Sources of pH Measurement Error
pH Sensors are electrochemical devices with limited lifespans. Changing process conditions directly impact each component within the combination electrode thus can be sources of error leading to inaccuracy and eventual death of the sensor.
The articles below review the impacts of temperature, coatings, liquid chemistry, and simple ageing due to sensor usage.
The Diffusion Potential of a pH Sensor
The diffusion potential of a pH sensor deals with the mobility of ions between the reference electrolyte and those occurring within the process liquid. As these ions move across the liquid junction a voltage potential develops that influences the pH sensor mV output.
Liquid Junction Contamination
Chemical reactions at the liquid junction between the KCl reference electrolyte and the measured solution must be avoided at all costs. This reaction can lead to junction contamination, increased electrical resistance, and pH measurement errors.
Alkaline Error
Acid Error in pH Measurement
At low pH values (< pH 2) the potential difference between measurement and reference electrode will not conform exactly to the Nernst equation. Through experiments it has been proven that the gel layer of the pH sensitive glass membrane will absorb acid molecules at very low pH values.
Temperature Influence and Temperature Compensation
The Aging of a Measurement Electrode
Every glass measurement electrode undergoes an ageing process, even when it is not in use. The ageing process is continuous and starts immediately after manufacture.
The Aging of the Reference Electrode
Reference electrodes do not age in principle, but their life may be considerably shortened by incorrect handling, during usage and storage.