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MODEL 3081 pH/ORP SECTION 13.0
pH MEASUREMENTS
designed, the liquid junction potential is usually small and rel-
atively constant. All three potentials depend on temperature.
As discussed in Sections 13.5 and 13.6, the factor relating
the cell voltage to pH is also a function of temperature.
The construction of each electrode and the electrical poten-
tials associated with it are discussed in Sections 13.2, 13.3,
and 13.4.
13.2 MEASURING ELECTRODE
Figure 13-2 shows the internals of the measuring electrode.
The heart of the electrode is a thin piece of pH-sensitive
glass blown onto the end of a length of glass tubing. The
pH-sensitive glass, usually called a glass membrane, gives
the electrode its common name: glass electrode. Sealed
inside the electrode is a solution of potassium chloride
buffered at pH 7. A piece of silver wire plated with silver
chloride contacts the solution.
The silver wire-silver chloride combination in contact with
the filling solution constitutes an internal reference elec-
trode. Its potential depends solely on the chloride concen-
tration in the filling solution. Because the chloride concen-
tration is fixed, the electrode potential is constant.
As Figure 13-2 shows, the outside surface of the glass
membrane contacts the liquid being measured, and the
inside surface contacts the filling solution. Through a com-
plex mechanism, an electrical potential directly proportion-
al to pH develops at each glass-liquid interface. Because
the pH of the filling solution is fixed, the potential at the
inside surface is constant. The potential at the outside sur-
face, however, depends on the pH of the test solution.
The overall potential of the measuring electrode equals the
potential of the internal reference electrode plus the poten-
tials at the glass membrane surfaces. Because the poten-
tials inside the electrode are constant, the overall electrode
potential depends solely on the pH of the test solution. The
potential of the measuring electrode also depends on tem-
perature. If the pH of the sample remains constant but the
temperature changes, the electrode potential will change.
Compensating for changes in glass electrode potential with
temperature is an important part of the pH measurement.
Figure 13-3 shows a cross-section through the pH glass.
pH sensitive glasses absorb water. Although the water
does not penetrate more than about 50 nanometers (5 x
10
-8
m) into the glass, the hydrated layer must be present
for the glass to respond to pH changes. The layer of glass
between the two hydrated layers remains dry. The dry layer
makes the glass a poor conductor of electricity and causes
the high internal resistance (several hundred megohms)
typical of glass electrodes.
13.3 REFERENCE ELECTRODE
As Figure 13-4 shows, the reference electrode is a piece of
silver wire plated with silver chloride in contact with a con-
centrated solution of potassium chloride held in a glass or
plastic tube. In many reference electrodes the solution is an
aqueous gel, not a liquid. Like the electrode inside the
glass electrode, the potential of the external reference is
controlled by the concentration of chloride in the filling solu-
tion. Because the chloride level is constant, the potential of
the reference electrode is fixed. The potential does change
if the temperature changes.
FIGURE 13-2. Measuring Electrode.
The essential element of the glass electrode is a pH-sen-
sitive glass membrane. An electrical potential develops at
glass-liquid interfaces. The potential at the outside surface
depends on the pH of the test solution. The potential at
the inside surface is fixed by the constant pH of the filling
solution. Overall, the measuring electrode potential
depends solely on the pH of the test solution.
FIGURE 13-3. Cross-Section through the pH Glass.
For the glass electrode to work, the glass must be hydrat-
ed. An ion exchange mechanism involving alkalai metals
and hydrogen ions in the hydrated layer is responsible for
the pH response of the glass.