CONDUCTANCE STUDY OF THE BORATE FORM OF AN ANIONIC HOMOGENEOUS MEMBRANE BY IMPEDANCE SPECTROSCOPY

To improve our understanding of the mechanism of boron extraction from B(OH)(3) aqueous solutions by means of electrodialysis across a homog eneous anionic ADP/216 membrane, the conductivity of the berate form o f the membrane was investigated using impedance spectroscopy. Measurem ents were carried out for concentrations ranging from 0.02 to 0.51 mol kg(-1) at four temperatures between 5 and 40 degrees C. The marked de crease in the membrane conductivity at higher concentrations was attri buted to the formation of univalent polyborate anions, the mobilities of which decrease with increasing number of boron atoms. Equilibrium c onstants for the formation of tri- and pentaborate species were estima ted. For this membrane, the specific conductivities at 25 degrees C of the mono- and triborate counter-ions were found to be (30 +/- 1) Ohm( -1) cm(-1) and (0.30 +/- 0.05) Ohm(-1) cm(-1) respectively. The activa tion energies for migration of these two species were proportional to the number of boron atoms.