Electrical Conductivity of Silver Bromide Membranes and the Diffuse Double Layer

摘要

Electrical impedance measurements have been made on a system consisting of an AgBr membrane separating identical aqueous solutions of AgNO3. Metallic silver electrodes immersed in the solutions complete a cell for dc resistance measurements related to the transport of Agplus;ions through the membrane/solution system. Measurements have been made at solute concentrations between4.0thinsp;times;thinsp;10minus;4Nand1.0Nand at temperatures ranging from 20deg;ndash;180deg;C, with pressurization of the conductance cell. The results obtained are interpreted in terms of a theoretical description which treats the specific resistance of the membrane as a sum of two parts, one of which is simply related to the resistivity of AgBr in bulk. The other component arises from the formation of diffuse double layers at the membrane/solution interfaces. Analysis of the data in terms of this theory permits the evaluation ofKthinsp;plus;thinsp;, the partition coefficient for Agplus;ions, which is equal to the ratio of the concentration of Agplus;ions in the aqueous solutions at the isoelectric point to the concentration of Agplus;interstitial ions in bulk AgBr. The concentration of Agplus;ions in aqueous solution at the isoelectric pointcisois also determined. It is found thatKplus;thinsp;equals;thinsp;7.4thinsp;times;thinsp;10minus;9explpar;0.97eVsol;kTrpar;, andcisothinsp;equals;thinsp;4.2thinsp;times;thinsp;10minus;6explpar;0.46eVsol;kTrpar; gmiddot;molesol;liter, whereTis the absolute temperature, andk, the Boltzmann constant, is equal to 8.61times;10minus;5eV/deg;K. Our values forcisoare higher than those reported by other authors working with freshly prepared sols and with systems similar to that employed in this study. These discrepancies are attributed to aging effects and to differing experimental conditions.