Myron L 6Pfc and 4P - Manual (Page 47)

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still produce an offset voltage, but using the same materials to connect
to the solution on the other side of the membrane causes the 2 equal
offsets to cancel.
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
Glass surface
Figure 33
KCl solution
Electrode wire
Electrode
wire
H + ions
Junction
Plug
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. 42.
4. The Myron L Integral
KCl solution
Figure 34
Junction plug
Platinum button
H + ions
Electrode wires
Glass
Glass
Surface
pH Sensor (6Pfc)
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 (6Pfc)
The basics are presented in pH and ORP, pg. 42.
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 6Pfc 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.