by Ben O'Donnell

Activity =

where is the activity coefficient of Hydrogen (H).



A pH meter measures the negative logarithm of the hydrogen ion activity, not just its concentration.

Ions in solution do not act according to their concentrations, which would be ideal, the activity coefficient is the deviation of this behavior from the ideal.(Harris 1999, 178).

Buffers are solutions that resist change in pH even when acids or bases are added or when they are diluted. They are very important to many bioanalytical procedures, from things like solution stability to analyte-ion binding due to pH as in capillary electrophoresis(Harris 1999, 178).

For some information on the biological significance of pH, go to the link listed below.

How a pH meter works(another description, from the web):

A pH meter consists of a reference electrode coupled to a working electrode. The reference electrode contains a saturated solution of a solid and a salt-for instance AgCl-which maintains a constant potential. The two electrodes measure the electric potential across a glass membrane. The glass membrane is located at the bottom of the pH electrode, with the reference electrode and working electrode contained within one tube. The working electrode is in contact with the glass electrode via a saturated solution of HCl and AgCl. The reference electrode is in contact with the working electrode through a salt bridge like solution of KCl and AgCl. This allows the two electrodes to interact electrically , which is how the electric potential of the glass membrane is measured. The glass membrane itself consists of a hydrated gel layer on the inner and outer surfaces, separated by a dry glass layer. H+ from the solution can diffuse into the gel membrane where it reacts with metal cations, which is known as ion-exchange equilibrium. The H+ then is the only ion that can be measured by this type of electrode, because it is the only ion that can bind significantly to the glass membrane(Harris 1999, 178). This is a picture of one kind of glass electrode, it has all of the basic parts described above, although it is hard to see.

The potential difference off the inner and outer surface of the glass membrane is measured in relation to the hydrogen ion and chloride ion concentrations on the inside of the glass membrane, since the concentrations of these ions are constant on the inside of the electrode. The only variable then is the concentration of hydrogen on the outside of the glass membrane. The equation for this potential is as follows:

The value of ß (electromotive efficiency) is accepted as 1 generally. The pH meter is then calibrated by using solutions with known values of pH(Harris 1999, 178).

Some Review articles from ChemAbstracts:

Studies of vanadate-organic ligand systems using potentiometry and NMR spectroscopy Author: Pettersson, Lage, and others Source: ACS Symp. Ser. American Chemical Society 711, no. Vanadium Compounds (1998): 30-50

The next two articles are older, but the field of pH and pH meters is not exactly cutting edge chemistry.

Behavior of the glass electrode and other pH-responsive electrodes in biological media Author: Bates, Roger G., and others Source: Ann. N. Y. Acad. Sci. 148, no. 1 (1968): 67-80

Sensitive measurements of changes of hydrogen ion concentration Author: Chance, Britton, and others Source: Methods Enzymol. 10 (1967): 641-50



Review articles and procedures from JChemEd online:

K. L. Cheng: A New Concept for pH-Potential Calculations
This paper discusses the concept of pH-potential calculations and indicates that the pH electrode is a capacitor, not a half-cell as currently believed. Citation: Cheng, K. L. A New Concept for pH-Potential Calculations J. Chem. Educ. 1999 76 1029.

Mary M. Walczak, Deborah A. Dryer, Dana D. Jacobson, Michele G. Foss, and Nolan T. Flynn: pH Dependent Redox Couple: An Illustration of the Nernst Equation
Electrochemical reactions depending on pH are ideal for illustrating the Nernst equation for undergraduate laboratories. An experiment utilizing the hydroquinone/quinone redox was developed which follows the potentials of the anodic and cathodic cyclic voltammetric waves as a function of solution pH. The 1mM hydroquinone solutions are prepared in phosphate/acetate mixed buffers with pH between 1-6. Cyclic voltammograms were obtained for each solution. As solution pH increases, the anodic and cathodic potentials shift cathodically as predicted by the Nernst equation. Plots of peak potential vs. pH were linear and showed that the hydroquinone/quinone redox reaction involves two electrons. J. Chem. Educ. 1997 74 1195.


Lai, S. T. F.; Burkhart, R. D. Evaluation of acid-base indicator constants using a limited pH range. J. Chem. Educ. 1976 53 500.

Finlayson, A. C. The pH range of the Mohr titration for chloride ion can be usefully extended to 4-10.5 (TF). J. Chem. Educ. 1992 69 559.

Dole, Malcolm. The early history of the development of the glass electrode for pH measurements (SBS). J. Chem. Educ. 1980 57 134.

Brower, Harold E. A glass electrode pH meter using a vacuum tube voltmeter. J. Chem. Educ. 1963 40 85.

Jackman, Donald C. A recipe for the preparation of a pH 7.00 calibration buffer. J. Chem. Educ. 1993 70 853.

Vitz, Ed; Betts, Thomas A. LIMSport (V): pH Data Acquisition: An Inexpensive Probe Calibration Software (CS). J. Chem. Educ. 1994 71 412.



Other Web sites dealing with pH and electrochemistry:

UBC WWW Site: A good site but pretty basic, no pun intended-it deals with the biological importance of pH, which was mentioned very briefly above.

Innovative Sensors: An excellent site that deals with a wide range of pH questions

A site demonstrating the process of calibrating a pH electrode.

Tulane University: A fun site where you are playing in the pH pool, need I say more?

pH Site at ?: This is a good site as well, with good presentation.

Back to Bioanalytical Home Page