
of the ions and
T is the temperature
Studies on fragmentation of neutral molecules with electrons In order to determine the proportions of particular ions which are derived from electron fragmentation of either monomer or dimer, measurements employing double Knudsen cells were carried out by the method reported by Gorokhov”. The measurement using a double Knudsen cell consists of producing various vapour pressures of the substance under investigation in the upper effusion cell, where a constant tem~rature is maintained, and of recording, in a spectrometer, the intensities of the ions formed. The intensity measurements of all ions observed were carried out with the upper cell maintained at a constant temperature and with the bottom cell maintained at several different temperatures. The intensity measurements of all ions observed enabled us to calculate the proportions of the ions derived from vapour ionization of either RbReO, (monomer) or (RbRe0,)2 (dimer).
(2) and c =
J,,,,
[I;M,D)-z~,,,,nill(n--nf)f,
(3)
is the intensity of the measured ion resulting from ionization of monomer and dimer,Z, is the intensity of measured ion resulting from ionization of the monomer,Zn is the intensity of Rb,ReO: ion resulting from ionization of the dimer and Znc is the intensity of the measured ion resulting from ionization of the dimer
K. SKUDLARSKI,
332
W. LUKAS
The values marked by superscripts (‘) correspond to the ion intensities measured with the bottom cell at higher temperatures, whilst those without superscripts relate to measurements at lower temperatures. The temperature of the upper cell was always constant and equal or higher than the temperature of the bottom cell. The temperature of the channel connecting both cells did not fall below that of the bottom cell. Factor C in eqn. (2) was calculated from each pair of measurements which differed in the bottom cell temperature. The average value C for each type of ion was calculated from the C values obtained. Using these values of C and eqn. (2) the proportions of each ion intensity from both monomeric and dimeric molecules were calculated from three independent experiments. The results are given in Table I. TABLE I THE ORIGIN OF IONS APPEARING IN THE MASS SPECTRUM EQUILIBRIUM WITH SOLID RbReO, AT 863 K
OF VAPOURS
IN
The energy of the ionizing electrons was 4.8 MJ/mole.
Type of
ion
Rb+ RbO+ Rb,ReO: RbReOa RbReO: RbReOi RbReO+ RbRe+ ReOa ReO: ReOi ReO ’ Re+
Relative intensity
100 0.23 0.25 0.22 0.051 0.009 0.022 0.11 0.048 0.039 0.084 0.06 0.044
Intensity of ions derived from dimer
11.3 0.03 1 0.25 0.020 0.005 0.0007 0.002 0.01 0.017 0.0107 _
Percentage of Intensity of ions derived intensity for from monomer particular ions derived from dimer
88.7 0.20 0.20 0.046 0.0083 0.020 0.10 0.03 1 0.029 0,084 _
11.3 13.5 100 8.9 9.4 8.15 8 9 35 25 0
Relative intensity spectrum of ions derived from dimer
mOnomer
100 0.27 2.2 0.18 0.044 0.006 0.018 0.09 0.15 0.09 _
100 0.23 0.23 0.052 0.0093 0.022 0.11 0.035 0.032 0.095
A comparison of the spectra for both monomer and dimer shows that the ionization of the dimer gives relatively three times more of the ReO: and ReO: ions than that of the monomer. The ReO: ion is derived from the monomer. The proportions of other ions in the spectrum are similar. Calculation of the RbReO, and (RbReOJ2
vapour pressures
In order to calculate the vapour pressures, the relationships between the molecular pressures in the Knudsen cell and the intensities of the ions formed from their ionization eqn. (1) as well as the Knudsen equation were used:
MASS SPECTROMETRIC
STUDIES
OF RbReO,
SUBLIMATION
333
where P is the gas pressure, G the mass of the substance evaporated from the Knudsen cell, q the area of the effusion hole, At the evaporation time at temperature T, M the mass of 1 mole of gaseous molecules, and K the Clausing factor, assuming K = 1. In our studies the ratio of the evaporating area to that of the effusion hole was at least 500. In practice, it was found that as in studies on caesium perrhenate6, with the size of effusion holes equal to about 0.05 mm2 only 0.2 mg RbReO, in the condensed phase was sufficient to produce saturated vapours in the cell. At the end of the measurements a few mg of the substance was always left in the cell. In order to determine the vapour pressures, a quantitative evaporation method and integration of ion intensities were employed in a slightly changed form. The first part of the measurement was carried out polythermally. The temperature of the Knudsen cell was raised gradually in steps of about 7 K and when the temperature became stabilized, the intensities of all ions were measured with the simultaneous recording of the exposure time at a given temperature and sample heating times. Having measured the intensities at about 2.5 different temperatures, the temperature was maintained constant for a period of time required to vaporize about three times as much RbReO, as was evaporated during the entire polythermal process carried out previously. The Gapour pressures of monomer and dimer were calculated from eqn. (1). The product of spectrometer sensitivity (s) multiplied by the effective ioni~tion cross section required in this equation was calculated from the same measurement by means of eqn. (5).
