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 K 1 o of the reaction: CO 2 + H 2O = HCO 3 − + H +, was determined from the e.m.f. of the cell: Pt/H 2, CO 2/NH 4HCO 3( m 1), NH 4Cl( m 2), CO 2( m 3)/AgCl/Ag. The activity coefficients of NH 4HCO 3 in NH 4Cl(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 NH 4Cl ranged from 0.02 to 0.3 mol·kg −1, whereas the molality of NH 4HCO 3 was varied from 0.04 to 0.73 mol·kg −1. Values of −lg K 1 o were calculated by means of the Harned extrapolation method. The average value (p K 1 o = 6.355) for all different ratios of m 1 m 2 agreed well with the adopted value (p K 1 o = 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 NH 4 + and HCO 3 tt-. With these results, the stoichiometric and equilibrium activity coefficients and the osmotic coefficient of pure NH 4HCO 3, as well as the trace activity coefficients of NH 4HCO 3 in NH 4Cl are calculated at various temperatures and selected molalities.