The effect of pressure on the formation of alkali metal borate ion pairs at 25°C

Abstract

Pressure dependent UV-visible spectrophotometric measurements were used to determine$\Delta \bar V^ *$ and$\Delta \bar \kappa ^ *$ for the formation of alkali metal borate ion pairs. The association constant for each ion pair was measured at 25°C and at ionic strengths of 0.1 and 1.0m over a pressure range of 1 to 2000 atm. The pressure dependence of the apparent association constants, K _A (P)/K _A (1), have been fitted to

$$[RT/(P - 1)]ln[K_A (P)]/[K_A (1)] = - \Delta \bar V^0 + \Delta \bar \kappa ^0 [(P - 1)/2]$$

to determine$\Delta \bar V^0 and \Delta \bar \kappa ^0$. The$\Delta \bar V^0$ for the alkali metal borate ion pairs range from 5–9 cm³-mol⁻¹. The association constants were also measured as a function of ionic strength at 1 atm. Extrapolation to I=0 yielded K _A of 2.12, 0.66, 0.76 and 1.12 for [LiB(OH)₄], [KB(OH)₄], [RbB(OH)₄] and [CsB(OH)₄], respectively. The trend generally indicates less ion pairing and a smaller volume change for the ion pair formation as the size of the cation increases. The concept of localized hydrolysis is used to explain the trend observed in the equilibrium constant of the ion pair as the cation size is changed.