|If a polarographic DO probe stops providing reliable readings and the probe’s response becomes sluggish some of the possible reasons are:
Electrolyte Solution and Membrane Needs to be Replaced
Refilling and membrane cap replacement are always the first step to restore probe’s performance. After extended use, electrolyte solution loses its ability to carry oxygen, and the membrane can become dirty, coated, or damaged.
- Remove the membrane cap, rinse electrode tip with DI water, and dry with a Kimwipe
- Fill the new membrane cap about ¾ full with new electrolyte solution
- Hold the electrode at a slight angle, and carefully insert the electrode into the cap
- Gently tap the cap to dislodge air bubbles
- Screw the membrane cap onto the probe until fully tightened. The electrolyte will overflow. Clean off the excess electrolyte before use
- Polarize the probe and check performance
Bubbles Behind the Membrane Cap
Check for air bubbles behind the membrane. If there are the bubbles and they cannot be removed by gently tapping, unscrew the membrane cap and refill it with fresh electrolyte as described above
Clean the Cathode and Anode
Remove the membrane cap from the probe, rinse electrode tip with DI water, and dry with a Kimwipe. Examine the anode and cathode. If cathode and anode look tarnished, use the following steps to clean them:
- A thin layer of silver chloride can build up on the anode after extended use (e.g., a year or more). To clean, soak the electrode in a 3% solution of ammonium hydroxide overnight. For quicker cleaning, remove the membrane and soak in a 14% solution of ammonium hydroxide for no more than 2 to 3 minutes. Longer soaking time in 14% ammonium hydroxide will damage the electrode. After either treatment, the probe should be flushed thoroughly with distilled water, and then made ready for service.
- To clean the cathode, hold the probe with the cathode facing upwards. Place a drop or two of distilled water on the cathode. With the frosted side of a polishing strip (supplied with the DO probe), gently polish the cathode surface for about 10 seconds using a circular pattern. Note: aggressive polishing will damage the electrode. Rinse electrode tip with DI water and then make ready for service.