A.S. Kerkar

Thermodynamic investigation of the vaporization of molybdenum trioxide

Abstract The vapour pressure of MoO3(s) was measured using a novel thermogravimetric system. The total pressures obtained in this investigation agree well with the best assessed values from data in the literature. The enthalpy of sublimation at 1000 K obtained from this work is 365.2 ± 5.0 kJ mol−1 in close agreement with the assessed value of 369.1 ± 0.8 kJ mol−1. The vapour pressure data for pure MoO3(s) was obtained in the context of our investigations on the vaporization of transition metal molybdates.

Thermodynamic stability of barium thorate, BaThO3, from a Knudsen effusion study

Abstract The Gibbs energy of formation of barium thorate was determined using the Knudsen effusion forward collection technique. The evaporation process could be represented by the equation BaThO 3 (s)=ThO 2 (s)+BaO(g) The vapour pressure of BaO(g) over the two-phase mixture of BaThO3(s) and ThO2(s) was obtained from the rate of effusion of BaO(g) and could be represented as ln (p/ Pa ) (±0.39)=−50526.5/T/ K +26.95 (1770≤T/ K ≤2136) The Gibbs energy of formation of BaThO3(s) could be derived from this data and represented as Δ f G°( BaThO 3 (s) )/ kJ mol −1 ±8.0=−1801.75+0.276T/ K

A microthermogravimetric system for the measurement of vapour pressure by a transpiration method

Abstract A novel automatic recording microthermogravimetric system for the measurement of vapour pressures at high temperatures by transpiration method was designed and fabricated. The performance of the system was checked by measuring the vapour pressure of anhydrous cadmium chloride in flowing dry argon in the temperature range 713–833 K. The vapour pressure of cadmium chloride in this range could be expressed as The enthalpy of sublimation of CdCl 2 derived from this equation at the mean temperature of investigation was found to be 167.2 ± 2.2 kJ mol −1 and compared very well with the value of 166.5 ± 4.7 kJ mol −1 reported by Skudlarski et al. (J. Chem. Thermodyn. 19 (1987) 857) from their recent Knudsen effusion mass spectrometric measurements. The present system can be used for measurement of vapour pressures up to 1425 K and is the first of its kind in which the mass loss due to vaporization is monitored continuously during the conventional transpiration experiment. The system described in this paper effects considerable saving of time in the experiment and minimizes the errors associated with mass measurements in the conventional transpiration method currently in use.

The palladium-platinum phase diagram

Abstract The palladium-platinum equilibrium phase diagram was determined using a “spot” technique. The two metals form a series of solid solutions over the entire composition range, which solid solutions coexist with their liquid solutions. The experimentally observed gap between the solidus and the liquidus was comparable to that calculated employing a regular solution model. The phase boundaries delineated in the present work, however, differ significantly from those in an earlier work which were obtained by the Pirani method.

Thermodynamic stability of solid SrThO3

Abstract The Gibbs energy of formation of strontium thorate was determined by the Knudsen effusion forward collection technique. The evaporation process from a mixture of tungsten and strontium thorate in the Knudsen cell could be represented by the following heterogeneous equilibria: 5SrThO3(s)+W(s)=Sr2WO5(s)+5ThO2(s)+3Sr(g), 1670 Δ f G 0 ( SrThO 3 ( s )) (±5.0 kJ mol −1 )=−1953.6+0.367·T , (1670 Δ f G 0 ( SrThO 3 ( s )) (±7.0 kJ mol −1 )=−1960.2+0.369·T , (2135

The standard free energy of formation of ZrMo2O8 by the transpiration method

The standard free energy of formation for zirconium molybdate (ZrMo2O8) was derived from its vapour pressure measured in the temperature range 1029 to 1142 K employing the transpiration technique. ZrMo2O8 vaporizes incongruently according to the reaction n〈ZrMo2O8〉 → n〈ZrO2〉 + 2(MoO3)n, n = 3,4,5. The standard free energy of formation of ZrMo2O8 calculated from the partial pressure of (MoO3)3 in the vapour above ZrMo2O8 + ZrO2 mixture could be expressed by the relation ΔfG°〈ZrMo2O8〉 =(−2525.5±4.9) +(0.6115±0.0045)Tkj/mol (1029) < T/K < 1142). The values of ΔfG°〈ZrMo2O8〉 obtained in this work agree very well with those reported recently from solid electrolyte galvanic cell measurements.

