Establishment of wide-range linear response curves in bulk optode sensors for cations based on ion exchange

Abstract

A linear curve is obtained when a decimal logistic transformation is applied to the sigmoidal calibration curve of ion-selective bulk optodes for the determination of cations based on ionophore–chromoionophore chemistry. In this way, problems derived from the use of a sigmoidal shape calibration functions are solved and the linear dynamic range is increased. The method is easier to use and eliminates the need for prior treatment of the sample. The influence of different parameters on the linearization of the theoretical model was studied: charge of the analyte, ionophore:chromoionophore ratio and complex stoichiometry, and the goodness of the linearization of the theoretical fit to experimental data for two experimental systems was checked. The procedure was applied to the determination of potassium and hardness using disposable sensors in different types of waters (tap, well, stream, rain, snow and sea), validating the results against a reference procedure. This new linear calibration proposed for cation determination increases the linear dynamic range up to 7–8 orders of magnitude. The main advantage is that it is possible to directly quantify samples with very different analyte contents in a fast and simple way.