Ionode IJ pH V5.1 - Manual (Page 10)

Ionode IJ Instruction Manual 10
The reference half-cell completes the circuit.
The primary IJ reference is a silver/silver
chloride
element in a gelled saturated
potassium chloride electrolyte. This makes
solution contact with the secondary sleeve
electrolyte through a low porosity wick. Usually,
the
sleeve electrolyte is concentrated
potassium chloride. However, as the IJ
reference has a double junction, it allows the
use of other suitable sleeve electrolytes.
Potassium chloride is the most commonly used
reference electrolyte because of the similar size
and mobility of the potassium and chloride ions.
This minimizes charge separation when the
sleeve electrolyte diffuses into the sample
Solution contact between the sleeve electrolyte
and the sample is through the porous junction
between the ground stem and the sleeve. This
constriction allows uninhibited movement of
ions between the sample and reference half cell
to assure a repeatable reference junction
potential. At the same time it does not grossly
contaminate the sample with reference ions.
In common electrode designs, a porous
restriction (typically a porous ceramic or fibre
wick) is used to slow the flow (see Figure 10a).
K
+
Cl
-
(a)
Cl
-
Cl
-
Cl
-
Cl
-
K
+
K
+
K
+
K
+
(b)
Figure 10