This formula was obtained by applying eqn. (1) and the Knudsen formula (2) to the process taking place at several different temperatures, when various gas molecules were ionized. s is the sensitivity constant of the spectrometer, q the area of the effusion hole, M the mass of 1 mole of gaseous RbReO,, R the gas constant, G the mass of vaporized RbReO,, CJ, the ionization cross section of RbReO,, LT,,the ionization cross section of (RbReO,),, XJwt the sum of intensities for all ions resulting from the monomer, CJn, the sum of intensities for all ions resulting from the dimer, T the temperature and At the sample heating time at temperature T. The sum of the intensities of all the ions derived from the monomer C JM, was assumed to be equal to the intensity of the rubidium ions derived from the monomer IRb*(M)andC4,,=I,,+(n,. The intensities of the Rb+ ions derived from the monomer and dimer as calculated from eqn. (2) were substituted for the intensities CJhlj and Cl,,,,. The intensities of particular ions RbO ‘, RbReOz, ReOT, where x = 1 to 4, were not integrated since their total intensity is only about 1% of the Rb+ ion intensity (Table I), and an increase in the accuracy of the pressure calculation due to the summation of intensities for all these ions would be insignificant in comparison with other sources of error. The sensitivity factor yr,/yM of the ion collector for various ions would be in this case equal to 1 since it does not depend on the origin of the Rbf ions measured (whether from a monomeric or dimeric molecule).
334
K. SKUDLARSKI, W. LUKAS
The values of the last terms of eqn. (5) corresponding to the long heating time at the highest temperature, were about three times greater than the sum of the other terms of the equation. The accuracy of the s(ay) product calculation was then almost the same as in a simple isothermal measurement. The same measurement enabled us to obtain simultaneously the relationship between the temperatures and intensities of particular ions. This made it possible to calculate the partial pressures of monomer and dimer from eqn. (1) over the entire temperature range concerned from the known origin of ions and known y~n/ycr~ ionization cross section ratio. The relative dimer/monomer electron ionization cross section ratio was assumed to be equal to 1.4, allowing for a change of this ratio for alkali halides with increasing weight of the alkali element”, and the values obtained previously for other perrhenates. For5 KReO, (~~~/(~y~ = 0.67, and for6 CsReO, {~~~/(~~~ = 1.8. The vapour pressures of monomer and dimer may be expressed by the equations : for RbReO, log PNlm:! = - (9750/T) + 11.33 for (RbReO& log P&Z = - (13450/T) + 14.62 Heats of sublimation were calculated from the equations log fa,,o;fMj. T= = f( l/T) and log IRbZReo4+ +T= f( l/T) where I”,,,,; (Mj is the intensity of RbReO: ions resulting from the monomer. For the monomer RbReO, AIi+‘so= 186.6 t_ 3.6 kJ/ mol, for dimer (RbReO,), AH,,,’ - 257.4k4.0 kJ/mol. The deviations given are standard errors of the regression factor from particular points of a single measurement. Taking into account other possible sources of errors of measurement it was assumed: for monomer RbReO, A&so’ -187-t- 15 kJ/mol and for (RbReO& AH&,, = 257$- 15 kJ/mol. The heat of dimer/m&omer dissociation II&,= 1165 25 kJ/mol. REFERENCES 1 2 3 4 5
D. Vorlander and G. Dalichau, Ber., 66 (1933) 1534. K. Neumann and V. Costeanu, Z. Phys. Chem., Al85 (1939) 65. V. P. Spiridonov, A. N. Hodchenkov and P. A. Akishin, Vesttl.MO&. Univ., Ser. II, 6 (1965) 34. M. Spoliti, B. G. Ward and F. E. Stafford, quoted in ref. 5. K. Skudlarski, J. Drowart, G. Exsteen and A. Vander Auwera-Mahieu, Trans. Faraday Sot., 63 (1967) 1146. 6 K. Skudlarski, Roczniki Chem., to be published. 7 G. A. Semenov, Probl. sovremen. khim. koordinac. soedin., Leningr. Univ., 3 (1966) 16. 8 K. Skudlarski and W. Lukas, Nukleonika, 17 (1972) 189. 9 K. Skudlarski, Prib. Tekh. Eksp., (2) (1971) 268. 10 W. Lukas, to be published. 11 P. A. Akishin, L. H. Gorokhov, Yu. S. Khodeev, J. Strukt. Khim., 2 (1961) 209. 12 A. Biichler and J. Berkow~t-Mattu~k, J. Chem. Phys., 39 (1963) 286. 13 R. F. Porter and R. C. Schoonmaker, J. Chem. Phys., 29 (1958) 1070. 14 J. Berkowiz and W. A. Chupka, J. Chem. Phys., 29 (1958) 653. 15 R. C. Schoonmaker and R. F. Porter, J. C&em. Phys., 28 (1958) 454.
