Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 94)

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Appendix: ISE Theory
PH6500, PC6500 and PCD6500 meters
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Analyte Addition and Analyte Subtraction
In the Analyte methods, the initial millivolt reading is taken with the electrode immersed in the
standard; then an aliquot of sample is added, and the final millivolt reading is taken. Sample
concentration is calculated from the change in the millivolt readings.
As with known addition, with analyte addition the sample and the standard solution contain the
same ion being measured. With analyte subtraction, the ion being measured is in the standard
solution, and not in the sample solution. For example, a fluoride standard and a fluoride ISE might
be used to measure to aluminum concentration of a sample. When the sample containing
aluminum is added to the standard it reduces the measured fluoride content of the standard. The
amount of reduction is proportional to the aluminum in the sample.
The analyte methods offer the same advantages as do the addition methods in that they
effectively deal with complex samples. A distinct advantage of the analyte subtraction method is
that it can be used in cases where an ISE does not exist for a given species, such as aluminum in
the example cited.