Probing the speciation of quaternary ammonium polybromides by voltammetric tribromide titration

Abstract

The speciation of quaternary ammonium polybromides (QBr_2n+1) was quantitatively determined by voltammetric tribromide titration on a Pt ultramicroelectrode (UME). The concentration of Br₃⁻ in a QBr_2n+1-water mixed solution (QBr_2n+1-WMS) was electrochemically estimated by measuring the steady state current associated with the electro-reduction of Br₃⁻ in a linear sweep voltammogram (LSV). The pBr₃⁻ titration curves of QBr_2n+1-WMSs show 2–4 plateaus, each of which relates to the formation of QBr_2n+1 from Br₃⁻ and Br₂. The values of pBr₃⁻ at these plateaus can be regarded as corrected equilibrium constants of QBr_2n+1, K′_eq(n), which is K_eq(n)/γ_±, where γ_± is a mean activity coefficient in QBr_2n+1-WMS. Based on the estimated K′_eq(n), fractional diagrams of QBr_2n+1 were obtained, which gave information on QBr_2n+1 speciation.