Kinetics of interchange between boric acid and tetrahydroxyborate has been studied by the band shape analysis of 11B NMR signals in aqueous solution of pH 8–10 at 20–52 °C and 0.1–250 MPa. The system was analyzed as two-site exchange B(OH)3 ⇌ B(OH)4-. The observed first-order rate constant toward the right (kX) is related to the second-order exchange rate constant (kex) by the equation kX = kexCT[OH-]/(kb + [OH-]), where kb = [B(OH)3][OH-]/[B(OH)4-], and CT is the total concentration of boron. Under the given conditions kex is (2–7) × 106 dm3 mol-1 s-1 and ΔH‡, ΔS‡, and ΔV‡ are 20.1 ± 1.0 kJ mol-1, -55.0 ± 3.1 J mol-1 K-1, and -9.9 ± 0.5 cm3 mol-1, respectively. The activation parameters reflect the transition state forming dimeric [(OH)3B(μ-OH)B(OH)3]-. The results are compared with those of related reactions.
ASJC Scopus subject areas