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 aging 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
The liquid junction (diaphragm) of the reference half-cell provides the conductive bridge between the reference element and the process media. This porous membrane is susceptible to contamination from chemicals in the process media such as sulfides and proteins.
Alkaline Error
Alkaline error deals with pH measurement error at high pH values (> 12pH). This article details the cause and effect of this error, as well as its impacts on the actual pH measurement.
Acid Error in pH Measurement
Strong acids (< 1pH) can induce measurement errors in glass membrane pH sensors. This article discusses why this error occurs and provides a graphic understanding of its effects.
Temperature Influence and Temperature Compensation
Temperature and pH directly influence each other three ways: the Nernst equation, the isopotential point, and the chemical composition of the process liquid. The article explores this relationship considering each type of dependency.
The Aging of a Measurement Electrode
Aging of the measurement electrode begins immediately after manufacture and occurs throughout the lifetime of the glass membrane pH sensor. This article reviews process related factors that decrease the longevity of the glass membrane.
The Aging of the Reference Electrode
In theory the reference electrode does not age on its own but is greatly affected by changes in the KCl electrolyte by liquid junction contamination and improper refilling of the electrolyte. This article discusses aging with respect to gel and polymer electrolytes.
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