pH meter

pH electrode

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Fundamentals of Electro-Analytical Chemistry Amazon

Fundamentals of Electro-Analytical Chemistry

by Paul M.S. Monk

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.