Extech Instruments PH220 User Manual page 8

Electrode Maintenance and Troubleshooting
Most pH measurement difficulties can be traced to the electrode. Careful handling and proper
storage of the electrode will optimize measurement accuracy and electrode life. Electrodes should
be rinsed to remove all traces of storage solution, process medium, or previous test solution.
Electrode Storage
The electrode should be stored in a soaker bottle or wetting cap filled with pH4 buffer. Never store
an electrode in distilled or deionized water as this will cause migration of the fill solution from the
electrode.
Electrode Cleaning
The type of electrode coating that occurs over time will determine the cleaning method. Soft
coatings can be removed by vigorous stirring or by using a squirt bottle. Organic chemical coating
(hard coatings) should be chemically removed. Only in extreme cases should the bulb be cleaned
mechanically.
Electrode Troubleshooting
Symptom
Long response time or
reading drift
Dry Bulb
Static Charge
Same readings in
different buffers and
samples
Erratic LCD display
Cause
Clogged Junction
Strong Alkaline measurement
Deteriorated Gel layer
Protein coating on electrode
surface
Oil, paint, dyes, suspended
solids etc. are on sensor
Organic solvents are coating the
sensor
Dehydrated membrane
Long term storage without
wetting
Wiping electrodes
Cracked or broken bulb
Samples have low ionic strength
(low sodium); e.g. distilled, de-
ionized, boiled, or lake water
(high pressure)
Recommendation
Soak in 4.07 M Potassium Chloride (KCl) @
140 o F (60 o C) for 30 minutes.
Soak in 0.1 M HCL overnight.
Replace electrode.
Soak in 1 gm Pepsin dissolved in 100ml of 0.1
m HCL for 30 minutes or as needed.
Rinse electrode alternately with a materials
solvent and a buffer 7.00.
Organic mole fraction should be less than 50%
to assure reasonable readings. Limit time of
measurement. Keep probe in the 7.0 buffer
between readings.
Read solution for dry bulb below:
Soak electrode tip in wetting cap filled with
1ml 7.00 buffer for 24 to 48 hours
Rinse electrode in 7.0 buffer and blot. Do no
wipe electrode.
Replace electrode. Use bulb guard. Avoid
plunging electrode to bottom of container
and spinning bars. Wetting cap will protect
bulb between tests.
For each 50 ml of sample add 1 drop (50uL) of
SAT. KCl (No alteration in pH will occur using
inert KCl).
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PH220-en-US_v3.5 3/21