Thermodynamics of the HBr+NiBr2+H2O system from 5°C to 55°C

Abstract

Abstract The emf of the following cell (A) without liquid junction was used to investigate the HBr+NiBr2+H2O mixed electrolyte system. Pt , H 2 ( g ,1 atm )| HBr (m A ), NiBr 2 (m B )| AgBr , Ag Measurements of the emf were performed for solutions at constant total ionic strengths of 0.05, 0.1, 0.25, 0.5, 1.0, 1.5 and 2.0 mol kg−1 at temperatures ranging from 5°C to 55°C, except for I=2.0 mol kg−1, where measurements were only made at 25°C. The mean activity coefficients of HBr in the mixtures were calculated using the Nernst equation. The thermodynamic properties of unsymmetrical mixtures are interpreted in terms of the simpler Harned's rule, as well as the comprehensive Pitzer's ion-interaction approach including higher order electrostatic effects for electrolyte mixtures. The Harned interaction coefficients αAB and βAB were determined from the data on the HBr+NiBr2 mixture. Literature osmotic coefficient data at 25°C have been used to determine the pure electrolyte Pitzer parameters (β(0), β(1), and Cφ) for NiBr2. The results were combined with the value of ΘH,Ni=0.069 obtained from the HCl+NiCl2+H2O system to determine ΨH,Ni,Br=0.06 from our emf data at 25°C. The effect of temperature (0–55°C) on the Pitzer parameters (β(0), β(1), and Cφ) for HBr and NiBr2 was determined from the present emf measurements, assuming that the higher order electrical terms (ΘH,Ni and ΨH,Ni,Br) were independent of temperature. They were fitted to a simple polynomial function of temperature (°C): β H , Br (0) =0.2085+8.55×10 −4 (t−25)−1.8×10 −4 (t−25) 2 β H , Br (1) =0.3477+18.0×10 −4 (t−25)+3.9×10 −4 (t−25) 2 C H , Br φ =0.001517−7.10×10 −4 (t−25)+1.05×10 −4 (t−25) 2 and β Ni , Br (0) =0.4451−0.0307(t−25)−3.1×10 −4 (t−25) 2 β Ni , Br (1) =1.49+0.0855(t−25)−6.2×10 −4 (t−25) 2 C Ni , Br φ =−0.00626+0.029(t−25)+8.45×10 −4 (t−25) 2 The values of the Pitzer parameters for NiBr2 were of the same order of magnitude as found for other bivalent bromides.