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While conventional pH
electrodes are fine for many routine pH measurements, they experience
problem when making pH measurements over wide temperature ranges.
Long times are required for thermal equilibration and during these
times pH readings appear to drift. Uncertainty about how long
to wait for stable readings leads to inaccurate results. These
problems are overcome by newer electrode designs such as the
pHASE
pH electrode described in this paper.
Conventional pH electrodes
may require3, 5 or even as long as 10 minutes to provide stable
readings when, for example, electrodes that have been at ambient
temperature are placed in a 60(C sample. The causes of this long
response time can be found in the design of both the pH and reference
half-cells.
Conventional pH electrodes
must have an unchanged potential regardless of the concentration
changes of the sample to which it is exposed. This constant potential
is achieved by placing the Ag/AgCl reference element in a fixed
chloride ion solution or gel. Positioning the reference element
this way protects it from the samples being measured. However,
when exposed to temperature changes, heat reaches the reference
element after being transferred through the electrode body and
its solution or gel, necessarily a slow process.
On the other hand, the
pH half-cell has a much faster response to thermal changes. Heat
only needs to transfer through the very thin glass pH membrane
rather than through the thick electrode body. So after a short
time the pH half-cell's potential represents the value at the
new temperature while the reference half-cell potential is representative
of some intermediate value. The result of this asymmetrical temperature
is erroneous, unstable readings.
Another source of error due to temperature changes are the
buffers that are used inside conventional design pH
half-cells. The pH of buffers changes versus temperature. So
even when thermal equilibrium is reached there is still an
error that is not overcome.
Many people are under
the false impression that temperature compensators will correct
for thermal effects. This is not true. Temperature compensators
only correct the "Nernstian" temperature effects.
Applications in Quality
Control, Research and Development, and other laboratory, plant
and field applications which require pH measurement in samples
of varying temperatures can benefit from the use of rapid temperature
equilibration electrodes like pHASE.
pHASE pH electrodes
are designed to overcome the effects of temperature changes that
occur with conventional electrodes (other than the "Nernstian"
effect; i.e.. temperature compensation is still needed). The pH
value of the special solution inside the pHASE half-cell is virtually
unchanged versus temperature so pHASE electrode provide more accurate
readings. Also, the locations of the pH and reference Ag/AgCl
elements have been arranged so that they respond equally to temperature
changes. It indicates how rapidly and accurately a pHASE style
electrode performs over wide changes of temperature.
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Among the many applications
that can benefit from using pHASE pH electrodes is verification
of process pH system values. One way of verifying the process
system's accuracy is to withdraw a sample from the system and
check it with a portable laboratory pH meter. Because these systems
are often at elevated temperatures, the errors notes above occur
when conventional pH electrodes are used.
In fact, by the time
a conventional electrode reaches thermal equilibration the sample
may have cooled. Since pH electrodes measure the hydrogen ion
activity, a change in temperature causes a change in apparent
pH concentration (the meter reading). This temperature effect
may cause operators to incorrectly think that the process systems
reading is in error. Use of a pHASE style pH electrode will allow
rapid thermal equilibration and assist users in eliminating thermal
effect errors.
There are many other pH measurement applications
that can benefit from the use if electrodes that have enhanced
speed, accuracy and stability features. Any pH measurement system,
regardless of cost, is only as good as the pH electrode providing
the measurement information. The improved data resulting from
the use of pHASE type electrodes can reduce maintenance, conserve
on chemical additions and improve product quality.
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