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

The emf of the following cell (A) without liquid junction was used to inves tigate the HBr + NiBr2 + H2O mixed electrolyte system. Pt,H-2(g,1 atm)\HBr(m(A)),NiBr2(m(B))\AgBr,Ag Measurements of the emf were performed for solutions at constant total ioni c strengths of 0.05, 0.1, 0.25, 0.5, 1.0, 1.5 and 2.0 mol kg(-1) at tempera tures ranging from 5 degrees C to 55 degrees C, except for I = 2.0 mol kg(- 1), where measurements were only made at 25 degrees C, The mean activity co efficients 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 Fitter's ion-interaction approach including higher order electrostatic effects for e lectrolyte mixtures. The Harned interaction coefficients alpha(AB) and beta (AB) were determined from the data on the HBr + NiBr2 mixture. Literature o smotic coefficient data at 25 degrees C have been used to determine the pur e electrolyte Pitzer parameters (beta((0)), beta((1)), and C-phi) for NiBr2 . The results were combined with the value of Theta(H,Ni) = 0.069 obtained from the HCl + NiCl2 + H2O system to determine Psi(H,Ni,Br) = 0.06 from our emf data at 25 degrees C. The effect of temperature (0-55 degrees C) on th e Pitzer parameters (beta((0)) beta((1)), and C-phi) for HBr and NiBr2 was determined from the present emf measurements, assuming that the higher orde r electrical terms (Theta(H,Ni) and Psi(H,Ni,Br)) were independent of tempe rature. They were fitted to a simple polynomial function of temperature (de grees C): beta(H,Br)((0)) = 0.2085 + 8.55 x 10(-4)(t - 25) - 1.8 x 10(-4) (t - 25)(2) beta(H,Br)((1)) = 0.3477 + 18.0 x 10(-4)(t - 25) + 3.9 x 10(-4)(t - 25)(2) C-H,Br(phi) = 0.001517 - 7.10 x 10(-4)(t - 25) + 1.05 x 10(-4)(t - 25)(2) and beta(Ni,Br)((0)) = 0.4451 - 0.0307(t - 25) - 3.1 x 10(-4)(t - 25)(2) beta(Ni,Br)((1)) = 1.49 + 0.0855(t - 25) - 6.2 x 10(-4)(t - 25)(2) C-Ni,Br(phi) = -0.00626 + 0.029 (t - 25) + 8.45 x 10(-4)(t - 25)(2) The values of the Pitzer parameters for NiBr2 were of the same order of mag nitude as found for other bivalent bromides. (C) 2000 Elsevier Science B.V. All rights reserved.