Hach POLYMETRON 8362 User Manual page 9

Transmitter 9135 - pH measurement in pure water
where :
E = the emf of the cell
Eo = the zero potential (isopotential) of the system :
depends on the internal construction of the
glass and reference electrodes
R = gas constant
T = temperature in Kelvin
a[H+] = activity of the hydrogen ion (assumed to be
equivalent to the concentration of hydrogen
ions)
F = Faraday constant
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.
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