Factors which limit the accuracy of pH measurements are - 

(1) Glass membrane: Coatings on the pH-sensitive membrane glass can disrupt the pH measurement significantly or even make measurement impossible. If pollutants, solids, biofilms, poorly soluble deposits, lime, or evenoils collect there, the measuring surface is no longer exposed to the measu-rement medium. This increases the electrical resist ance at this location and incorrect measurements can result. The glass membrane should be cleaned regularly during the periodic calibration that is required anyway.

(2) Diaphragms: The diaphragm of the pH electrode, which can consist of porous ceramic, a porous Teflon ring, a small hole, or a gap, needs to fulfill"contradictory functions". On the one hand, the diaphragm provides electrolytic contact between the reference electrolyte and the measurement through outflow of electrolyte (usually potassium chloride) and on the other hand, the diaphragm must prevent overly fast leaching of the electrolyte. If parts of the diaphragm are blocked by sparingly soluble compounds, the pH measurement is low due to increased resistance and even the zero point of the pH electrode can be shifted. A poorly soluble deposit forms in the diaphragm when anions from the measurement medium encounter silver ions from the reference electrolyte and form a sparingly soluble salt. This is the case, for instance, for sulfides, bromides, and iodides. This problem can be avoided by using a high-quality electrode with a cartridge reference system as the contact element (4). In this case, the reference electrolyte (3) is silver-ion free.

(3) Reference electrolyte and contact element of the reference electrode

(4): In unfavorable situations, the measurement medium can contain electrode poisons that can reach the contact element via the diaphragm and the