What does the Selectivity Coefficient mean in the Interferences section in ISE User Manuals?

Document ID

Document ID TE2688

Published Date

Published Date 27/05/2017
Question
What does the Selectivity Coefficient mean in the Interferences section in ISE User Manuals?
Summary
Meaning of selectivity coefficient in Interferences section of ISE User Manuals
Answer
The selectivity coefficients is the approximate apparent increase caused by the interfering ion. A selectivity coefficient of 0.1, for example, implies that the electrode is ten times more responsive to the primary ion than to the interfering ion. If, for example, the solution contains equal concentrations of both ions, the interfering ion will cause the apparent measured concentration of the primary ion to be about 10% too high. At a selectivity coefficient (K) value of 1, there is equal response to both ions.

A practical example is if you are using the Fluoride ISEF121, there is a potential interference with hydroxyl (OH-) below pH of 8. The selectivity coefficient is 0.1. Therefore, for every one unit of OH- in solution, there is an apparent increase in fluoride by 0.1 units. If fluoride in the solution is actually at 1ppm and OH- is also at 1ppm, then the fluoride reading will appear to be 1.1 mg/L. We want to avoid these interferences as much as possible through the use of ISA and other sample pre-treatment steps to get an accurate measurement.