Probing the speciation of quaternary ammonium polybromides by voltammetric tribromide titration.

The speciation of quaternary ammonium polybromides (QBr(2n+1)) was quantitatively determined by voltammetric tribromide titration on a Pt ultramicroelectrode (UME). The concentration of Br-3(-) 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 Br-3(-) in a linear sweep voltammogram (LSV). The pBr(3)(-) titration curves of QBr(2n+1)-WMSs show 2-4 plateaus, each of which relates to the formation of QBr(2n+1) from Br-3(-) and Br-2. The values of pBr(3)(-) at these plateaus can be regarded as corrected equilibrium constants of QBr(2n+1), K-eq(n), which is K-eq(n)/(+/-), where (+/-) 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.