membrane sites

in an ion-selective electrode
https://doi.org/10.1351/goldbook.M03826
Membranes frequently contain built-in 'fixed' charged sites (e.g. immobile –(SO3) in polystyrenesulfonate), or intentionally added, hydrophobically trapped, 'mobile' sites [e.g. tetraphenylborate in plasticized poly(vinyl chloride)]. Such membranes with charged sites are named sited membranes. Ions of opposite sign in the @[email protected] are '@[email protected]'. Ions of the same sign as sites are not present in significant quantities, and are @[email protected] Sited membranes are 'permselective' to @[email protected] i.e. only @[email protected] exchange into the @[email protected] and therefore have some @[email protected] in the @[email protected] bulk.
Source:
PAC, 1994, 66, 2527. (Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations 1994)) on page 2532 [Terms] [Paper]