pH meter

pH electrode

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Instant Notes in Analytical Chemistry Amazon

Instant Notes in Analytical Chemistry

by D. Kealey, P.J. Haines

pH meter

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.