Electrode Potentials in Fused Systems VII. Effect of Ion Size on Membrane Potentials

Concentration cells exhibit ing lass membrane potentials have been measured for the AgBr-NaBr, AgC1-KC1, and AgC1-CsC1 systems. It is shown that cat-ions from the melt diffuse into the glass and that sodium ion impurit ies in KC1 and CsC1 may transport a large part of the current. The possibi l ity of anion transport through glass is suggested. A l though the l i te ra ture on glass membrane poten-t ials in aqueous systems is very large (1), v i r tua l ly noth ing has been publ i shed on this subject in mol ten salt systems. Severa l di f ferences to be expected be-tween the behav ior of glass membranes in aqueous solut ions and in mol ten salts are wor th noting. In the mol ten salt there is no molecu lar solvent, and therefore the ionic concentrat ions are ext remely high. When a glass is immersed in an aqueous elec-t ro ly te only its surface layers are in equ i l ib r ium wi th the solut ion (2). At the high temperatures usua l ly employed for mol ten salt studies, glasses are con-s iderab ly more permeab le to ions, and it is to be ex-pected that the glass wi l l come to equ i l ib r ium with the mel t at some approx imate ly un i fo rm ionic con-centrat ion in the glass. [For example, it has recent ly been shown (3) that even anions can migrate through fused si l ica under an appl ied potent ia l.] This does not imply, however, that the re lat ive ionic composit ion of the glass wi l l be the same as in the mel t when equ i l ib r ium is reached. Thus, sodium ions diffuse pre ferent ia l l y to potass ium ions into the glass f rom a KNO~-NaNO3 mel t conta in ing only 1