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Arblasteros 06A SC.Docx ACCEPTED MANUSCRIPT 18/05/2020 www.technology.matthey.com Johnson Matthey’s international journal of research exploring science and technology in industrial applications ************Accepted Manuscript*********** This article is an accepted manuscript It has been peer reviewed and accepted for publication but has not yet been copyedited, house styled, proofread or typeset. The final published version may contain differences as a result of the above procedures It will be published in the JANUARY 2021 issue of the Johnson Matthey Technology Review Please visit the website https://www.technology.matthey.com/ for Open Access to the article and the full issue once published Editorial team Manager Dan Carter Editor Sara Coles Editorial Assistant Yasmin Stephens Senior Information Officer Elisabeth Riley Johnson Matthey Technology Review Johnson Matthey Plc Orchard Road Royston SG8 5HE UK Tel +44 (0)1763 253 000 Email [email protected] ArblasterOs_06a_SC.docx ACCEPTED MANUSCRIPT 18/05/2020 A Re-assessment of the Thermodynamic Properties of Osmium Improved value for the enthalpy of fusion John W. Arblaster Droitwich, Worcestershire, UK E-mail: [email protected] The thermodynamic properties were reviewed by the author in 1995. A new assessment of the enthalpy of fusion at 68.0 ± 1.7 kJ mol-1 leads to a revision of the thermodynamic properties of the liquid phase and although the enthalpy of sublimation at 298.15 K is retained as 788 ± 4 kJ mol-1 the normal boiling point is revised to 5565 K at one atmosphere pressure. Introduction The thermodynamic properties of osmium were reviewed by the author in 1995 (1) with a further review in 2005 (2) to estimate a most likely value for the melting point at 3400 ± 50 K to replace the poor quality experimental values which were currently being quoted in the literature. More recently Burakovsky et al. (3) have estimated a value of 3370 ± 75 K in good agreement with the above selected value. In the 1995 review the enthalpy of fusion was unknown but was estimated from a relationship between the entropy of fusion and the melting point which showed a high degree of correlation for the platinum group of elements. However the derived entropy of fusion value for osmium was based on values for the other platinum group elements available at that time but since then the values for both palladium and platinum have been revised so that the entropy of fusion value for osmium would also be revised leading to a new estimate of 68.0 ± 1.7 kJ mol-1 for the enthalpy of fusion. This would then require that the thermodynamic properties of the liquid phase to also be updated. A comment is included on an independent much lower estimate of the enthalpy of fusion. Wherever possible measurements have been corrected to the ITS-90 temperature scale and to the currently accepted atomic weight of 190.23 ± 0.03 (4). Low Temperature Solid Phase Selected values in the normal and superconducting states are based on the specific heat measurements of Okaz and Keesom (0.18 to 4.2 K) (5) including a superconducting transition temperature of 0.638 ±0.002 K, an electronic specific heat coefficient (γ) of 2.050 ±0.003 mJ -1 -2 mol K and a limiting Debye temperature (ΘD) of 467 ±6 K. Specific heat values up to 5 K in both the normal and superconducting states are given in Table I. Above 4 K selected specific heat values are initially based on the measurements Naumov et al. (6 to 316 K) (6). However above 280 K these measurements show an abrupt increase of 0.5 J mol-1 K-1 and a further abrupt increase of 0.3 J mol-1 K-1 above 300 K. Naumov et al. attempted to accommodate these values but the selected specific heat curve showed an unnatural sharp change in slope above 270 K. Therefore the selected values of Naumov et al. above 250 K were rejected and instead specific heat values to 298.15 K were obtained by joining smoothly with the high temperature enthalpy measurements of Ramanauskas et al. (7). In the original review of the low temperature data only the specific heat values were given consisting above 50 K of 10 K intervals to 100 K and then 20 K intervals above this temperature as well as the value at 298.15 K. This minimalist approach is now considered to be unsatisfactory and therefore Johnson Matthey Technol. Rev., 2021, 65, (1), xx-yy 1/13 Doi: 10.1595/205651320X15898131243119 ArblasterOs_06a_SC.docx ACCEPTED MANUSCRIPT 18/05/2020 comprehensive low temperature thermodynamic data are now given at 5 K intervals from 5 