Kinetics of the formation of trace amount of tetrafluoroboric acid

Abstract

The rate constants for the formation of trace amount of tetrafluoroboric acid using nonstoichiometric amounts of boric acid and hydrofluoric acid have been measured at several temperatures. It was observed that the rate constants at a given temperature using the same hydrofluoric acid to boric acid ratio are much higher in the presence of sulfuric acid than those measured in its absence in the reaction mixture. In the range 0 to 7·5 × 10 −2 M H 2SO 4 the observed rate constant was found to be proportional to the hydrogen ion concentration raised to about three-tenth power. A mechanism for the catalysed reaction is proposed. The values for the activation energy, ΔH ‡ and ΔS ‡ for the reaction carried out in the presence and absence of H 2SO 4 have also been computed. With a 2500 fold hydrofluoric acid over boric acid the reaction could be brought to equilibrium in about 30 min provided it is carried out at 60°C or more and in the presence of sulfuric acid.