Biomedical Engineering Reference
In-Depth Information
Ion-selective electrode
Reference electrode
Emf
Ag/AgCl
Ag/AgCl
Reference
electrolyte
Inner
electrolyte
Frit
Bridge electrolyte
Ion-selective
membrane
Capillary
Sample
FIGURE 4.4
Schematic diagram of a membrane electrode measuring circuit and cell assembly.
The electromotive force (emf) across the cell is a sum of several potentials; many of
them are sample independent:
emf
E const
E M
E J
(1)
where E M is the membrane potential and E J is the liquid junction potential, formed at
the sample-salt bridge interface. The latter is often considered constant and neglected,
or it may be estimated according to the Henderson formalism.
Solvent polymeric membranes, conventionally prepared from a polymer that is
highly plasticized with lipophilic organic esters or ethers, are the scope of the present
chapter. Such membranes commonly contain various constituents such as an ionophore
(or ion carrier), a highly selective complexing agent, and ionic additives (ion exchang-
ers and lipophilic salts). The variety and chemical versatility of the available mem-
brane components allow one to tune the membrane properties, ensuring the desired
analytical characteristics.
The theory of ion-selective electrode response is well developed, due to the works
of Eisenman, Buck and others [23]. Three models used for the description of the ISE
response through the years, namely kinetic, membrane surface (or space charge) and
phase boundary potential (PBP) models, although being seemingly contradictory,
give similar results in most cases [7]. The fi rst two sophisticated models are out of the
scope of the present chapter, as the PBP model, despite its simplicity, satisfactorily
explains most of the experimental results and thus has become widely applicable. The
 
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