Describe the working principle of a ph meter and ph electrode?

Answer 1

pH METER
Principles of operation of a pH meter
A pH meter is essentially a voltmeter with a high input impedance which measures the voltage of an electrode sensitive to the hydrogen ion concentration, relative to another electrode which exhibits a constant voltage. The key feature of the pH-sensitive electrode is a thin glass membrane whose outside surface contacts the solution to be tested. The inside surface of the glass membrane is exposed to a constant concentration of hydrogen ions (0.1 M HCl).
Inside the glass electrode assembly, a silver wire, coated with silver chloride and immersed in the HCl solution, is called an Ag/AgCl electrode. This electrode carries current through the half-cell reaction. The potential between the electrode and the solution depends on the chloride ion concentration, but, since this is constant (0.1 M), the electrode potential is also constant.
A reference electrode is needed to complete the electrical circuit. A common choice is to use another Ag/AgCl electrode as the reference. The Ag/AgCl electrode is immersed in an 0.1 M KCl solution which makes contact with the test solution through a porous fiber which allows a small flow of ions back and forth to conduct the current. The potential created at this junction between the KCl solution and the test solution is nearly zero and nearly unaffected by anything in the solution, including hydrogen ions.
Using the pH Meter: Allow the meter a few minutes to stabilize after you plug it in. When you are not using the meter, keep the electrode immersed in pH 7.0 buffer to a depth of about one inch. The meter must be calibrated by using standards of known pH before an unknown is measured. Since the unknowns are acidic, the pH 4.00 and pH 7.00 standards should be used.
An accurate pH reading depends on standardization, the degree of static charge, and the temperature of the solution.


Glass electrode        Reference electrode      Combined electrode



1. The pH meter should be standardized each time it is used with a buffer of known pH, preferably one closest to the desired final pH. To calibrate the pH meter, expose the hole in the electrode, rinse the electrode with deionized water, and place the electrode in a standard solution, e.g., pH 7. Turn the selector to "pH". Adjust the pH meter to the appropriate pH. Rinse electrode with deionized water and place in a second standard buffer solution. The choice of the second standard depends on the final he standard pH buffers used should be 7 and 10. If the final pH desired, for example, if the final pH desired is 8.5, t pH desired is 5.5, the standard pH buffers used should be 4 and 7. Turn the selector to "pH". Adjust the temperature k**b to the second standard pH. Rinse the electrode with deionized water, and return the electrode to the soaking solution.
2. When rinsing the electrode, never wipe the end, but blot gently since wiping can create a static electric charge, which can cause erroneous readings,
3. Make sure the solution you are measuring is at room temperature since the pH can change with a change in temperature.
4. The pH-sensitive glass membrane is very thin and very easily broken. Do not touch the membrane with anything harder than a Kim-Wipe and do that very gently. Do not drop the electrode or bump it on the bottom of the beaker when immersing it in a solution.
5. The glass membrane must be thoroughly hydrated to work properly. Do not allow the electrode to remain out of water any longer than necessary. When the electrode is not in use, keep it immersed in the pH 7.00 buffer. Do not put the electrode down on the desk.