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.