MASS SPECTROMETRIC 16 17 18 19 20
STUDIES
OF RbReO,
SUBLIMATION
A. V. Gusarov and L. N. Gorokhov, Zh. Fiz. Khim., 42 (1968) 860. L. N. Gorokhov and J. Panchenkov, Zh. Fiz. Khim., 44 (1970) 269. R. E. Porter, J. Chem. Phys.. 35 (1961) 318. L. N. Gorokhov, T/estn. Mask. liniu., Ser. II, 6 (1958) 231. J. Berkowitz, H. A. Tasman and W. A. Chupka, J. Chem. Ph~~s., 36 (1962) 2170
335
Journal qf the Less-Common Metals, 31 (1973) 329-335 lF\ Elsevier Sequoia S.A., Lausanne Printed in The Netherlands
MASS SPECTROMETRIC STUDIES ON THE SUBLIMATION RUBIDIUM PERRHENATE
KRZYSZTOF
SKUDLARSKI
Institute ,for Inorganic (Poland) (Received
November
and WOJCIECH
Chemistry and Metallurgy
OF
LUKAS
of Rare Elements, Technical University, Wroclaw
29, 1972)
SUMMARY
The equilibrium sublimation of RbReO, over the temperature range 71&863 K has been studied. RbReO, was evaporated from single and double silica Knudsen cells and the effusing vapours were examined by means of a MI-1305 mass spectrometer specially adapted for high temperature studies. Molecules of RbReO, monomer and those of the dimer (RbReO,), were found in the vapours. The partial pressures of monomer and dimer, their sublimation heats and the heat of thermal dissociation of the dimer were determined.
INTRODUCTION
Studies on the composition and partial pressures of the gaseous phase in equilibrium with heated inorganic compounds are now usually carried out by means of suitably adapted mass spectrometers. Studies on the compositions of vapours evolved from inorganic oxy-acid salts have been developed only during the last ten years. Potassium perrhenate, KReO,, was found by Vorlander and Dalichau’ to boil at 1650 K without visible decomposition. Neuman and Costeanu’ determined the vapour pressure over KReO, at temperatures from 780 to 855 K on the assumption that only monomeric molecules exist in the vapours. Spiridonov, Hodchenkov and Akishin3 investigated KReO, vapour by electron diffraction, and Spoliti, Ward and Stafford4 examined CsReO, vapour by infrared spectroscopy. Investigations on the composition of the gaseous phase over perrhenates have been carried out for NaReO, and KReO, by Skudlarski, Drowart, Exteen and Vauder Auwera Mahieu’, by Skudlarski6 for CsReO, and by Semonov7 for TlReO,. The gaseous phase formed over each of these salts contains monomeric and dimeric molecules. In addition to the above perrhenates lithium, rubidium, silver and copper perrhenates also evaporate with the salt molecules retained in the gaseous phase*. The purpose of the present work was to investigate the composition of the gaseous phase over solid RbReO,, to determine the partial pressures of the individual components of the vapour and to calculate the enthalpies of sublimation and dimerization reactions.
