Improvement of Al3+ ion conductivity by F doping of (Al0.2Zr0.8)4/3.8NbP3O12 solid electrolyte for mixed potential NH3 sensors

New Al³⁺ ion conducting solid electrolytes (Al_0.2Zr_0.8)_4/3.8NbP₃O_12-xF_2x(0?≤x?≤?0.4) with Nasicon-structure are successfully prepared by solid state reaction method. The influences of the doped F^- content on the properties of the (Al_0.2Zr_0.8)_4/3.8NbP₃O_12-xF_2x samples are investigated using X-ray powder diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The results show that F^- doping can effectively improve the sinterability and the total conductivity of the (Al_0.2Zr_0.8)_4/3.8NbP₃O_12-xF_2x samples. Among the solids series, (Al_0.2Zr_0.8)_4/3.8NbP₃O_11.7F_0.6 shows the highest conductivity of 1.53?×?10^?3 S?cm^?1at 500?°C, which is approximately 7.9 times higher than that of the undoped (Al_0.2Zr_0.8)_4/3.8NbP₃O₁₂. The ion transference number of the samples is higher than 0.99 at 300–700?°C. On the basis of the promising properties, a mixed-potential type NH₃ sensor based on (Al_0.2Zr_0.8)_4/3.8NbP₃O_11.7F_0.6 electrolyte and In₂O₃ sensing electrode has been developed. The sensing performance of the sensor is evaluated. The mixed-potential type sensor can work at relatively low temperatures of 200–350?°C and an excellent sensitivity of 99.71?mV/decade at 250?°C is obtained. The sensor also displays excellent stability and reproducibility, accompanied by low cross-sensitivities to CO₂, CH₄ and H_2.     

Thermodynamic properties of hydrobromic acid in ethanol + water solvent mixtures from electromotive force measurements at 15–55°C

From the electromotive force measurements of cells of the type: Pt, H₂ (gas, 1 atm); HBr (m), ethanol + water solvent mixtures; AgBr, Ag, at nine different temperatures in the range 15–55°C and in the molality range of HBr from 0.005 to 0.1 mol kg^?1, the standard potentials of silver—silver bromide electrode have been determined in twenty ethanol + water solvent mixtures covering the whole range of solvent composition. The temperature variation of the standard potential was utilized to calculate the standard thermodynamic functions for the cell reaction, the primary medium effects of various solvents upon HBr, and the standard quantities for the transfer of HBr from the standard state in water to the standard states in the respective solvent media. The results have been interpreted in regard to the acid-base properties and structure of the solvents.     

Thermodynamics of ionic-association in aqueous solutions of Ca and Mg organic salts using ion-selective electrode technique—I. Formates, acetates, propionates and butyrates

The divalent selective electrode together with high precision solid state, digital pH -mv -meter makes broader application of potentiometry in physical and inorganic chemistry a certainty. The above set-up is used to determine the stoichiometric constants, K, for Ca and Mg ions association with formates, acetates, propionates and butyrates at 25°, 35° and 45°C in aqueous media. The K-values were converted to infinite dilution K_A values were found to be 8.4 LM⁻¹, 10.4 LM⁻¹, 19.1 LM⁻¹ and 19.3 LM⁻¹ for calcium salts of formate, acetate, propionate and butyrate respectively. Also K_A values for Mg salts of formate, acetate, propionate and butyrate were found to be 7.8 LM⁻¹, 9.5 LM⁻¹, 13.1 LM⁻¹ and 13.1 LM⁻¹ respectively. Other thermodynamic parameters such as ΔG°, ΔH° and ΔS° are also obtained from the variation of K_A with temperature for each salt. The data are interpreted relative to each other on basis of pK_a of the corresponding organic acid. Their temperature behaviour is similar to those salts derived from strong acids such as sulphates, rather than weak acids.