De-Chloridization Of The Cathode - Hach ORBISPHERE 32301 Manuel D'utilisation De Base

Section 4 Hydrogen sensor servicing
A hydrogen sensor service consists of the following operations:
1. De-chloridization of the cathode. This process removes the chloride film from the silver cathode
surface.
2. De-greasing the sensor. This is only necessary under certain, fairly unusual circumstances.
3. Re-chloridization of the cathode. Here a fresh layer of silver chloride is grown on the cathode
surface.
4. Activation of the anode. The anode surface is polished and treated with concentrated nitric acid.
5. Electrolyte and membrane replacement.

4.1 De-chloridization of the cathode

1. Remove the membrane and electrolyte from the sensor. Refer to the operator manual delivered
with the sensor for details on how to do this.
2. Fix the sensor vertically onto a clamp or stand so that its head is facing upwards. If the sensor
has a permanently attached cable, introduce the LEMO plug into the sensor receptacle (No. 6 in
Figure 1 on page 5). Sensors with detachable cables may connect directly to the sensor cable
(No. 3).
3. For the supplied regeneration cell, push the tube of the regeneration cell over the sensor head
until it reaches a stop. Connect the red banana plug into the counter electrode socket (No. 7).
4. For the DG33619 regeneration cell , screw the regeneration cell in the sensor head. Connect the
blue banana plug into the counter electrode socket (No. 7).
5. Pour standard ORBISPHERE model 29011 chloridizing solution into the regeneration cell so that
it covers the electrode .
6. Turn the selector knob (No. 4) to the dechloridize position and press the TIMER switch (No. 5).
The warning light will come on and remain on for 60 seconds while de-chloridization takes place.
7. Observe the solution in the regeneration cell. From being a clear liquid it should fill with small
hydrogen bubbles given off at the cathode. This has the effect of darkening the solution. If at the
end of the 60 seconds de-chloridizing period an abundant development of bubbles from the
cathode is not obvious then press the TIMER switch once more and continue to do so until
bubbles appear. An abundant evolution of bubbles is a sign that de-chloridization is probably
complete.
8. Look closely at the cathode. It should now be silver-white with no darker patches. If this is not the
case discard the solution and replace with new solution then continue to activate the de-
chloridizing process. Any darker patches will disappear with time.
9. With de-chloridization of the cathode complete, disconnect the counter electrode and sensor, and
empty the solution from the regeneration cell.
10. Rinse the interior of the regeneration cell and sensor with water.
11. Remove the regeneration cell from the sensor and rinse the sensor head again with water.
4.2 De-greasing the sensor (optional procedure)
Under most operating conditions the sensor is not exposed to greasy samples and with careful
handling no problems should be experienced with grease. In such circumstances no sensor de-
greasing is necessary and you should proceed to the next section.
If, however, the sensor has been used with dirty samples or exposed in some way to grease then the
de-greasing procedure described in this section should be followed. You will learn from experience
whether in your particular case de-greasing is necessary on a regular or occasional basis or not at
all.
The procedure is as follows:
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