Electrochemical and spectroscopic characterisation of barium acid salts of 3,5-disulfophenylphosphonic acid

Abstract

Barium salts of 3,5-disulfophenylphosphonic acid (H₄Y) having formulae Ba₂Y, Ba_1.8H_0.4Y, BaKHY and BaH₂Y have been prepared and characterised by X-ray powder diffraction, thermogravimetry, IR spectroscopy, ac conductivity and e.m.f. measurements. Ba₂Y is orthorhombic, with a=8.126, b=4.691, c=8.649 Å; Ba_1.8H_0.4Y and BaKHY have similar cell parameters to these. BaH₂Y is also orthorhombic, but with all cell parameters close to double those of the former salts. The anhydrous salts are stable in air up to at least 300 °C. The ac conductivity (σ) has been measured at 100 °C and controlled relative humidity (RH) in the range 50–80%; at 70% RH, σ increases from ca. 10^–8 up to ca. 10^–2 S cm^–1 according to the sequence: Ba₂Y<Ba_1.8H_0.4Y<BaKHY<BaH₂Y. Electromotive force measurements have shown K⁺ and H⁺ to be the main charge carriers in BaKHY and BaH₂Y, respectively. In both salts protons are localized on the phosphonate group at 53% RH, but IR spectroscopy provides evidence for a degree of proton transfer to water molecules in BaH₂Y at 75% RH.