Gibbs energy of formation of bismuth(III) oxide
Graphical abstract
Introduction
The Gibbs energy of formation of bismuth(III) oxide has been measured at high temperatures by many investigators using electrochemical cells [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. However, these studies were carried out using open cells and conventional two-compartment cell designs. Because of the relatively high vapor pressures of bismuth and oxides of bismuth [11], [12], [13] in equilibrium with Bi + Bi2O3 at high temperature as shown in Fig. 1, possible deviation from equilibrium in open cells is a source of concern. Hence, a closed cell was used in this study to avoid problems caused by continuous vaporization. Polarization of electrodes caused by trace hole or electronic conduction in the solid electrolyte and consequent electrochemical permeability the solid electrolyte is a second source of concern. An innovative cell design is developed in this study to deflect this flux to a buffer electrode and avoid electrode polarization. In previous studies air [2], [5], [8], Cu + Cu2O [1], [7], Ni + NiO [6] and Fe + Fe1-xO [3] were used as secondary standards for oxygen chemical potential. The oxygen partial pressure in air is dependent on altitude and weather conditions such as humidity and wind velocity. In this study measurements are made using the primary reference standard for oxygen, O2 gas at 0.1013 MPa. The oxygen electrode is non-polarizable.
The heat capacity of α-Bi2O3 has been measured in the temperature range from 60 to 298 K by Anderson [14] and from 11 to 50 K by Gorbunov et al. [15]. The results from these two measurements are in reasonably good accord. The standard entropy of α-Bi2O3 at 298.15 K computed using the low temperature heat capacity data is 150.0 (±1.6) J/mol K. Although the early solution calorimetric measurements of the enthalpy of formation of α-Bi2O3 at 298.15 K by Ditte et al. [16] and Mixter [17] gave discordant results, Mah [18] later provided a reliable value using combustion calorimetry.
Section snippets
Materials
Bismuth metal and bismuth(III) oxide used in this study were 99.99 mass % pure and were supplied by Apache Chemicals. Spectrochemical analysis of Bi2O3 showed the major impurities which were Ca (43 mass ppm), Pb (32 mass ppm), Si (12 mass ppm), B (7 mass ppm) and Fe (5 mass ppm). High-purity (99.999 mole%) oxygen gas was dried by passage through columns containing silica gel and magnesium perchlorate. High-purity (99.999 mole%) Ar gas used in this study was also dried the same way. Residual
Results
The reversible emf values of the cell are recorded in Table 1 and plotted as a function of temperature in Fig. 4. There is change of slope at 1002 and 1078 K. The break in the slope at 1002(±2) K indicates α–δ transition of bismuth(III) oxide and the break at 1078(±2) K indicates the monotectic reaction. The liquid oxide in equilibrium with the metal at and above the monotectic temperature is nonstoichiometric and contains dissolved Bi as indicated in the phase diagram of the system Bi-O [5].
In
Conclusion
Measurements using a new design of the electrochemical cell, with an enclosed electrode to prevent volatilization and an auxiliary electrode to deflect the electrochemical oxygen flux through the electrolyte, has enabled more accurate measurements of the standard Gibbs energy of formation of Bi2O3 as a function of temperature. The results obtained in this study are fully consistent with calorimetric information available in the literature. A complete thermodynamic data Table for Bi2O3 has been
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