pH meter

pH electrode

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Ewing's Analytical Instrumentation Handbook Amazon

Ewing's Analytical Instrumentation Handbook

by Jack Cazes

cleaning pH electrode

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

Cleaning of the clogged junction:

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.