How does a pH electrode works?
Most pH electrodes consists of 2 juctions, glass bulb and a reference electrode(built-in) containing potassium chloride(KCL). The glass bulb is sensitive to hydrogen ions and will generate a small voltage across the 2 junctions. The pH meter measures this small voltage and translate it to pH values.
This picture shows the sensitive glass bulb and the protective cover.
The glass bulb should be kept moist with pH7 buffer solution or potassium chloride. In the event that the bulb is dry, rinse the bulb in distilled water and allow the bulb to saturate in pH7 buffer solution for 15mins before calibration.