Reconditioning the DO Sensor
Polarographic Sensors - Model # 605203
Due to the chemical reaction taking place under the membrane, deposits will form on the gold cathode and silver
anode. The gold cathode will begin to appear dull and the silver anode will turn dark in color. This discoloration is
normal; however, it is recommended that you remove the deposits as needed. Perform the following cleaning
procedures to remove the deposits if 1.) You have troubles calibrating the sensor or the DO readings are
unstable; and 2.) Changing a membrane does not correct the problem.
Silver Anode:
After extended use, a layer of Silver Chloride (AgCl) builds up on the silver anode reducing the sensitivity of the
sensor. The anode must be cleaned to remove this layer and restore proper performance. The cleaning can be
chemical and/or mechanical:
Chemical cleaning: Remove the membrane cap and rinse the electrodes with deionized or distilled water. Soak
the sensing electrode section of the sensor in a 14% ammonium hydroxide solution for 2 to 3 minutes or in a 3%
ammonia solution overnight for 8-12 hours (most household ammonia cleaners are typically around 3%). Rinse
heavily in cool tap water followed by a thorough rinsing with distilled or deionized water. The anode should then
be thoroughly wiped with a wet paper towel to remove the residual layer from the anode. Trapping residual
ammonia under the new membrane cap can quickly tarnish the electrode and/or give false readings.
Note: Chemical cleaning should be performed as infrequently as possible (1 or 2 times per year depending on
use). First attempt a membrane change and recalibrate. If a new membrane does not resolve the problem, then
proceed with cleaning.
After performing a chemical cleaning, perform a mechanical cleaning on both the anode and cathode.
Mechanical cleaning: In order to sand the silver anode along the shaft of the sensor, remove the membrane and
hold the sensor in a vertical position. Wet 400 grit wet/dry sand paper with a small amount of clean water then
gently wrap it around the sensor anode and twist it a few times to lightly sand the anode (the goal is to sand off
any build-up without scratching or removing layers of the anode itself). Usually, 3 to 4 twists of the sanding disk
are sufficient to remove deposits. However, in extreme cases, more sanding may be required to remove all of the
deposits.
After completing the sanding procedure, repeatedly rinse the electrode with clean water and wipe with lens
cleaning tissue to remove any grit left by the sanding disk. Thoroughly rinse the entire tip of the sensor with
distilled or deionized water and install a new membrane.
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