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MODEL 54e pH/ORP SECTION 6.0
THEORY OF OPERATION
SECTION 6.0
THEORY OF OPERATION
6.1 THE pH SENSOR ASSEMBLY
The pH measurement is accomplished by means of a
measuring electrode (usually made of glass) which
develops a potential directly related to the hydrogen ion
concentration (pH) of the solution in which the elec-
trode is immersed. A second electrode, called the ref-
erence electrode, is necessary to complete the electri-
cal circuit and to serve as a constant reference poten-
tial against which the potential of the glass electrode
can be compared. Together, the two electrodes com-
prise the pH sensor.
The Glass Electrode
The glass electrode is a thin-walled bulb of glass con-
taining a pH-buffered solution and the elements of a
half-cell (typically silver/silver chloride). The glass
membrane provides a means of collecting hydrogen
ions so the potential can be measured. The hydrogen
ions on the surface of the glass membrane are in equi-
librium with the hydrogen ions in the solution being
measured. As the hydrogen ion concentration in the
solution changes, the concentration of hydrogen ions
on the electrode changes, creating a change in poten-
tial on the glass electrode.
The Reference Cell
The purpose of the reference cell is to maintain a sta-
ble reference potential regardless of a change in sam-
ple pH. The cell is comprised of the reference elec-
trode, reference fill solution and porous reference junc-
tion. The reference cell also serves to complete the
voltage measuring circuit. Within the reference elec-
trode, a silver chloride element is surrounded by a con-
centrated potassium chloride solution. The internal ele-
ment maintains electrical contact with the pH sensitive
glass electrode through the porous liquid junction (a
multi-capillary hardwood plug) of the internal reference
electrode.
The external wood reference junction and the gelled
reference fill solution provide protection for the internal
reference electrode to prevent contamination from
harsh chemical environments. The internal liquid junc-
tion can become blocked if the process contains any
material that reacts with the filling solution to form a
precipitate. The double-junction reference electrode is
designed to avoid blocking of the internal liquid junc-
tion. It is essentially a complete electrode within an
electrode outer body, using two liquid junctions.
A non-reactive electrolyte (potassium chloride gel) is
the filling solution in the outer body, and only this solu-
tion is in contact with the process. Clogging of the outer
junction is minimized, since neither potassium ions nor
chloride ions form insoluble compounds with the
majority of materials found in process streams.
Contamination of the inner junction is not likely with this
configuration.
The Temperature Compensation Element
In addition to the glass electrode and reference cell, a
third element required in the pH measurement system
is the temperature compensation element. Its purpose
is to compensate for "apparent" pH changes due to the
pH response of a glass electrode increasing with tem-
perature. At values around 7 pH, the variation with tem-
perature is zero. Thus, no compensation is required at
7 pH. The error caused by varying temperatures is
greater at high/low pH values; in other words, as you
move away from a neutral pH of 7. This compensation
is done automatically by the Model 54e pH/ORP unless
specifically disabled.
The Preamp
Because of the high resistance of the glass measuring
electrode, a preamplifier must be used to transform the
extremely low level, high impedance signal to a low-
impedance signal. The Model 54e pH/ORP includes a
switchable preamplifier that may be used if the dis-
tance to the pH sensor is less than 15 feet. When
longer runs of cable are necessary, the preamp can be
located in the pH sensor itself or in a remote junction
box. Either design allows transmission of the pH signal
long distances without suffering degradation or inter-
ference from outside voltage sources.
6.2 CONTINUOUS SENSOR DIAGNOSTICS
The Model 54e pH/ORP verifies the integrity of the
glass and reference segments of the pH sensor by con-
tinuously measuring the impedance between each
segment and the solution ground.