Standard thermodynamic properties of Bi₂O₃ by a solid-oxide electrolyte EMF technique

Highlights

• The EMF of galvanic oxygen concentration cells including solid zirconia electrolyte were measured accurately.

• An improved experimental apparatus and data analysis method have been applied.

• △_fG^∘, Δ_f$H_{298}^{\circ }$ and $S_{298}^{\circ }$ values for Bi₂O₃ have been determined using the measured EMF data.

Abstract

The standard Gibbs free energies of formation of Bi₂O₃ were determined by the EMF technique using the following galvanic cell over the temperature range from (1012 to 1235) K:

$$(-)\text{Pt,}{\text{Cr}}_2{\text{O}}_3\text{,Bi}(\text{l})\text{,}{\text{Bi}}_2{\text{O}}_3(\text{s,l})|\text{YSZ}|{\text{O}}_2\text{,Pt}(+)\text{,}$$

where YSZ denotes stabilized ZrO₂ with 0.085 mass fraction of Y₂O₃. The standard Gibbs free energy of formation of Bi₂O₃ was derived from the cell above is described as linear functions of temperature as:

$${\mathrm{\Delta }}_{\text{f}}{G}^{\circ }/(\text{kJ}·{\text{mol}}^{-1})=-591.511+0.2934T/\text{K}\pm 0.492(1012\le T/\text{K}\le 1098)\text{Bi-liquid;}{\text{Bi}}_2{\text{O}}_3-\text{solid,}$$

$${\mathrm{\Delta }}_{\text{f}}{G}^{\circ }/(\text{kJ}·{\text{mol}}^{-1})=-582.682+0.2853T/\text{K}\pm 0.481(1098\le T/\text{K}\le 1235)\text{Both phases in liquid.}$$

The Δ_f$H_{298}^{\circ }$ and $S_{298}^{\circ }$ values of α-Bi₂O₃ were calculated to be −567.31 kJ ⋅ mol^-1and 148.77 J ⋅ K⁻¹ ⋅ mol⁻¹ using the c_p functions presented in the literature and the measured data. The results were compared to the corresponding values of the literature.

Introduction

The demand and consumption of the metals increases simultaneously with the exhaustion of high-grade copper ores. The rising consumption of raw materials has increased the impurity concentrations in the available ores and concentrates. Several impurities, such as Pb, As, Sb and Bi, have harmful impact in the copper electro-refining process and to the properties of copper [1], [2]. A low bismuth concentration causes phenomenon called bismuth-induced embrittlement due to its low solubility in copper. Bi₂O₃ is a promising substance as an ionic conductor and as piezoelectric material for energy applications. Furthermore, currently bismuth has several applications in cosmetics, pigments and it can also be used as an alloying element [1], [2], [3], [4]. Accurately measured stability functions of bismuth oxide are required, in order to extract bismuth from the material streams during the copper and lead smelting and refining processes.

The Bi – O system contains the following oxides: BiO, Bi₂O₃, Bi₂O₅ and BiO₂. Furthermore, Bi₂O₃ is observed to be the most stable phase in the bismuth – oxygen system [5] and it can form six different polymorphs, viz. α-Bi₂O₃ with a monoclinic structure, metastable β-Bi₂O₃ with a tetragonal structure, δ-Bi₂O₃ with a cubic structure, metastable γ-Bi₂O₃ with bcc-structure, metastable ω-Bi₂O₃ with a triclinic structure and the newly found metastable ε-Bi₂O₃-phase with an orthorhombic structure [6], [7], [8], [9], [10], [11]. The transition temperature between the stable α-Bi₂O₃ and δ-Bi₂O₃ phases has been evaluated by numerous authors [11], [12], [13], [14], [15], [16], [17], [18]. Risold et al. have selected T = 1002 K as the recommended value for the transition temperature for α-Bi₂O₃–δ-Bi₂O₃ phase transformation in their compilation [11].

The melting point of δ-Bi₂O₃ has been measured by several authors [8], [19], [20], [21]. Risold et al. selected T = 1098 K as the recommended value of the melting point in their assessment [11]. The melting enthalpy of Bi₂O₃ has been determined by numerous authors [8], [12], [14], [21], [22]. Risold et al. have selected 15.9 kJ ⋅ mol⁻¹ as the recommended value for the melting enthalpy of Bi₂O₃ from the available literature data [11].

The aim of this study is to improve the reliability and accuracy of the thermodynamic functions of Bi₂O₃ by using an improved experimental technique and data processing for measured values of the solid oxide electrolyte EMF technique. The electrochemical cell arrangements of the previous EMF studies for the standard Gibbs free energy of formation of Bi₂O₃ are presented in table 1 and compared to the experimental set up of this study.