Myron L 6Psi and 4P - Manual (Page 48)

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The problem is, on the other side of the membrane is an unknown test
solution, not potassium chloride. The outside electrode, also called
the Reference Junction, is of the same construction with a porous
plug in place of a glass barrier to
allow the junction fluid to contact
the test solution without significant
migration of liquids through the plug
material. Figure 33 shows a typical
2 component pair. Migration does
occur, and this limits the lifetime of a
pH junction from depletion of solution
inside the reference junction or from
contamination. The junction may
be damaged if dried out because
insoluble crystals may form in a
layer, obstructing contact with test
solutions. See pH and ORP, pg. 43.
4. The Myron L Integral pH Sensor (6Psi)
The sensor in the Ultrameter II
(see Figure 34) is a single
construction in an easily
replaceable package. The sensor
body holds an oversize solution
supply for long life. The reference
junction “wick” is porous to provide
a very stable, low permeable
interface, and is located under the
glass pH sensing electrode. This
construction combines all the best
features of any pH sensor known.
5. Sources of Error (6Psi)
The basics are presented in
pH and ORP, pg. 43.
a. Reference Junction
The most common sensor problem will be a clogged junction because
a sensor was allowed to dry out. The symptom is a drift in the “zero”
setting at 7 pH. This is why the Ultrameter II 6Psi does not allow more
than 1 pH unit of offset during calibration. At that point the junction is
unreliable.
b. Sensitivity Problems
Sensitivity is the receptiveness of the glass surface. A film on the surface
can diminish sensitivity and cause a long response time.
Glass surface
Figure 33
KCl solution
Electrode wire
Electrode
wire
H + ions
Junction
Plug
KCl solution
Figure 34
Junction plug
Platinum button
H + ions
Electrode wires
Glass
Glass
Surface