Evaluating Test Report Certificate Uncertainty in Submersible Environmental Sensors
Evaluating sensor certificates requires auditing expanded uncertainty, coverage factors, and environmental test bounds to ensure data defensibility.
Liquid Junction Bias
Electrochemical Potential
Potentiometric measurements in aqueous solutions require a stable electrical connection between the reference electrode and the sample. This interface generates an unwanted potential known as liquid junction bias, which arises from the differing diffusion rates of ions across the boundary. The effect is present in all electrochemical cells where the internal fill solution of the electrode contacts a different sample solution.
Ionic Mobility
Different ions move at different speeds through the porous junction depending on their size and charge. When highly mobile ions like hydrogen are present in the sample, they diffuse faster than the larger ions of the reference fill solution, creating a charge separation. This separation of charge produces a voltage that adds to the measured cell potential and shifts the apparent concentration reading.
The magnitude of this voltage depends on the concentration gradient and the salinity of the solutions.
Error Propagation
In low-conductivity waters, the absence of supporting electrolytes makes the junction potential highly unstable and sensitive to flow rate changes. This instability leads to fluctuating pH readings that do not represent the true state of the sample. The error can become large enough to mask the actual ionic changes of interest.
Reference Verification
Metrologists minimize this error by using potassium chloride as the reference electrolyte, as the potassium and chloride ions have nearly equal mobility. They verify the junction performance by testing the sensor in two different buffer solutions of known pH and calculating the offset. If the calculated liquid junction bias exceeds the system tolerance, the junction must be cleaned or the fill solution replaced.
This testing ensures that the electrochemical readings remain stable during field operations.