to 50 K and at 10 K intervals above this temperature up to 290 K and then the value at 298.15 K as given in Table II. High Temperature Solid Phase In the high temperature region, after correction for temperature scale and atomic weight, the enthalpy measurements of Ramanauskas et al. (1155 to 2961 K) (7) were fitted to the following equation with an overall accuracy of ± 200 J mol-1 (0.4 %): -1 -4 2 -7 3 -11 4 H°T - H°298.15 K (J mol ) = 26.1938 T + 1.32318 x 10 T + 3.85960 x 10 T + 3.99978 x 10 T + 150378/T – 8336.36 (i) This equation was used to represent selected enthalpy values from 298.15 K to 3400 K. Equivalent specific heat and entropy equations corresponding to the above equation are given in Table III, the free energy equations in Table IV, transitions values associated with the free energy functions in Table V and derived thermodynamic values in Table VI. The actual equation given by Ramanauskas et al. to represent the enthalpy measurements over the experimental temperature range agrees with Equation (i) to within 0.2%. The only other enthalpy measurement, were obtained by Jaeger and Rosenbohm (693 to 1877 K) (8) and compared to the selected values vary from 1.6% low at 693 K to an estimated 1.2% low at 1600 K to 1.4% low at 1877 K. Liquid Phase Selected values of the enthalpies and entropies of fusion of the groups 8 to 10 elements with a close-packed structure are given in Table VII. Only the enthalpy of fusion of osmium is unknown. References 21 to 25 represent the latest reviews on the thermodynamic properties of the platinum group elements by the present author. From an evaluation of the entropies of fusion of the elements, Chekhovskoi and Kats (9) proposed that the entropy of fusion ΔS°M and the melting point TM could be related by the equation ΔS°M = A TM + B. In the previous review (1) different values were proposed for the entropies of fusion of palladium (8.80 J mol- 1 K-1) and platinum (10.45 J mol-1 K-1) leading to an estimate of the entropy of fusion for osmium of 20.6 J mol-1 K-1. With the revised values it is clear that, although of the right order, the entropy of fusion of nickel is discrepant and has therefore been disregarded. The other six values were fitted to the equation with A = 6.6954 x 10-3 and B = - 2.7630 and a standard deviation of the fit of ± 0.193 J mol-1 K-1. However in order that the derived entropy of fusion of osmium has a similar accuracy to those of the input values then the accuracy is expanded to a 95% confidence level leading to an entropy of fusion of 20.0014 ± 0.387 J mol-1 K-1 and based on a melting point 3400 ± 50 K to an enthalpy of fusion of 68,005 ± 1,653 J mol-1. Based on neighbouring elements then a liquid specific heat of 50 J mol-1 K-1 was proposed in the original paper (1) and therefore the enthalpy of liquid osmium can now be expressed as: -1 H°T - H°298.15 K (J mol ) = 50.0000 T + 816.2 (ii) Equivalent specific heat and entropy equations corresponding to the above equation are given in Table III, the free energy equation in Table IV and derived thermodynamic values in Table VI. It should now be possible to accurately determine the melting point and enthalpy of fusion of osmium since the metal is available in high purity in a coherent form whilst the enthalpies Johnson Matthey Technol. Rev., 2021, 65, (1), xx-yy 2/13 Doi: 10.1595/205651320X15898131243119 ArblasterOs_06a_SC.docx ACCEPTED MANUSCRIPT 18/05/2020 of fusion of other high melting point elements such as rhenium (3458 K) and tungsten (3687 K) have been successfully determined. Gas Phase Based on a standard state pressure of one bar the thermodynamic properties of the monatomic gas were calculated from the 295 energy levels listed by Van Kleef and Klinkenberg (10) and Gluck et al. (11) using the method outlined by Kolsky et al. (12) together with the 2018 Fundamental Constants (13). Derived thermodynamic values are given in Table VIII. Enthalpy of Sublimation No temperature scales were given with the measurements of the vapour pressures by Panish and Reif (14) and Carrera et al. (15). Normally the experimental temperature values would therefore be accepted but in the case of such values above 2000 K the difference from the current scale, ITS-90, becomes significant. Since the measurements were carried out in 1962 and 1964 then they would ultimately be associated with the IPTS-1948 temperature scale and were therefore corrected to the ITS-90 scale on this basis. Derived enthalpies of sublimation are given in Table IX.
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