Page 380 - Instrumentation Reference Book 3E

Potentiometry and specific ion measurement 363

Center
conductor
(white)

; Toofened membranf
Shield
(black insulation 3 Cap araldited Io bung
over braided shield) 4 Aging Ci wire
5 Tube
6 Topbung
7 2 pH filling
8 7 pH filling
-

Shielded
electrode cable
Glass-to-metal seal
('"total glass" construction)
-
Min. amount of air
Silver Internal solution to be trapped
chloride pH-sensitive membrane -4
bead
Figure 17.1 6 pH measuring electrode. Courtesy
the Foxboro Company.
Figure 17.17 industrial electrode. Courtesy Kent
industrial Measurements Ltd. Analytical Instruments
suspended solids, viscous liquids, or reaction
products of the process stream. Where this is a
danger. the faster flow type of plug must be used. mended for low temperature applications unless
Many rodtine measurements can, however, be provision is made to prevent freezing or precipi-
made with the non-flowing electrode, thus avoid- tation in the electrode and any associated hard-
ing the necessity of refilling, or arranging a pres- ware.
surized continuous supply. Flowing types of When materials such as sulfides: alkali phos-
junctions are usually required where an accuracy phates or carbonates. which will react with silver,
of h0.02 pH units (hl or 2 mV) is required, where are present in the process stream, either non-
frequent or large temperature or composition flowing electrodes, or electrodes containing
changes occur, or where the process fluid is such potassium chloride at lmol per liter should be
that it is prone to foul the junction. used. The diffusion rate of silver can be neglected
The temperature of operation will influence the in the non-flowing type, and the solubility of
choice of concentration of the filling solutions. silver chloride in potassium chloride at a concen-
Potassium chloride solution having a concentra- tration of 1 mol per liter is only 1 or 2 percent of
tion of 4mol per liter saturates and starts to pre- that in a solution at 4mol per liter.
cipitate solids at about 19"C, and will freeze High temperatures with wide fluctuations are
at -4 "G, while if the concentration is reduced to best handled by potassium chloride solution at
1 mol per liter the solution will freeze at -2 "C 1 mol per liter.
without becoming saturated. Thus, no precipita-
tion will take place in the solution of lower con-
centration. Although not damaging, precipitated 17.7.2 Measurement of pH
potassium chloride and associated silver chloride
will tend to clog reference junctions and tubes, Glass electrode Almost all pH measurements
decreasing electrolyte flow rate and increasing the are best made with a glass electrode (the earliest
risk of spurious potentials. For these reasons, of the ion-selective electrodes), the e.m.f. being
flowing reference electrodes are not recom- measured relative to a reference electrode. The