Calibration method:
It is very difficult to determine the value of E* both experimentally as well as theoretically. Thus, calibration method is used to eliminate this factor. Within this method, first test solution is comprised of a standard buffer solution with the pH precisely known. Therefore, for the standard, we could write following expression:
(Ecell )s = E* - 0.0592 (pH)s
This step is followed by measurement of cell potential for unknown solution. The expression will be
(Ecell) u = E* - 0.0592 (pH) u
For compute E* subtract Eq. (3.9) from Eq. (3.10), we get
(pH)u = (pH)s - (Ecell ) u - (Ecell )s/0.0591
Eq. (3.11) has been adopted throughout the world as the operational definition of pH.