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By Mike Ross
General
Performance
The purpose
of this article is to provide a better understanding of how to evaluate
the performance of both new and used pH electrodes. Also discussed
are some of the many factors which can affect pH electrode performance.
Theoretically,
when placed in 7.00 buffer at 25 (C a pH electrode produces zero
millivolts which the pH meter reads as 7.00 pH. The difference between
these perfect readings and the electrode's actual reading is called
offset error. A perfect pH electrode, at 25 (C produces 59.12mV
per pH unit. The difference between this perfect reading and the
electrode's actual reading is called the span error. These theoretical
values are not always achieved, even with brand new electrodes.
New pH electrode performance specifications should meet the following
criteria:
Offset: 7.00 +/- 0.2 pH
(+/- 12mV) SPAN: Better than 95% of theory; i.e. between 56.2 and
59.2 mV
Normal
Aging
As electrodes
are used they will experience some shifts in these new electrode
specifications. OFFSETS will change and SPAN error will increase;
i.e., the span will become shorter. By using the calibration controls
these errors can be corrected. If an electrode is able to be calibrated
and it is stable and responsive, it is still a functional electrode
and may be used in service even though it no longer meets "new"
electrode specifications.
As described
later in this article, an electrode's response time becomes longer
as it ages. Even though the electrode can be calibrated, sluggish
response can limit its life. Also, certain application conditions,
elevated temperatures, for example, will cause electrodes to have
shorter service lives.
Speed
of Response
An electrode's
speed of response is affected by several things; mainly, by the
impedance (resistance) of the pH glass measuring surface. The type
of pH responsive glass used and the size, shape and thickness of
its surface determines impedance characteristics. When selecting
pH electrodes there are tradeoffs to consider. Here is a comparison
of the three most common shapes:
SPHERICAL BULB
HEMI-SPHERICAL BULB
FLAT pH
GLASS
A spherical shaped bulb
will provide 95% response in less than one second. It has low impedance
and fast response but is relatively fragile. pH electrodes with
spherical shaped surfaces are designed so that the bulb is recessed
inside the electrode body. Such designs protect the glass bulb against
breakage.
A hemispherical
shaped bulb is a stringer shape mechanically and, as a result, it
has a higher impedance and slightly slower response. These shapes
are often used in a fully exposed manner.
A flat measuring
surface is the most durable of all the shapes. It makes good sample
contact, is easily cleaned, is very strong mechanically but has
the highest impedance and the slowest speed of response-95% in less
than 5 seconds.
Coatings
can mimic a sluggish speed response problem, therefore a used pH
glass measuring surface should be cleaned before assuming that the
electrode is no longer functional. Normally, the electrode may be
cleaned with whatever chemical the coating material is soluble in
provided the chemical will not attack the electrode's materials
of construction. The glass surface should never be abraded;when
wiping the surface always use clean, non abrasive materials and
cloths.
Effects
of Temperature
The impedance
issue previously discussed is also a factor when measuring samples
at temperatures other than 25(C. For every 10(C decrease in temperature
the glass impedance will increase about 2.5 times. Therefore a spherical
electrode (which has lower impedance) will offer better speed of
response in lower temperatures than a flat electrode will.
Use of electrodes
in elevated temperatures will cause pH electrodes to experience
shorted service life than if used at ambient temperature. High temperatures
accelerate both the natural aging of pH glass and chemical attack
of the glass. These factors cause impedance to increase and the
surface to loose its ability to respond to hydrogen ion activity
(which is what a pH electrode actually measures!).
High
and Low pH Measurements
Very strong
acidic or caustic solutions will accelerate aging of a pH electrode
leading to shorter service life. Some of these solutions will actually
etch the pH glass surface with a resulting loss of response to hydrogen
ions.
Low Ionic
Strength Measurements
pH measurement
in low ionic or low conductivity solutions may create several problems
for standard pH electrodes. Typical difficulties include:
Slow sluggish or drifting
readings
Unrepeatable readings.
Premature electrode demise.
Special
design electrodes have been developed and are often used in low
ionic strength applications. They feature:
Reference
junctions- the porous material that contacts the sample- are made
of porous polyethylene so that there is a very large surface area
where the junction contacts the sample.
The reference
junction is peripheral; that is, it surrounds the glass stem onto
which the pH bulb is blown. This design minimizes streaming current
effects which can generate spurious reference junction potentials.
The built-in
reference electrode is of a double junction design. The inner chamber
contains the usual high (3.5M or higher) salt concentration solutions
so that stable outputs are generated. The outer chamber, which contacts
the sample through the porous reference junction, is filled with
0.1M KCl. This lower ionic strength material more closely matches
that of the sample and further reduces spurious potentials.
Single
vs. Double Junction References
For many
applications, a single junction reference electrode is satisfactory.
However, if samples contain proteins, sulfides, heavy metals or
any other material which interacts with silver ions, unwanted side
reactions may occur. These reactions can lead to erroneous reference
signals or to precipitation at the reference junction leading to
a short service life.
A double
junction reference design affords a barrier of protection to combat
the above interactions. When in doubt about using single or double
junction designs, the safest approach is to use the double junction;
they can be used anywhere a single junction design can be used.
Conversely, single junction designs should not be used where double
junction designs are needed.
Storage/Shelf
Life Since pH electrodes have limited
lives, it is important to keep one or more spare electrodes on hand.
An important aspect of the performance of any spare pH electrode
is will it work when go to use it. Electrodes supplied in soaker
storage bottles have longer shelf lives than those supplied dry
or with small vinyl caps. The special solution in soaker bottles
allows the electrode to be instantly ready for use as well as ensuring
a long storage life without reference junction or pH glass dehydration.
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