330
K. SKUDLARSKI,
W. LUKAS
EXPERIMENTAL
~~surements were carried out in a Soviet made MI-1305 mass spectrometer with a 60” deflection angle and 200 mm radius of curvature, specially adapted for thermochemical work9*“. The Knudsen cells were made of silica glass. A detailed discussion of the measuring procedure has been given previously’. RbReO, was obtained by the neutralization of rubidium hydroxide with perrhenic acid, RbOH was obtained from RbCl by reaction with Ag,O, and HReO, from NH4Re0, by ion exchange. RESULTS
AND DISCUSSION
The mass spectrum of rubidium perrhenate contains the ions: Rbf, RbO+, RbRe+, RbReO+, RbReOT, RbReOz, RbReO:, RbzReOa, Re+, ReO+, ReO:, ReO:, ReO:. The Rb” ions exhibit the highest intensities; the intensities of other ions are much lower. By analogy with the evaporation of alkali metal borates”, halides13, 14, hydroxide$-i7, and cyanidesIs, it may be assumed that, as with NaReO,, KRe045 and CsRe046, all the above mentioned ions result from the monomeric RbReO, and dimeric (RbReO,), molecules. This conclusion is also supported by the fact that as in the case of the ions formed from the ionization of caesium perrhenate vapours6, all other ions, apart from RbReOf, possess a certain excess kinetic energy. The excess kinetic energy was also previously found in the Na’ and K+ ions formed during the electron ionization of sodium and potassium perrhenate vapours5. The mass spectrometer used did not permit the precise determination of the appearance potentials of the ions observed. It was, however, possible to observe that, as in the case of CsReO,, all the ions, apart from Rb+, have appearance potentials higher than that of RbReO:. These potentials decrease in the sequence: RbRe’ > RbReO+ > RbReO: > RbReO: > Rb,ReO: > RbReO:. The above mentioned ions therefore originate from larger molecules which are disintegrated by electrons, most probably from the monomer RbReO, and the dimer, (RbReO,),. Figure 1 illustrates a temperature relationship of the 1. T product which is proportional to the partial pressure of those molecules from which the ions in question are formed. P = IT,/crys
(1)
where P is the pressure, I the intensity.of the ion stream, T the temperature, c the ionization cross section, y the sensitivity factor of the ion coilector, and s the sensitivity constant of the mass spectrometer, This figure shows that the line corresponding to the Rb,ReO: ions has significantly the greatest slope. The other lines have slopes close to that of the RbReOf line though somewhat higher. This is further evidence that the majority of all ions, other than Rb,ReO: originate from the same primary molecule as RbReO:, most probably from the RbReO, monomer. On the other hand, the Rb,ReOf ions come from another primary molecule, probably from the (RbReO,), dimer.
MASS SPECTROMETRIC
STUDIES
Rb'
0 + Rho'
OF RbReO,
SUBLIMATION
331
\b
Fig. 1. Dependence of log IT upon l/T. I is the relative of the gas from which the ions are formed.
intensity
of the ions and
T is the temperature
Studies on fragmentation of neutral molecules with electrons In order to determine the proportions of particular ions which are derived from electron fragmentation of either monomer or dimer, measurements employing double Knudsen cells were carried out by the method reported by Gorokhov”. The measurement using a double Knudsen cell consists of producing various vapour pressures of the substance under investigation in the upper effusion cell, where a constant tem~rature is maintained, and of recording, in a spectrometer, the intensities of the ions formed. The intensity measurements of all ions observed were carried out with the upper cell maintained at a constant temperature and with the bottom cell maintained at several different temperatures. The intensity measurements of all ions observed enabled us to calculate the proportions of the ions derived from vapour ionization of either RbReO, (monomer) or (RbRe0,)2 (dimer).
(2) and c =
J,,,,
[I;M,D)-z~,,,,nill(n--nf)f,
(3)
is the intensity of the measured ion resulting from ionization of monomer and dimer,Z, is the intensity of measured ion resulting from ionization of the monomer,Zn is the intensity of Rb,ReO: ion resulting from ionization of the dimer and Znc is the intensity of the measured ion resulting from ionization of the dimer
K. SKUDLARSKI,
332
W. LUKAS
The values marked by superscripts (‘) correspond to the ion intensities measured with the bottom cell at higher temperatures, whilst those without superscripts relate to measurements at lower temperatures. The temperature of the upper cell was always constant and equal or higher than the temperature of the bottom cell. The temperature of the channel connecting both cells did not fall below that of the bottom cell. Factor C in eqn. (2) was calculated from each pair of measurements which differed in the bottom cell temperature. The average value C for each type of ion was calculated from the C values obtained. Using these values of C and eqn. (2) the proportions of each ion intensity from both monomeric and dimeric molecules were calculated from three independent experiments. The results are given in Table I. TABLE I THE ORIGIN OF IONS APPEARING IN THE MASS SPECTRUM EQUILIBRIUM WITH SOLID RbReO, AT 863 K
OF VAPOURS
IN
The energy of the ionizing electrons was 4.8 MJ/mole.
