Note: following procedures are a last resort. They may work, they may won't. You may try them before throwing electrode away.
First of all - clean the electrode as described in electrode cleaning section, then:
- Soak the electrode for 4-8 hours in 1M HCl solution.
- Rinse it and move to pH 7 buffer for an hour.
- Give it a try.
If the electrode is still not working:
- Fill the electrode with filling solution.
- Move to the fume hood!
- Place the electrode in the 10% nitric acid solution on a hotplate. Heat to boiling, and keep it in the solution for 10 minutes.
- Place 50 mL of filling solution in a second clean beaker. Heat, although boiling is not necessary.
- While the electrode is still hot, transfer it to the beaker of heated filling solution. Set aside to cool.
When the electrode has cooled, test the electrode as described in the testing electrode parameters section. This rejuvenating procedure is particularly effective with gel filled combination electrodes. Do not be concerned if a small amount of the gel protrudes through the reference frit during the boiling in nitric acid step. This is both acceptable and useful.
If this procedure does not result in a pH electrode that responds quickly and has a slope of 55 - 61 mV/pH unit, the electrode is unrecoverable and should be thrown away. Remember, the procedure was proposed for the electrode that was to be thrown away anyway.
Some manufacturers suggest the electrode may be reactivated by treating with a diluted solution of hydrofluoric acid followed by subsequent conditioning in electrolyte. Before considering the procedure, take into account that hydrofluoric acid is extremally dangerous! Safer (but still dangerous) approach can be to use some slightly acidic solution containing fluorides, like 20 wt% ammonium bifluoride, NH4HF2 - put glass bulb part of the electrode in the solution for a minute followed by 15 seconds bath in 6 M hydrochloric acid. Rinse the electrode well and soak for 24 hours in a pH buffer with pH < 7.