The pH 7 or zero point of an electrode is the pH value at which the total output electrode voltage is equal to 0 mV. In theory,
the zero point of a pH electrode happens at the pH value of 7.00.
However, in practice, there is almost always a zero point offset. For
this reason, a zero point calibration should always be performed prior
to starting a pH measurement.
To perform a zero point adjustment, you need, for obvious reasons, a buffer solution of the pH-value of 7.00.
Most pH meters
can be switched between pH and mV reading modes. This is a good feature,
because during a zero point adjustment the mV reading will give you
valuable information about the used pH electrode.
As a thumb of rules, if the zero point offset:
< ±15 mV; Go ahead and calibrate.
> ±15 mV; First clean the electrode and then calibrate.
> ±35 mV; You need to change the electrode, it is no longer usable.
A large zero point offset will introduce a large error in your pH measurement, if you calibrate and measure at different temperatures.
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