The calibration of a glass electrode is based on the fact that, under certain conditions, it responds linearly to hydrogen ion concentration (1), but see Concentration or activity? below.
E = E0 + s log[H+] (1)
The required conditions include constant ionic strength and constant temperature. E0 and s are calibration constants. This equation will usually apply over a pH range of about 2.5 - 11, depending on the quality of the electrode.
The program GLEE is available at http://hyperquad.co.uk/glee.htm to assist with electrode calibration. This program takes data from the titration of a strong acid with a strong base. When the strengths of the acid and the base are known the hydrogen ion concentration is known so the electrode potential can be calibrated in terms of hydrogen ion concentration.
In order to calibrate the electrode in conditions that are close to the conditions used for stability constant determination, the titration vessel is filled with a large volume of the ionic medium solution and a small volume of acid. This is then titrated with a small volume of base.
Because the volume change during the titration is small the ionic strength is sufficiently constant for equation (1) to apply. The results of the calibration calculation are as follows.
The values of E0 and Slope factor are used as data in Hyperquad potentiometric data files. For more details concerning GLEE see the program's documentation.
Electrode calibration at pH values below about 2.5 can be performed with the spreadsheet program StrongH. This calibration is based on titration with strong acid and takes into account deviations from the linear behavior implied by equation (1) due to variations in junction potential.
In theory the electrode response should follow the Nernst equation (2).
E = E0 + RT/nF lne{H+} (2)
Thus, it responds to hydrogen ion activity rather than concentration. In practice the electrode can be calibrated to respond to hydrogen ion concentration, as discussed above. Alternatively, if the electrode is calibrated by using two or more buffer solutions of know activity then the response is in terms of activity.
Stability constants determined with the two calibration methods will not be exactly the same because the concentration method implicitly assigns values of unity to the activity coefficients. It is important, therefore, that the method of electrode calibration is stated explicitly in any report.
Contents > Experimental:Errors | Potentiometry > Electrode calibration | Very low pH calibration
