pH meter

pH electrode

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Analytical Chemistry in a GMP Environment Amazon

Analytical Chemistry in a GMP Environment

by James M. Miller, Jonathan B. Crowther

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.