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There are many ion selective electrodes which might have different causes for a similar issue. It is advised to read first thoroughly the manual of the electrode to be able to solve the problem. In the manuals section of this support wiki will you find extended manuals that are very useful.
Adding ionic strength adjuster (ISA) to samples and standards ensure that they have a similar ionic strength. This is important because the ionic strength of a solution affects the relationship between activity and concentration (i.e. the measured value). By bringing the ionic strength of all samples and standards to a similar level, the effect of variable ionic strengths on the activity (and therefore the concentration) is minimized, thus reducing measurement error. Many electrode slope issues and measurement instabilities are solved when using ISA.
ISE’s are not perfect. They do suffer from interferences of other ions to a greater or lesser degree.
The best advice is to consult the ISE table. If there is an ion present in the samples that is listed as an interferent then you will need to think about a few points:
There is no temperature compensation foreseen for the ion selective measurements although these are sensitive to temperature changes. A difference of 1°C can cause an error of 2%. It is advised to maintain all solutions, standards and samples, at the same temperature during calibrations and measurements. Repeat the calibrations when the temperature has changed to ensure minimal measurement errors.
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