The First Acidity Constant of Carbon Dioxide in Solutions of Ammonium Chloride from e.m.f. Measurements at 278.15 to 318.15 K.

Abstract The equilibrium constant K1o of the reaction: CO2 + H2O = HCO3− + H+, was determined from the e.m.f. of the cell: Pt/H2, CO2/NH4HCO3(m1), NH4Cl(m2), CO2(m3)/AgCl/Ag. The activity coefficients of NH4HCO3 in NH4Cl(aq) and in pure water at 278.15, 288.15, 198.15, 308.15, and 318.15 K were calculated. The values of m 1 m 2 were approximately 1, 2, 3, and 6. The molality of NH4Cl ranged from 0.02 to 0.3 mol·kg−1, whereas the molality of NH4HCO3 was varied from 0.04 to 0.73 mol·kg−1. Values of −lgK1o were calculated by means of the Harned extrapolation method. The average value (pK1o = 6.355) for all different ratios of m 1 m 2 agreed well with the adopted value (pK1o = 6.360) for (sodium bicarbonate + sodium chloride + water) and (potassium bicarbonate + potassium chloride + water) at 298.15 K. The values of lg γ were interpreted by the mixed-electrolyte equations of Pitzer and Kim to yield Pitzer's ion-interaction parameters for NH4+ and HCO3tt-. With these results, the stoichiometric and equilibrium activity coefficients and the osmotic coefficient of pure NH4HCO3, as well as the trace activity coefficients of NH4HCO3 in NH4Cl are calculated at various temperatures and selected molalities.