A lot of people clean their pH electrode with a strong acid, such as HCl. This can be OK, but nevertheless there are some things to consider.
If you put a combined pH electrode in an acid solution, the concentration of protons (H+) of the acid can be about 10 million times higher than the proton concentration in the reference electrolyte. This causes a huge concentration gradient and results in an important diaphragm potential. As protons are very mobile ions, they move very fast into the electrolyte trying to reach an equilibrium.
After cleaning with acid you mostly rinse the electrode with water. Then, you try to calibrate your electrode in pH buffer solutions. Inside the reference electrolyte, there is still a high concentration on protons. Now, the diffusion goes in the other direction as the protons are looking for equilibrium again. Doing so, they cause again a diaphragm potential. As pH measurement (and calibration) is in essence a potential measurement, you easily understand that you are introducing calibration errors. These errors can be as big as a few tenths of a pH unit, up to 0.5pH or more.
The only way to minimize these errors is to use the right type of electrode (with internal or external overpressure to avoid diffusion into the electrolyte) and to have patience. After acid cleaning, put the electrode in a bucket of tap water for several minutes, up to half an hour or even longer. Calibrate afterwards.06/02