HOW TO CLEAN pH ELECTRODES | HOW TO REJUVENATE pH ELECTRODES

HOW TO CLEAN pH ELECTRODES

Cleaning of the electrode (note that in case of gel electrodes replacing of the reference solution is usually impossible):

General
Soak in 0.1M HCl for half an hour.
Drain and refill the reference solution.
Soak the electrode in filling solution for one hour.

Inorganic:
Soak in 0.1M tetrasodium EDTA solution for 15 minutes.
Drain and refill the reference solution.
Soak the electrode in filling solution for one hour.

Protein:
Soak in 1% pepsin / 0.1M HCl for 15 minutes.
Drain and refill the reference solution.
Soak the electrode in filling solution for one hour.

Grease and Oil:
Rinse with detergent or ethanol solution.
Drain and refill the reference solution.
Soak the electrode in filling solution for one hour. Electrode response may be enhanced by substituting a mixture of 1:1 pH 4 buffer and filling solution for the soaking solution.

Cleaning of the clogged junction:
Pollution by sulfides:
Use a solution of 8% thiocarbamide in 1 mol/L HCl.
Keep the electrode in the above solution till junction's color turns pale.
Pollution by silver chloride:
Use concentrated ammonia solution.
Keep the electrode in the above solution for about 12 hours.
Rinse and put into pH 4 buffer for at least 1 hour.
Other contamination have to be removed by cleaning with distilled water, alcohol or mixtures of acids. If nothing else helps you may consider use of ultrasonic cleaner as last resort.



HOW TO REJUVENATE pH ELECTRODES

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 .