Experiment ##: Ion-Selective Electrodes

Experiment ##: Ion-Selective Electrodes
Pre-Lab
1. Describe how an ion selective electrode measures ion concentration in solution.
2. Write down the cell potential E cell (mV) expression for a fluoride ISE. Draw a sketch graph of E cell vs. ion
concentration.
3. Write down the E cell (mV) expression for a calcium ISE. Draw a sketch graph of E cell vs. ion concentration.
4. A calcium ISE has a K value equal to 0.03 mV. What is the concentration of a solution measuring 53.3 mV when
using this electrode?
5. Why is it important that a refrigerated sample or standard be removed from storage 30 minutes prior to
measurement?

Experiment ##: Ion-Selective Electrodes
Theory
An ISE consists of a ‘sensing’ halfcell consisting of a polymer, crystalline or glass membrane (Table X.X). The most
commonly used ISE is the pH probe which uses a glass membrane. Polymer membranes are coated with an organic
molecule that selectively absorbs the ion being measured. Crystalline membranes contain crystal lattice holes
generated with an impurity through which ions may selectively pass [1, 2, 3]. Both types also contain a ‘reference’
halfcell.
Polymer membrane Ca 2+ , BF 4 - , NO 3 - , ClO 4 - , K + , Mg 2+
Crystalline membrane Br - , Cd 2+ , Cl - , Cu 2+ , F - , I - , Pb 2+ , Ag/S 2- , SCN -
Glass membrane H + , Na + , Ag + , Pb 2+ , Cd 2+
Table X.X.: Polymer, crystalline and glass ISEs.
The potential difference generated between the sensing electrode and the reference electrode is proportional to
the ions concentration.
The ions concentration generates a potential E (mV) according to the Nernst equation:
E cell = E 0 + 2.303 RT log C
nF
Where R = gas constant = 8.316 J mol -1 K -1 , T = temperature (K), n = charge on the analyte ion, F = Faraday’s
constant = 96487 C eq -1 , and C = concentration of ion. This simplifies to:
E cell = K + 59.2 log [M n+ ]
n
And, E cell = K - 59.2 log [A n- ]
n
Where E = potential (mV), M = metal element, A = non-metal element, n = integer charge, K = constant. This
equation conforms to the standard equation for a line ‘y = mx + b’ with slope (m) = ± 59.2/n and intercept (b) = K.

Fig. X.X: Components of a typical Ion-Selective Electrode.
Ionic Strength Adjuster (ISA): It is likely that samples will be higher ionic strength than standards. Variable ionic
strength of solutions varies electrode response; also the calibration curve is affected. ISA adds ionic strength (it
does not add ion under study). ISA swamps ionic effects of host solutions and gives uniform ionic strength in all
solutions. Calcium ISA does not buffer a solution or complex interfering ions like fluoride ISA. Fluoride ISA buffers
to a much narrower range (pH 5.2 to 5.7) which reduces interference from OH- and formation of toxic HF. Fluoride
ISA also complexes Al 3+ and M 2+ with citrate. These metal ions would otherwise bind with F - too strongly for it to be
measured correctly.

Experiment ##: Ion Selective Electrodes
Objective
To use an ion selective electrode to measure the concentration of an ion in solution.
Apparatus
An electrode, an output device, ionic strength adjuster and ion standards.
Method
1. Set up electrode and allow to equilibrate in high concentration standard (~30 mins). The pH range for most ISEs
is 3-10. Neutralize samples outside this range to bring them in range.
2. Samples and standards should be at the same temperature for precise measurement. Make up standards of 1,
10, 100, 1000 ppm. Add 2mL of ISA per 100 mL of standard.
3. Analyze with ISA from low to high (0.1, 1, 10, 100, 1000 ppm). Be sure that the ISE is not resting on the bottom
of the sample. Agitate samples whilst measuring them, this supplies a constant flow of ions to the sensor. Hold the
ISE and wait approximately 30 seconds for the voltage reading to stabilize. Rinse the ISE thoroughly with DI water
and shake off excess solution, do not ever touch the membrane it is very fragile.
(Note: if the voltage reading fluctuates, calculate the mean value.)
4. Construct a calibration curve of E cell (mV) vs. log C (where C = [M n+ ] or [A n- ]), ensure that the slope is at or near
59.2/n (± 2 mV).
5. Analyze unknown water samples and use the calibration curve to calculate the ion concentration (mg/L).
6. For the calcium ion calculate the concentration of samples in ppm CaCO 3 .
Calcium hardness as CaCO 3 (ppm CaCO 3 ) = 2.5 x [Ca 2+ ]
7. Store ISEs in dilute ISA solution. For short periods in between use ISEs are best stored moist. Dry before storing
for long periods of time.

Results
Standard
Concentration
(ppm)
Potential (mV)
Sample ID Potential (mV) [ ____ ] (ppm) Notes

Calibration Curve
Conclusion
1. Discuss the sample ion concentrations, are they as expected?
2. What kind of ion concentration values could you use for comparison?
3. How many ISE readings of a single sample should you take in order to improve accuracy?
4. Describe what you would look for on an ISE calibration curve in order to assess whether an ISE is
faulty or in need of repair.

References
[1] Frank and Ross (1966) Electrode for Sensing Fluoride Ion Activity in Solution. Science, Vol. 154,
pp.1553.
[2] Frant, MS. (1994) History of the Early Commercialization of Ion-Selective Electrodes. Analyst, Vol.
199, pp. 2293-2301.
[3] Meyerhoff, ME and Opdycke, W.N. (1986) Ion Selective Electrodes. Advances in Clinical Chemistry,
Vol. 25, pp. 1-47.