Page 382 - Instrumentation Reference Book 3E

Potentiometry and specific ion measurement 365
high pH ranges is probably to condition them and
store them in borax buffer solution.

Electrical circuits for use with glass electrodes
For measurement of pH the e.m.f. in millivolts
generated by the glass electrode compared with
that of the reference electrode has to be converted
to a pH scale, that is, one showing an increase of
Cable spacer one unit for a decrease in e.m.f. of approximately
\, I1 /sea‘ 60mV. The pH scale requires the use of two
controls-the calibration control and the slope
Electrode
cap top control. The latter may not always be identified
as such on the pH meter, as it often acts in the
same way as the temperature compensation con-
trol. The slope and temperature compensation
controls adjust the number of millivolts equiva-
lent to one pH unit. The calibration control
relates the measured e.m.f. to a fixed point on
the pH scale.
A typical pH measuring system (glass electrode
and reference electrode immersed in a solution)
may have a resistance of several hundred
megohms. To obtain an accurate measurement
of the e.m.f. developed at the measuring elec-
KCI crystals trode, the electrical measuring circuit must have
AgCl granular a high input impedance and the insulation resist-
ance of the electrical leads from the electrodes to
the measuring circuit must be extremely high
(- lo5 MR-a “Megger” test is useless). The lal-
ter is best achieved by keeping the eiectrode leads
as short as possible and using the best moisture-
resistant insulating materials available (e.g., poly-
Reservoir cap
thene or silicone rubber).
Fill reservoir with The usual method of measurement is to convert
3.8~ KCI solution the developed e.m.f. into a proportional current
by means of a suitable amplifying system. The
Figure 17.1 9 Sealed silver/silver chloride electrode essential requirements of such a system have been
Courtesy Kent industrial Measurements Ltd. Analytical met completely by modern electronic circuits, and
Instruments. one system uses an amplifier with a very high
negative feedback ratio. This means that the
greater part of the input potential is balanced by

Protective
Reference
internal solution plastic skirt
reference element

t I Outer --L..” ^^^^ \ \ Glass
I
17 mm diameter Plastic E’
body
Figure 17.20 Combined reference electrode and glass electrode for pH measurement. Courtesy ABB Instrument Group