Your new pH or ORP sensor will ship with a small cap over the tip of the sensor. This cap will contain an electrolyte solution designed to keep the pH sensitive glass membrane and porous diaphragm hydrated. This cap is referred to as a “watering cap” and should be saved for future usage.
When inspecting a new pH sensor the watering cap should have visible liquid present. There may be salt crystals around the cap seal. These crystals are non-toxic (Potassium Chloride) and are completely normal. The cap should be hand-tightened and any excess salt can rinsed off with tap water.
Example of a pH watering cap which is shipped with all pH sensors
The glass membrane on the pH sensor has a thin gel layer that must remain hydrated in order to keep the pH sensor functional. If the gel layer is allowed to dry out then the sensor response to changing pH will become slow or potentially non-existent. Typically 24 to 48 hours will create a notable effect in response; however any prolonged loss of hydration should be avoided. If drying out has occurred then test the sensor response in pH buffer solutions. A sensor should respond to a 3pH change (say from 4 to 7pH) in < 30 seconds. Alternately, if the sensor cannot pass calibration then it should be questioned.
The response of the pH sensor can sometimes be improved through alternating exposure to strong base and acids. The procedure is as follows:
Immerse sensor for 10 min in 0.1 – 1M NaOH, then for 10 min in 0.1 – 1M HCl. After regeneration, place the sensor in storage solution for a further 15 min.
When the sensor is not in use, then keep the sensor tip hydrated in Hamilton storage solution (Ref 238931) or 3M KCl. Storage in acidic or basic solutions should be avoided.