Discussion
Why must a pH meter have an extremely high input impedance?
Answer: A
A thin glass membrane is a resistance of 50 to 500 megohms, so any appreciable current drawn would drop most of the potential inside the electrode itself. Input impedances of ten to a thousand gigohms are therefore normal, which is why the cable and connector must be kept scrupulously clean and dry: a film of moisture across the connector is a resistance in parallel with the input and will pull the reading towards zero. Corrosion, reference current and temperature stability are real concerns but none of them sets the impedance requirement.
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