Selectivity of ISE
When answering the question 'what is the selectivity of an ISE?' it is useful to use the selectivity coefficient.
The higher the selectivity coefficient value of, k, the more selective the ISE is towards X relative to A.
If we use the example of the ZP sodium ISE, then we have the following selectivity coefficients:
These selectivities can be interpreted by understanding the ratios of these ions within the application, e.g. if the Na/K ratio is > 20 then there is no significant interference of potassium ions with the ZP sodium sensor.
Similarly if the Na/NH4+ ratio >10, then again there is no significant interference from ammonium and finally if the Na/Mg or Ca ratio is >1 then there is no significant interference of these ions to the ZP sodium sensor.
Whether the ZP sodium ISE is specific enough relative to ions such as potassium depends on the application of interest. If we consider sea water or blood then the sodium to potassium ratio is intrinsically high. In the case of blood the sodium concentration is approximate 135 mM whilst the potassium concentration is approximately 4 mM , therefore the Na/K ratio = 34, so the ZP sodium ISE sensors is not effected by potassium within this application.
The ZP potassium ISE uses a very selective ionophore, and so the selectivity coefficient of the ZP potassium sensors means an excellent selectivity towards potassium in the prescence of other ions, as reflected in the selectivity coefficients:
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