Transmitter 9135 - pH/redox measurement
Page 5
For every unit change in pH (or decade change in ion
concentration) the emf of the electrode pair changes
by 59.16 mV at 25°C. This value is known as the
Nernstian slope of the electrode.
The pH electrode pair are calibrated using solutions of
known and constant hydrogen ion concentration,
called buffer solutions. The buffer solutions are used to
calibrate both the electrode isopotential and slope.
Redox measurement
The potential which will be measured in a redox
system with a measuring chain consisting of a redox
electrode and a reference electrode is called the redox
potential. It depends on the ratio of activities of the two
partners of a redox system and of the numbers of
transferred electrons. In many cases the pH of the
solution will influence the potential, too.
Peters ‘s equation can determine the redox potentials.
The stability and reversibility of a redox system
strongly influence the reproductibili ty of the measured
redox potential.
Standard potentials of a redox system will be found for
aOx = a Red and for pH = 0 which corresponds again
to a standardized hydrogen ion activity aH+ = 1 mole
per litre.
