Application of the Standard Additions Procedure to Potentiometry

The application of standard additions to potentiometric measurements is complicated by the fact that the measured signal is proportional to the logarithm of the analyte concentrations. Thus, a conventional standard additions plot would be nonlinear. It is possible to linearize the potentiometric standard additions plot by mathematically transforming the signal ("correcting" it). Once this is done, then the analyte concentration can be determined by using the usual standard addition equations.

Example

The concentration of cadmium in three samples of seawater was determined potentiometrically using the method of standard additions. The volume of each sample was initially 50.00 mL, and the concentration of the standard added to the samples was 10.00 mg/mL. The following data were collected.

Calculate a confidence interval for the concentration of cadmium in seawater. Assume that the slope of the calibration curve of Emeas vs log[Cd] is 29.0 mV/decade.

Answer: 0.202 +/- 0.065 mg/mL (95% CI)

The potentiometric standards addition plot do not exhibit homogeneous variance, so that the residuals cannot be used to determine the standard deviation of the point estimate of analyte concentration. Instead, it is required to perform the standard additions procedure on multiple samples in order to obtain a confidence interval, as shown in the previous example.

In order to do standard additions for potentiometry, the slope of the calibration curve (E vs logC) is required. For best accuracy, this value should be measured, but the theoretical (Nernstian) value is sometimes used. The accuracy of the standard additions results will depend on the value used for this slope.