Introduction:
The use of electrodes to measure voltages that provide
chemical information is called potentiometry (Harris). An ion-selective
electrode (ISE) is a membrane electrode that responds selectively
to a specific ion in the presence of other ions. The ISE is constructed
of two electrodes, the reference electrode and the ion-selective
electrode. Activities (concentrations) in the reference are held
constant so that the measured cell potential is only due to the
potential difference across the ionic conducting membrane. Check
out this
for a schematic of the ISE setup. If all substances could move
across the conducting membrane equally well the potential difference
would be zero. The ISE limits the movement of ions by manipulating
ionophores. Ionophores are charged or neutral compunds that complex
ions reversibly and transfer them across the membrane. The ISE
membrane acts in the same way as biological cell membranes, allowing
only specific ions to pass. Membrane potentials are responsible
for the operation of the nervous system in living organisms. Chemists
make use of membranes to construct sensors for many ions in solution.
The most common ISE is the pH electrode, but many other ions and
gases in solution can be measured. Hydrogen, sodium, potassium,
and fluorine ions are commonly detected by ISE. Measurement by
ISE is usually simpler and faster than other analytical techniques.
ISE's are inexpensive and are adaptable to both field and laboratory
work. Ion activity determinations can me made in water, pharmaceuticals,
food, and biological samples. For a good description of the different
kinds of ISE's check out Encyclopedia
Britannica. To get an idea of commercially available electrodes,
their methods of calibration, their membranes, and their sources
of error check out Orion
Research Inc. This is also a good place for links to other
ISE sites. If you want to try out a calibration by standard addition
yourself look at the spreadsheet lab for determination of calcium. If anatomy gets you
going check out a lab based on data from occluding the coronary
artery of a pig. It will show you how to determine potassium
and hydrogen ion concentrations.
Doing the math: (Modified
from Scimedia)
Potential is measured under conditions of no current flow.
The potential that develops in the electrochemical cell is the
result of the free energy change that would occur if the
chemical phenomena were to proceed until equilibrium had been
achieved.
DGrxn = -nFErxn
For these electrochemical cells the potential difference
between the cathode electrical potential and the anode electrical
potential is the potential of the electrochemical cell.
Ecell = Ecathode - Eanode
If the reaction is conducted under standard state conditions,
this equation allows the calculation of the standard cell potential.
When the reaction conditions are not standard state, we must utilize
the Nerst equation to determine cell potential. The logarithmic
term is the reaction quotient Q.
Ecell = Eo (RT/nF) ln(Aa/Ab)
Physical phenomena which do not involve explicit redox reactions,
but whose initial conditions have a non-zero free energy, will
also generate a potential. An ion gradient across a semi-permeable
membrane is an example of this. A calibration at a known activity
solves for the constant term.
Emem = (constant) (RT/ZiF) ln (Ai)
An ideal ISE would respond only to one kind of ion, but normally
there are many interfering species. The selectivity coefficient
gives the relative response if the ISE to different species with
the same charge.
KA,X = response to X
response to A
Because of this interference we must modify the equation for Emem to take into account the potential difference due to interfering species.
Emem = (constant) (RT/ZiF) ln (Ai+Â KA,XAX)
ISE Laboratories: References from Journal of Chemical Education
1. Diamandis, E. P.; Koupparis, M. A.; Hadjiioannou, T. P. Kinetic studies with ion-selective electrodes: determination of creatinine in urine with a picrate ion selective electrode: a laboratory experiment. J. Chem. Educ. 1983 60 74.
2. Papastathopoulos, D. S.; Karayannis, M. I. Construction and evaluation of a solid-state iodide selective electrode: A chemical instrumentation laboratory experiment. J. Chem. Educ. 1980 57 903.
3. O'Reilly, James E. Determination of iodide in milk with an ion selective electrode. J. Chem. Educ. 1979 56 279.
4. Lloyd, Baird W.; O'Brien, Frank L.; Wilson, W. David.
Student preparation and analysis of chloride and calcium ion selective
electrodes. A multipurpose experiment. J. Chem. Educ. 1976 53,
328.
5. Covington, A. K.; Thain, J. M. The dissociation constant of HF. An undergraduate experiment with the lanthanum fluoride ion selective electrode. J. Chem. Educ. 1972 49 554.
ISE Review Articles: References from Chemical Abstracts
1.Design features of ionophores for ion-selective electrodes
Author: Pretsch, Ernoe, and others Source: Pure Appl. Chem. 60,
no. 4 (1988): 567-74
2. Electrochemical flow measurements in environmental analysis.
Author: Stulik, Karel Source: Pure Appl. Chem. 59, no. 4 (1987):
521-30
3. Ion-selective electrodes Author: Arnold, Mark A., and
others Source: Anal. Chem. 58, no. 5 (1986): 84R-101R
4. Applications of ion-selective electrodes in nonaqueous and
mixed solvents Author: Pungor, E., and others Source: Pure Appl.
Chem. 55, no. 12 (1983): 2029-65
5. Membrane electrode probes for biological systems. New sensors
expand measurement horizons Author: Rechnitz, G. A. Source: Science
190, no. 4211 (1975): 234-8
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