Determination of the Gibb's energy of formation of CaTeO3 and CaTe2O5 by the transpiration technique

Abstract The vapour pressure of two compounds, 〈CaTeO 3 〉 and 〈CaTe 2 O 5 〉, in the pseudo-binary CaOTeO 2 system was measured employing the microthermogravimetric transpiration assembly built in our laboratory. Both compounds vaporized incongruently, giving TeO 2 vapour according to the reactions 〈CaTeO 3 〉 = 〈CaO〉 + (TeO 2 ) and 〈CaTe 2 O 5 〉 = 〈CaTeO 3 〉 + (TeO 2 ) respectively. The vapour pressure of (TeO 2 ) above the mixtures of 〈CaTeO 3 + CaO〉 was measured in the temperature range 1169 to 1247 K. The Gibbs energy of formation of CaTeO 3 derived from the vapour pressure data could be expressed as a function of temperature by the equation Δ f G° 〈 CaTeO 3 〉 (±12.83 kJ mol −1 ) = − 975.08 + 0.242T (1169 T K CaTe 2 O 5 exhibited the reversible crystallographic phase transition at 1077±1 K as recorded by DTA. The Gibbs energy of formation of CaTe 2 O 5 derived from its vapour pressure measured below and above this phase transition could be expressed as a function of temperature in terms of the following equations: Δ f G° 〈 CaTe 2 O 5 〉 (±13.11 kJ mol −1 ) = − 1357.3 + 0.464T (1007 T K Δ f G° 〈 CaTe 2 O 5 〉 (±13.26 kJ mol −1 ) = − 1313.9 + 0.423T (1082 T K The phase transition temperature T tr and the enthalpy of transition ( ΔH ° T tr ) deduced from these equations were found to be 1048±30 K and 43.4±10 kJ mol −1 respectively.

Vaporization behaviour and thermodynamic stability of HfMo2O8

Abstract The standard Gibbs energy of formation for hafnium molybdate (HfMo2O8) was derived from its vapour pressure measured in the temperature range 1023 to 1185 K, employing the transpiration technique. HfMo2O8 vaporizes incongruently according to the reaction n〈HfMo2O8〉 → n〈HfO2〉 + 2(MoO3)n (n = 3, 4, 5). The standard Gibbs energy of formation of HfMo2O8 calculated from the partial pressure of (MoO3)3 in the vapour phase above the HfMo2O8 + HfO2 mixture could be expressed by the relation ΔfG°〈HfMo2O8〉 (kJ/mol) = −(2498.78 ± 4.06) + (0.6039 ± 0.0037)T (1023

Vaporization behaviour and thermodynamic stability of zirconium tellurate ZrTe3O8

Abstract The standard Gibbs energy of formation for zirconium tellurate ZrTe3O8 was derived from its vapour pressure measured in the temperature range 1008 to 1146 K, employing the transpiration technique. ZrTe3O8 vaporizes incongruently, according to the reaction 〈ZrTe3O8〉 ai 〈ZrO2〉 + 3(TeO2). The standard Gibbs energy of formation (ΔfG°) of 〈ZrTe3O8〉 calculated from the partial pressure of TeO2(g) in the vapour phase above the 〈ZrO2〉 + 〈ZrTe3O8〉 mixture can be represented by the relation ΔfG°〈ZrTe3O8〉(± 14.3 kJ mol−1) = −2168.3 + 0.801T(K) (1008 ≤ T/K ≤ 1146). The standard enthalpy of formation ΔfH°(298.15K) for 〈ZrTe3O8〉 derived from these data employing the estimated heat capacity for the compound was found to be −(2153.0 ± 18.3) kJmol−1, in good agreement with the value of −(2131.2 ± 9.64) kJmol−1 determined by isoperibol calorimetry.

Gibbs energy of formation of Ba(OH)2 vapor species using the transpiration technique

Abstract The Gibbs energy of formation of Ba(OH) 2 was determined employing the heterogeneous reaction between solid BaO and water vapor and generating gaseous barium hydroxide species according to the reactions BaO(s)+H 2 O(g)=Ba(OH) 2 (g). The equilibrium vapor pressure of Ba(OH) 2 species under a controlled pressure of water vapor was measured using an automatic recording transpiration apparatus. From the vapor pressure results, the equilibrium constant of the reaction obtained could be represented as ln K p (±0.04)=−16792.3/T+4.94 , K p = p (Ba(OH) 2 )/ p (H 2 O) (1346⩽ T /K⩽1451). The Gibbs energy of formation of Ba(OH) 2 derived from these data could be expressed as Δ f G °( Ba ( OH ) 2 , g )(±2 kJ mol −1 )=−667.29+0.118 T (1346⩽T/ K ⩽1451) . From the study, the second-law and third-law values of Δ f H°(Ba(OH) 2 ,g, 298.15 K) were worked out to be −(645.1±8.0) kJ mol −1 and −(643.9±2.5) kJ mol −1 , respectively.