Why do online and laboratory pH values differ?

Differences between online and laboratory pH values are common and do not necessarily indicate a sensor problem. Sampling, process conditions, and the measurement setup can all influence the results.

• Sample handling: Mixing, open containers, and temperature changes can affect pH.
• Sample changes during transport: CO₂ absorption, degassing, oxidation, or contamination can alter the pH before lab analysis. 
• Mixing effects: Online sensors measure continuously in the actual process, while lab samples may not fully represent the process conditions. 
• Sensor condition: Fouling, aging, or junction blockage can affect either the online or laboratory sensor.
• Different sensor systems: Inline and offline electrodes may have different glass, reference electrolytes, diaphragms, junction potentials, and temperature behavior.