The impact of ion association in the optode phase to the dynamic range and the sensitivity of the response of ion-selective bulk optodes

•Impact of ion association in optode phase to its response is studied theoretically.•Association results in a shift of the optical response curve along the solution pH.•Neglecting association may cause a significant error in the optical pH measurement.•Proper choice of the ionic additive allows for tuning the optode dynamic range.•Theory is verified using K+/pH optode as a model system.

It is demonstrated theoretically how the ionic association of the metal cation and/or protonated form of the neutral indicator (chromoionophore) with the anionic sites in the polymeric optode phase alters the dynamic range and the sensitivity of the optode response. Explicit response equations are derived for several limiting cases where the metal ion or the protonated indicator form ion pairs with the anionic sites. The optode response is also simulated numerically with no assumptions on the association degree of any type of the species present in the membrane phase. Simulations show that association in the optode phase may result in significant errors when optode is used as an analytical tool. The results are confirmed experimentally using pH/pM optodes based on chromoionophore ETH2439 as a model system. It is demonstrated that by selection of a proper ionic additive one can optimize the optode response range for the desired concentration range.

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