To calibrate, two buffer solutions, usually pH 7.02 and 4.00 or thereabouts, are taken from the refrigerator (where they should be stored in order to prevent microbial growth that affects the pH value). The solution has to be warmed up to the ambient temperature of the calibration pH solution. After initial cleaning of the combined glass electrode with distilled water, the electrode is inserted into the standard pH 7 buffer solution with agitation and the instrument is adjusted, taking about 20 s to reach the pH for the calibration buffer.
The glass electrode is removed from the first buffer and cleaned thoroughly with distilled water, and then it is inserted into the second standard pH solution, the pH 4 buffer. After 20 s of agitation and a stable reading, the pH meter is then set to this second standard buffer value.
The glass electrode is taken out, cleaned again with water, and the procedure starting with the pH 7 buffer is repeated until the values of both calibrating buffers give a stable reading.