Type of
ion
Rb+ RbO+ Rb,ReO: RbReOa RbReO: RbReOi RbReO+ RbRe+ ReOa ReO: ReOi ReO ’ Re+
Relative intensity
100 0.23 0.25 0.22 0.051 0.009 0.022 0.11 0.048 0.039 0.084 0.06 0.044
Intensity of ions derived from dimer
11.3 0.03 1 0.25 0.020 0.005 0.0007 0.002 0.01 0.017 0.0107 _
Percentage of Intensity of ions derived intensity for from monomer particular ions derived from dimer
88.7 0.20 0.20 0.046 0.0083 0.020 0.10 0.03 1 0.029 0,084 _
11.3 13.5 100 8.9 9.4 8.15 8 9 35 25 0
Relative intensity spectrum of ions derived from dimer
mOnomer
100 0.27 2.2 0.18 0.044 0.006 0.018 0.09 0.15 0.09 _
100 0.23 0.23 0.052 0.0093 0.022 0.11 0.035 0.032 0.095
A comparison of the spectra for both monomer and dimer shows that the ionization of the dimer gives relatively three times more of the ReO: and ReO: ions than that of the monomer. The ReO: ion is derived from the monomer. The proportions of other ions in the spectrum are similar. Calculation of the RbReO, and (RbReOJ2
vapour pressures
In order to calculate the vapour pressures, the relationships between the molecular pressures in the Knudsen cell and the intensities of the ions formed from their ionization eqn. (1) as well as the Knudsen equation were used:
MASS SPECTROMETRIC
STUDIES
OF RbReO,
SUBLIMATION
333
where P is the gas pressure, G the mass of the substance evaporated from the Knudsen cell, q the area of the effusion hole, At the evaporation time at temperature T, M the mass of 1 mole of gaseous molecules, and K the Clausing factor, assuming K = 1. In our studies the ratio of the evaporating area to that of the effusion hole was at least 500. In practice, it was found that as in studies on caesium perrhenate6, with the size of effusion holes equal to about 0.05 mm2 only 0.2 mg RbReO, in the condensed phase was sufficient to produce saturated vapours in the cell. At the end of the measurements a few mg of the substance was always left in the cell. In order to determine the vapour pressures, a quantitative evaporation method and integration of ion intensities were employed in a slightly changed form. The first part of the measurement was carried out polythermally. The temperature of the Knudsen cell was raised gradually in steps of about 7 K and when the temperature became stabilized, the intensities of all ions were measured with the simultaneous recording of the exposure time at a given temperature and sample heating times. Having measured the intensities at about 2.5 different temperatures, the temperature was maintained constant for a period of time required to vaporize about three times as much RbReO, as was evaporated during the entire polythermal process carried out previously. The Gapour pressures of monomer and dimer were calculated from eqn. (1). The product of spectrometer sensitivity (s) multiplied by the effective ioni~tion cross section required in this equation was calculated from the same measurement by means of eqn. (5).
This formula was obtained by applying eqn. (1) and the Knudsen formula (2) to the process taking place at several different temperatures, when various gas molecules were ionized. s is the sensitivity constant of the spectrometer, q the area of the effusion hole, M the mass of 1 mole of gaseous RbReO,, R the gas constant, G the mass of vaporized RbReO,, CJ, the ionization cross section of RbReO,, LT,,the ionization cross section of (RbReO,),, XJwt the sum of intensities for all ions resulting from the monomer, CJn, the sum of intensities for all ions resulting from the dimer, T the temperature and At the sample heating time at temperature T. The sum of the intensities of all the ions derived from the monomer C JM, was assumed to be equal to the intensity of the rubidium ions derived from the monomer IRb*(M)andC4,,=I,,+(n,. The intensities of the Rb+ ions derived from the monomer and dimer as calculated from eqn. (2) were substituted for the intensities CJhlj and Cl,,,,. The intensities of particular ions RbO ‘, RbReOz, ReOT, where x = 1 to 4, were not integrated since their total intensity is only about 1% of the Rb+ ion intensity (Table I), and an increase in the accuracy of the pressure calculation due to the summation of intensities for all these ions would be insignificant in comparison with other sources of error. The sensitivity factor yr,/yM of the ion collector for various ions would be in this case equal to 1 since it does not depend on the origin of the Rbf ions measured (whether from a monomeric or dimeric molecule).
