In this article we will discuss about the pH meter and its operation.
pH meter is a potentiometer that measures the voltage between two electrodes dipped in a solution. One of these electrodes is a calomel electrode and the other a glass electrode. The calomel electrode has a constant electrical potential and is the external reference electrode. However, the glass electrode is the standard test electrode and its electrical potential depends on the pH of the test solution.
The electromotive force (emf) of ccsnplete cell E= Eref – Eglass. Where Eref = potential reference electrode or Calomel electrode which at normal temperature is +250 V.
Eglass = Potential of the test electrode or glass electrode which depends on the solution tested.
Operation of pH Meter:
1. Soak new /dry electrodes in water / buffer pH 6-7 overnight or in 0.1 (M) HCl for 12-24 hours. (Note: store electrodes always in distilled water).
2. Fix a “temperature compensations knob” to the temperature of the solution (Note: This is necessary since equilibrium constant of a reaction i.e. in acid base titration does vary with temperature).
3. Remove beaker with water, rinse and mop with tissue paper.
4. Dip electrodes into standard buffer with known pH making sure that the electrodes do not touch the bottom or sides of the beaker.
5. Adjust the pH meter to standard pH using pH adjust knob.
6. Remove standard buffer, rinse electrodes with distilled water mop with tissue paper and dip in solution with unknown pH.
7. Read the pH on the dial.
8. Wash and store the electrodes in distilled water.
This electrode contains Hg, HgCl2 and saturated solution of KCl, each one of these compounds exist in ionized state, though the extend of ionisation may vary. This electrode is dipped in concentrated solution of KCl. The electrical contact between the calomel electrode and the test solution is obtained by the KC1 salt bridge through a fine capillary in the glass casing known as porus plug.
This electrode is made of thick glass that is impermeable to H+ ions and therefore its potential is independent of pH.
This electrode contains Ag, AgCl2 and 0.1 (M) HCl the tip of which is made of special thin (0.05 to 0.1 mm) borosilicate glass which is permeable to H+ ions alone, but not to other cations or anions.
When electrodes are placed in a solution with high H+ ion concentration (low pH) the calomel electrode will not respond as it is not permeable to H+. The H+ ions pass through
the glass membrane and neutralise the electrons of the electrodal reaction. Hence electrons flow from calomel to glass.
In cases where test solution has high concentration of OH– ions (high pH), H+ moves out of glass bulb making momentary negative charge. Hence electrons flow from glass to calomel.
This passage of ions leads to the development of an electric potential across the glass and calomel electrodes resulting in a flow of current between these electrodes. The magnitude of the current depends on the concentration of H+/OH ions present in the solution. This current is directly fed into a calibrated dial and pH of the solution can be read.