How to Care for pH Meter Electrodes

pH meter electrodes measure the degree of acidity by comparing the aqueous solutions. The electrodes are immersed in with the reference half-cells built inside the electrodes. These half-cells are coated with silver chloride or AgCl.

It is very important that the reference half-cells situated inside the electrodes are functional, and the most effective way to ensure that is to provide optimal care for the pH meter electrodes. This includes calibrating it from time to time, using pH calibration buffers, such as potassium chloride or KCl.

With AgCl inside the electrode and KCl outside the electrode, a concentration gradient develops between the two aqueous environments over the difference in the concentration of hydrogen ions. This leads to a development of potential difference that can be measured in millivolts, that is used to measure the pH in turn.

In addition to pH, the electrode can also convert the millivolts signal into a temperature sensor, as the temperature can affect the pH calibration of the electrode. Most modern pH meter electrode has an in-built temperature sensor that detects the temperature automatically.

pH calibration buffers like KCl are standardized solutions of fixed pH values that are calibrated up to an accuracy of 0.001 pH. These buffer solutions are certified for quality control after calibrating twice or even thrice to assure of its reliability, even though calibrating the buffer solution once with a pH reference buffer is typically sufficient to guarantee the ability and accuracy of the pH calibration buffer in measuring a range of pH values.

KCl is used in a concentration of 4 moles per cubic decimeter to immerse the pH meter electrode in a beaker. After immersing the electrode in KCl, it is not uncommon to observe KCl crystals forming outside the pH meter electrode. Rest assured, the crystals are not known to interfere with the ability of the electrode to obtain accurate measurements and can be easily addressed by rinsing and blot drying the electrode before use. It is also important to ensure that while immersing the pH meter electrode in KCl, the glass bulb is entirely covered by the KCl because it needs to be kept moist.
Related: Working Principle of pH Meter

However, before the electrode is soaked in KCl, it needs to be washed with distilled water. This stage of washing the electrode has to be quick as prolonged exposure of the pH meter electrode to distilled water can compromise with its ability to effectively measure the pH, by damaging the special glass membrane that contains the reference half-cells.

Other factors that can threaten the effectiveness of the pH meter electrode include the entry of KCl into the glass bulb of the electrode, which should be prevented by the diaphragm that forms a protective barrier between the glass bulb and the surrounding KCl solution.

However, the diaphragm can be dysfunctional and then KCl can enter the glass bulb and alter the equilibrium balance of AgCl inside the glass bulb, adversely affecting the calibration of the ion concentration and potential difference of the electrode, which can severely compromise the effectiveness of the pH meter electrode.