Chem301 Tutorial: Potentiometry Calibration

	Tutorial: Calibration Curves

Potentiometric Analysis using the Calibration Curve Method

In potentiometric analysis, the measured signal increases with the logarithm of the analyte concentration. This functional relationship creates complications when calculating confidence intervals for the analyte concentration. Two of these complications are:

don't forget to use the logarithm of the standard concentrations in the calibration curve
even with the assumption that the measurements are normally distributed, it is the logarithm of the point estimate that is normally distributed.

Example

In the potentiometric determination of Pb²⁺ in solution, the following calibration data was collected.

[Pb²⁺], ppm	E_meas, mV
15	-338.5
35	-329.8
89	-316.5
150	-312.2
230	-303.7
400	-296.4
500	-295.5
650	-292.5

Another solution, whose concentration of Pb²⁺ was not known, yielded a measured potential of -300.8 mV. Report the concentration of lead in the solution in the form of a confidence interval.

Answer: 319 ppm (best point estimate); 243 - 420 ppm (95% CI)