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
membrane are '
counter-ions'. Ions of the same sign as sites are not present in significant quantities, and are
co-ions. Sited membranes are '
permselective' to
counter-ions i.e. only
counter-ions exchange into the
membrane and therefore have some
mobility in the
membrane bulk.
Source:
PAC, 1994, 66, 2527
(Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations
1994))
on page 2532