Question

Why $KCl$ used in pH meters?

Answer 1

In the pH meter, we need to understand how it functions to understand the need of $KCl$ solution. The pH meter displays the pH of any solution through the potential difference, which in a way needs potential difference, and in order to get that, there must be sufficient carriers in the form of ions which can in the completion of circuit but also do not interfere with the pH of the solution. $KCl$ would serve that purpose.

Complete answer: In the pH meter, the basic elements are a glass electrode, a reference electrode, the electrolyte solution, and the test solution(whose pH is to be calculated). Hence, in order to understand why $KCl$ is used as an electrolyte, we need to understand the role of electrolytic solution in the pH meter.
The working of the pH meter is such that the glass electrode and the reference electrode are dipped in the electrolytic solution which in turn are connected to the test solution(whose pH is to be found) through a porous ceramic membrane. There is then a voltmeter which displays the volts in terms of pH.
Hence, we need such a solution in the electrolytic solution which would provide enough ions to complete the circuit and should also not participate or disturb or change the pH of the test solution, since it is in a way, connected to it.
Thus, we used $KCl$ solution, since it is a good source of ions in the form of $C{l^ - }$ ions. Also, $KCl$ is neutral and thus does not participate or change the pH of the solution to be tested.
Thus, due to these two factors, $KCl$ is used in the pH meter.

Note:
The term we use for the quantification of selection of such electrolyte is termed as ionic mobility and transference number. We can also use $NaCl$ in place of $KCl$ if we only needed the electrolyte to be neutral, but since the atomic size of ${K^ + }$ and $C{l^ - }$ ions are the same, thus having high transference number and thereby ionic mobility, they are used as an electrolytic solution in the pH meter.