Activity coefficient of hydrochloric acid in HCl/MgCl2 mixtures and HCl/NaCl/MgCl2 mixtures from 5 to 45°C

Magnesium, the dominant bivalent cation in natural seawater, exerts a substantial influence on the patterns of ion interactions in this saline medium. Mean activity coefficients of hydrochloric acid in mixtures of this acid with magnesium chloride at four ionic strengths, namelyI=0.1, 0.3809, 0.6729, and 0.8720 mole-kg^–1, were obtained from emf measurements of cells without liquid junction at nine temperatures from 5 to 45°C. The three highest ionic strengths correspond to seawater of salinities 20, 35, and 45, respectively. In addition, mixtures of HCl, NaCl, and MgCl₂ were studied atI=0.6729, the molal ratio of NaCl to MgCl₂ being maintained at 7.202 as in natural seawater. The Harned coefficients _{1 2} were found to decrease slowly with increase in temperature. The trace activity coefficient of HCl in solutions of MgCl₂ as well as in NaCl MgCl₂ mixtures was found to be nearly identical with that measured earlier in synthetic seawater of the same ionic strength but containing NaCl, MgCl₂, KCl, and CaCl₂.