USF Wallace & Tiernan DEPOLOX 4, Manual

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9.3 MAINTENANCE OF pH PROBE
Depending on the degree of contamination of the sample water, the pH probe has to
be cleaned in regular intervals. The frequency of cleaning has to be determined
empirically, but it is recommended to clean or calibrate it every 4 weeks. To prevent
incorrect measurement, both the glass diaphragm and the ceramic membrane have to
be cleaned.
Clean the glass diaphragm mechanically with a moistened, clean and soft napkin
or cotton swab.
Do not clean the probes with a dry napkin or cotton swab as this method tends to
produce a uniform cover of dirt on the probe surface.
Remove oil or grease with acetone, denaturated alcohol or another type of sol-
vent.
After cleaning it, rinse the probe thoroughly with water.
To remove calceous deposits from the glass diaphragm and the membrane, submerge
the probe in hydrochloric acid of 10 % strength. Rinse thoroughly with distilled water.
Probe ageing is frequently the cause of a sluggish pH-value indication or a declining
slope. pH-probes have a service life of 1 to 3 years provided that they are serviced
regularly.
9.4 pH MEASUREMENT USING THE PHENOL RED METHOD
Checks of the pH-value by means of comparators or photometers using the phenol red
method may show deviations of 0.5 pH units between the value displayed on the
DEPOLOX 4 amplifier and the value obtained from the sample water discharge from
the DEPOLOX 4. The cause for such a deviation is the degassing of
carbonic acid in
the sample water drain hose.
For comparative measurements the sample water should be taken through the
drain valve. Do not forget to shut the drain valve after drawing with the sample.
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