Determination of borate ion-pair stability constants by potentiometry and non-approximative linearization of titration data

Abstract

Borate anions, B(OH) − 4, are known to associate with alkali and alkaline-earth metal cations in sea-water. The borate cation ion-pairs are of the general form MB(OH) ( n−1)+ 4, where M n+ is the cation. In this work, the cation borate stability constants ( K* MB) have been evaluated for Na +, Li +, Mg 2+, Ca 2+ and Sr 2+ where K* MB = [MB(OH 4) ( n−1)+ ]/[M n+ ][B(OH) − 4]. The K* MB values were obtained from values found for the stability constant of boric acid (K* B) in various electrolyte media at 25° and an ionic strength of 0.7. Acid—base potentiometric titrations were performed in the electrolyte media with a standard Pt/H 2 electrode and a junctionless Ag/AgCl reference electrode to monitor the emf. A non-approximative equation was used to linearize the titration data. The values obtained were: K* Lib = 0.89 ± 0.02, K* NaB = 0.44 ± 0.01, K* MgB = 13.6 ± 0.7, K* CaB = 11.4 ± 0.15, K* SrB = 3.47 ± 0.06. The values for K* MB correlate with the charge-density parameter z 2/( r + 0.85), where r is the radius of the cation. The speciation of boron in sea-water was predicted from the K* MB, data for the major cations present.