Acidic polyol-boric acid complex chemistry and its application to boron analysis

¹¹B NMR measurements showed that boric acid reacts with the polyol compounds, chromotropic acid, Tiron and 2,3-dihydroxynaphthalene-6-sulfonate, to form complexes with 1: 1 and 1: 2 stoichiometries. The equilibria of the complexation depend on the pH in the solution. The 1: 2 complex is formed in an acidic or a neutral solution, while the 1: 1 complex occurs in a higher pH range. Kinetic studies revealed that the reaction of 1: 1 complex formation is much faster than that of 1: 2 complex formation. A proton catalyzes both the reactions of 1: 2 complex formation and its decomposition. On the basis of equilibrium and kinetic information, a novel method of on-line absorptiometric determination for trace amounts of boron was developed using a chromotropic acid presorbed anion- exchange column. On-line reaction and separation were achieved by controlling the pH conditions in solutions to accelerate 1: 2 complex formation in the concentration process at pH 3 and to stabilize the complex in the separation process at pH 8. The 1: 2 complex was satisfactorily separated from excess free ligand and matrix components by changing the concentration of NaClO₄ in the eluent; its peak height on a chromatogram monitored at 350 nm was used for boron determination. The sensitivity could be enhanced by increasing the sample amount introduced, and the detection limit (3σ) was 0.05 μg dm^-3 of boron when 13.4 cm³ of the sample was used. The method has been successfully applied to the determination of trace boron in river water, ion-exchanged water and steels. Based on the electrochemical activities of respective species in the boron-chromotropic acid and boron-Tiron systems, methods for monitoring the boron concentration in RO desalination processes were also proposed.