The ionization of boric acid in NaCl, NaCaCl and NaMgCl solutions at 25°C

Abstract

The stoichiometric pK ★ for the ionization of boric acid has been determined from emf measurements in pure NaCl solution and in NaCl solution with small amounts of added CaCl 2 and MgCl 2 from I = 0.5 to 6.0 molal at 25°C. Our results in NaCl media at low concentrations are in good agreement (±0.01 pK units) with the results of Owen and King (1943). From the values of the pK★ we have determined the Pitzer parameters β 0 , β 1 and C ø for the interactions of Ca 2+ and Mg 2+ with B(OH) 4 −. Ion pairing constants for the interaction of borate ion with Ca 2+ and Mg 2+ were also determined from I = 0.5 to 6 m and values were extrapolated to pure water. These interactions are weak and the pK's in water and in NaCl solution are in reasonable agreement with values of other workers. Pitzer terms (β [MA]Cl 0) for the interaction of [MA] ion pairs with Cl − were also determined. The pK HA ★ data can be fitted within experimental error with one less parameter using the combined ion pairing/Pitzer approach than when using the Pitzer formalism alone. We, thus, favor the combined approach for calculating the ionic interactions in these solutions.