pH probe recommendations
For ultra-pure water, as for dirty water, pH measurement requires an electrode with a liquid junction (annular ring or glass sleeve) that diffuses a lot.
For dirty water, it will avoid electrode contamination. The impurities will not penetrate the electrode because of the higher outflow at the liquid junction.
For pure waters, it is to have a faster response.The high diffusion liquid junctions make it possible to establish a concentration gradient of KCl at the interface between the junction and the pure water. This makes the transition of ionic strength much less brutal, and thus reduce the junction potential to a minimal value, while stabilizing the measurements.