334
K. SKUDLARSKI, W. LUKAS
The values of the last terms of eqn. (5) corresponding to the long heating time at the highest temperature, were about three times greater than the sum of the other terms of the equation. The accuracy of the s(ay) product calculation was then almost the same as in a simple isothermal measurement. The same measurement enabled us to obtain simultaneously the relationship between the temperatures and intensities of particular ions. This made it possible to calculate the partial pressures of monomer and dimer from eqn. (1) over the entire temperature range concerned from the known origin of ions and known y~n/ycr~ ionization cross section ratio. The relative dimer/monomer electron ionization cross section ratio was assumed to be equal to 1.4, allowing for a change of this ratio for alkali halides with increasing weight of the alkali element”, and the values obtained previously for other perrhenates. For5 KReO, (~~~/(~y~ = 0.67, and for6 CsReO, {~~~/(~~~ = 1.8. The vapour pressures of monomer and dimer may be expressed by the equations : for RbReO, log PNlm:! = - (9750/T) + 11.33 for (RbReO& log P&Z = - (13450/T) + 14.62 Heats of sublimation were calculated from the equations log fa,,o;fMj. T= = f( l/T) and log IRbZReo4+ +T= f( l/T) where I”,,,,; (Mj is the intensity of RbReO: ions resulting from the monomer. For the monomer RbReO, AIi+‘so= 186.6 t_ 3.6 kJ/ mol, for dimer (RbReO,), AH,,,’ - 257.4k4.0 kJ/mol. The deviations given are standard errors of the regression factor from particular points of a single measurement. Taking into account other possible sources of errors of measurement it was assumed: for monomer RbReO, A&so’ -187-t- 15 kJ/mol and for (RbReO& AH&,, = 257$- 15 kJ/mol. The heat of dimer/m&omer dissociation II&,= 1165 25 kJ/mol. REFERENCES 1 2 3 4 5
D. Vorlander and G. Dalichau, Ber., 66 (1933) 1534. K. Neumann and V. Costeanu, Z. Phys. Chem., Al85 (1939) 65. V. P. Spiridonov, A. N. Hodchenkov and P. A. Akishin, Vesttl.MO&. Univ., Ser. II, 6 (1965) 34. M. Spoliti, B. G. Ward and F. E. Stafford, quoted in ref. 5. K. Skudlarski, J. Drowart, G. Exsteen and A. Vander Auwera-Mahieu, Trans. Faraday Sot., 63 (1967) 1146. 6 K. Skudlarski, Roczniki Chem., to be published. 7 G. A. Semenov, Probl. sovremen. khim. koordinac. soedin., Leningr. Univ., 3 (1966) 16. 8 K. Skudlarski and W. Lukas, Nukleonika, 17 (1972) 189. 9 K. Skudlarski, Prib. Tekh. Eksp., (2) (1971) 268. 10 W. Lukas, to be published. 11 P. A. Akishin, L. H. Gorokhov, Yu. S. Khodeev, J. Strukt. Khim., 2 (1961) 209. 12 A. Biichler and J. Berkow~t-Mattu~k, J. Chem. Phys., 39 (1963) 286. 13 R. F. Porter and R. C. Schoonmaker, J. Chem. Phys., 29 (1958) 1070. 14 J. Berkowiz and W. A. Chupka, J. Chem. Phys., 29 (1958) 653. 15 R. C. Schoonmaker and R. F. Porter, J. C&em. Phys., 28 (1958) 454.
MASS SPECTROMETRIC 16 17 18 19 20
STUDIES
OF RbReO,
SUBLIMATION
A. V. Gusarov and L. N. Gorokhov, Zh. Fiz. Khim., 42 (1968) 860. L. N. Gorokhov and J. Panchenkov, Zh. Fiz. Khim., 44 (1970) 269. R. E. Porter, J. Chem. Phys.. 35 (1961) 318. L. N. Gorokhov, T/estn. Mask. liniu., Ser. II, 6 (1958) 231. J. Berkowitz, H. A. Tasman and W. A. Chupka, J. Chem. Ph~~s., 36 (1962) 2170
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