Unfortunately the selectivity coefficient is not constant and depends on several factors including the concentration of both elements, the total ionic strength of the solution, and the temperature. Thus the selectivity coefficient cannot be used to make accurate corrections for the interfering ion in any simple manner. According to the British Standards Specification for Ion-Selective Electrodes, all manufacturers should provide details of the selectivity coefficients together with the conditions under which these were measured, including concentrations of both ions, for all ISEs which they supply. This should assist potential users in properly evaluating the significance of this limitation for their own particular application - but all this information is not always available. Probably the most common example of interference is that of potassium on ammonium and vice-versa.
Potassium is a very common element and is present in most natural substances.
The selectivity coefficient for K of the NH4 electrode is approximately 0.1. A further complication is that NH4 also has a slight effect on the K electrode which may become significant at high NH4 to K ratios. Until recently, these interferences have limited the use of the ammonium ISE to only those applications where a high NH4 to K ratio can be guaranteed. Another typical example of interference is that of the chloride ion on the nitrate electrode. In this case the selectivity coefficient is only about 0.003.