Experiment ##: Ion-Selective Electrodes

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.