Page 25 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GJB Revised Pages
Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

564 Analytical Chemistry

an interfering ion of charge Z adjusted for response with a
selectivity factor K 1,2 representing the ratio of selectivity
for ion 1 over ion 2. All selective electrodes suffer from in-
terferences caused by competing species, and these must
be recognized and controlled in any quantitative assay.
It is possible to produce hybrid potentiometric elec-
trodes suitable for the detection and measurement of cer-
tain gases and organic molecules. An ion-selective elec-
trode can be used as a transducer to measure changes in ion
activity caused by enzyme–substrate reactions or dissolu-
tion of gases at small aqueous volumes trapped at the se-
lective electrode surface. For example, an enzyme trapped
in a hydrophilic water-swollen gel can be attached to the
surface of a glass electrode. When placed in a substrate-
containing sample solution, the enzyme–substrate reac-
tion may produce a certain amount of hydronium ion
product, which can be quantitatively detected as a local
FIGURE 16 Construction and hydration properties of an ion- pH alteration by the glass electrode. Further extension of
selective glass electrode. this technology makes use of enzyme-linked immunoas-
say. An antibody that is attached to an enzyme provides
immunochemical selectivity in a competitive binding or
Glass electrodes. Glass electrodes are ion-
sandwich assay strategy, while the enzyme provides the
selective electrodes based on the chemical properties of a
electrochemical signal.
glass membrane of defined chemical composition. Alter-
ation of the glass chemistry to contain variable quantities
of Na 2 O, CaO, SiO 2 , and Al 2 O 3 produces chemically 3. Voltammetry and Polarography
active binding sites in the glass that have hydronium,
sodium, or simple cation selectivity. The construction of Voltammetry refers to a broad range of techniques that
a typical electrochemical cell based on a glass electrode compare current–voltage relationships between a work-
is shown in Fig. 16, including a representation of the ing and a reference electrode. The best known of these
physical properties of the sensing membrane. The internal techniques is polarography, which is based on an elec-
solution contains a fixed concentration of the cation of trolytic reduction or oxidation at a microelectrode when
interest and therefore fixes the internal electrode surface the rate of the redox reaction is controlled by analyte dif-
potential, while that in the external solution varies. The fusion to the working electrode surface. Standard work-
electric potential develops only in each hydrated gel ing electrodes are easily polarized microelectrodes such
layer based on an ion-exchange principle that leads to as the mercury drop, rotating platinum, glassy carbon,
a phase boundary potential. The entire glass membrane
is nominally 50–100 µm in thickness, but the hydrated
glass accounts for little of this, being only 5–100 nm TABLE X Common Ion-Selective Electrodes
thick. Conduction within the dry glass is due to the cation Typical
of lowest charge and is not related to penetration by a Membrane type Construction ion selectivity
substantial amount of cations.
+
Glass Thin, specially formulated H , monovalent
glass Group I cations
Ion-selective electrodes. The glass electrode is
−
−
Solid state Pellets or polymer matrix Cl ,Br ,I ,Cu 2+ ,
−
one example of an ion-selective electrode; three other spe- of insoluble salts Cd 2+ ,Pb 2+
cialized types exist as defined by their membrane con- −
Single crystal of LaF 3 F
struction. A summary of these is given in Table X. All doped with EuF 2
these devices are governed by modifications of the Nernst Liquid ion Hydrophobic charged ion- Ca 2+ ,Mg 2+ ,NO ,
−
3
−
equation, as shown for the fluoride electrode at 25 C, exchange liquid trapped ClO ,Cl −
◦
4
in porous lipophilic
0.059 n/z diaphragm
E cell = const − log a 1 + K 1,2 a 2 ,
n Neutral carrier Neutral chelating agent in K ,Na ,Rb +
+
+
where the cell potential E cell is related to a constant in- liquid trapped in porous
cluding internal and external reference electrode poten- diaphragm or supported
in polymer matrix
tials, the activity a 1 of the analyte, and the activity a 2 of