Introduction; Theory - Hach POLYMETRON 9135 Operator's Manual

Transmitter 9135 - pH/redox measurement

Introduction

Page 4
PH Measurement

Theory

pH is the negative logarithm of the hydrogen ion
activity and a measure of the acidity, or alkalinity of a
solution.
pH is normally measured using a glass electrode and a
reference electrode.
The glass electrode acts as a transducer, converting
chemical energy (the hydrogen into activity) i n to an
electrical energy (measured in millivolts). The reaction
is balanced and the electrical circuit is completed by
the flow of ions from the reference solution to the
solution under test.
The electrode and reference solution together develop
a voltage (emf) whose magnitude depends on the type
of reference electrode, the internal construction of the
glass electrode, the pH of the solution and the
temperature of the solution. This voltage is expressed
by the Nernst equation :
E = Eo - (2.3 RT/F)xlog a[H+]
E = Eo -(slope) xlog a [H+]
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
pH = - log a [H+]