New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 78

PBP model considers the membrane potential as a sum of the potentials formed at the
membrane-solution interfaces (phase boundary potentials), and generally neglects any
diffusion potential within the membrane:

E M E const E PB (2)

where E PB is the phase boundary potential at the membrane-sample interface, which
can be derived from basic thermodynamic considerations:

a
a

(aq)
(org)

EE RT
zF

I

0

ln

(3)

PB

I

where R , T and F are, correspondingly, the universal gas constant, absolute tempera-
ture, and the Faraday constant, z is the ion charge and a I (aq) and a I (org) are the activi-
ties of the ion I in the aqueous and organic (membrane) phases. The standard potential,
E 0 , is a function of the standard chemical potentials in respective phases:

0 0

µ (aq) µ (org)

(4)

E

0

zF

If a I (org) does not depend on a I (aq) and is therefore constant, Eq. (3) reduces to the
well-known Nernst equation:

EE RT
zF

0

ln a (aq)

(5)

I I I

and ISE demonstrates a linear response to the logarithmic ion activity with the
Nernstian slope of 59.2 mV decade 1 for z 1 at 298 K (see Fig. 4.5). For Eq. (5)

to hold, the respective ion activity in the membrane phase must be constant, which is
usually ensured by the lipophilic ion exchanger present in the membrane. For neutral
carrier-based membranes, the ion exchanger carries a charge opposite to the analyte
ion, while for charged carrier sensors the same charge is normally required (Fig. 4.6)
[24]. Oddly, in the early stages of polymeric sensors research, no ion exchangers were
used and it was later found that intrinsic ionic impurities played the role of the ion
exchanger in such membranes [25]. Nowadays added ion exchangers are strongly rec-
ommended, as they are also benefi cial in terms of selectivity optimization and reduc-
tion of the membrane electrical resistance.
While ionophore-free membranes based on classical ion exchangers are still in use
for the determination of lipophilic ions, such sensors often suffer from insuffi cient
selectivity, as it is governed solely by the lipophilicity pattern of ions, also known for
anions as the Hofmeister sequence. This pattern for cations is Cs Ag K

NH 4 Na Li Ca 2 Mg 2 ; and for anions: ClO 4 SCN I Sal

HPO 2 . While the ion
exchanger fi xes the concentration of hydrophilic analyte ions in the membrane on the
basis of the electroneutrality condition within the membrane, the second key membrane
component is the ionophore that selectively binds to the analyte ions. The selectivity of

SO 2 NO 3 Br NO 2 Cl OAc HCO 3