USING GALVANIC CELLS

A galvanic cell consist of two half cells of different potentials.
A digital voltmeter measures the potential difference between them.
A salt-bridge sometimes completes the circuit.
One half cell contains a reference electrode and otherwise closed unit in the electrical contact with the other half cell
Reference electrodes maintain constant known potentials.
An example is the saturated calomel electrode
This apparently sealed probe has a glass fretted space which provides electrical contact with the surrounding liquid.
The other half cell which contains the indicator electrode has a potential which changes with the properties of the solution it dips into.
Examples of commonly used indicator electrodes are
The Silver Electrode
The Platinum Electrode
The Glass Electrode whose potential varies with the concentration of Hydrogen Ions and so is used in PH measurement
And there is the other
Ion Selective Electrode whose potential depend on the concentration of Ions like fluoride and Nitrate
Two half cells can be combined into a single probe as in the measurement of Ph
Such probes are usually more robust than single electrodes
It is essential that electrodes are in good condition and free from surface contamination.
If you have any doubt about this, ask a member of staff.
Handle electrode carefully. They are often expensive and easily broken.
Take care to mount fragile electrode well cleared of magnetic stirrer baths.
Electrodes are very sensitive. So when taking measurement on a series of samples, It is important to rinse the electrode and then the bridge used thoroughly with distilled water.
Then dry them before taking the next reading.
Alternatively, they may be rinsed with some of the next solution to be measured before taking readings using a fresh sample of that solution.
At the end of the set of measurements, wash the electrodes with distilled water and return them to their storage containers.
Electrodes left lying around can become contaminated or dry out and become unusable.
When using a digital voltmeter, the correct scale of the meter must be chosen and the meter must be calibrated.
So for Ph measurements, the meter’s PH scale is selected.
And the scale is calibrated using standard buffer solutions of accurately known ph.
The PH of any sample solution can then be read.
Salt bridges contain metal salts in an agar gel
They come pre-pared or else can be made as required as illustrated by the bridge shown here.
It is important the salt in the bridge does not react with the ions in the cells.
For example, potassium chloride in a salt bridge will react with silver nitrate in a cell solution.
If this happens, both the bridge and the solution are ruined.
At the end of an experiment, rinse the end of the slat bridge with distilled water; remove the excess water with tissues
And store the salt bridge in a storage container such as desiccators but without desiccant.