Instruction manual

Model 14 pH/Redox Transmitter
Chapter 1: Introduction
1.1 Introduction
The Model 14 pH/Redox transmitter has been designed
for measuring and continuously controlling pH or
redox potential and temperature in industrial process.
1.2 pH Measurement
Theory
The pH of a solution is the measurment of the acidity,
or alkalinity of that solution. It is determined by the
negative logarithm of the hydrogen ion activity: pH = -
log10 a
[H+]
Two electrodes, glass electrode and a reference are
used to measure pH. The glass electrode acts as a
transducer, converting chemical energy (the hydrogen
activity) into an electrical energy (measured in milli-
volts). The reaction is balanced and the electrical cir-
cuit is completed by the flow of ions from the reference
solution through the measured solution.
The electrode and reference solution together develop
a voltage (emf) whose magnitude depends on four
things:
• the type of reference electrode
• the internal construction of the glass electrode
• the pH of the solution
• the temperature of the solution.
This electrode voltage is expressed by the Nernst equa-
tion:
E = E
o
- (2.3 R I) log a
[H+]
E = E
o
- (slope) log a
[H+]
Where:
E = the emf of the cell
E
o
= 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 is calibrated using solutions
of known and constant hydrogen ion concentration,
called buffer solutions. The buffer solutions are used to
calibrate both the electrode’s isopotential and slope.
1.3 Redox Measurement
A redox measuring system consists of a redox and a ref-
erence electrode. The measured redox potential is the
ratio of electrode activities and the number of trans-
ferred electrons. In many cases the pH of the solution
will influence the potential, too.
The half-cell potential e
B
, of the reference electrode
will strongly influence the potential E of the measuring
chain. To remove this influence the potential of the
measuring electrode can be related to the hydrogen
electrode. If e
B
is the half-cell potential of the reference
electrode used, the calculation is made by
e
(
H
)
= E + e
B
Such standardized redox potentials provide informa-
tion to some extent on the oxidizing or reducing power
of a redox system. Increasing positive values express an
increasing power of oxidation. The more negative the
potential, the stronger the reducing power will be. The
range of practical interest is between +1500 and -1000
mV. Standard potentials of a redox system will be found
1-1