Selectivity coefficients divalent cations

In this equation, E is the potentiometric response, is the standard potential, and is the potentiometric selectivity coefficient [23]. The Nickolsky Eisenman equation is only valid if ions of the same valency are compared [24]. If that is not the case, if a divalent cation is interfering the measurement of a monovalent ion or vice versa, a new selectivity factor AT is recommended [24,25] which more accurately describes the degree of interference log/Tfj is derived graphically from the horizontal distance of the separately measured calibration curves towards the two ions, i and j, of interest (Figure 5) and is formulated as follows ... 

Divalent Ion-Hydrogen Ion Selectivities. Selectivity coefficients determined at equivalent ionic fractions of 0.5 for the alkaline earth ions, Co2" ", and Zn " " are listed in Table II along with corresponding polymer water contents (7). Again, the normal order of selectivities is seen for the alkaline earth ions for a low charge density exchange site environment. The order of standard hydration free energies for these cations is Zn2" " > Co2" "... 

A selectivity scale for divalent cations is given in Table 13.1-4, Again the arbitrary value of unity is given to the lithium ion, Bonner and Smith corrected their value of the equilibrium constant by allowing for activity coefficients by using the method developed by Argersinger and Davidson.7 Thus,... 

Table 3.5 Extraction coefficients of divalent cations from 4M sodium nitrate into di(2-ethyl-hexyl) phosphoric acid in benzene, and selectivity coefficients of PVC ISEs with dialkylphosphate sensors (24)...

To address the theoretical limitation of the Nikolsky-Eisemnan equation, a more general description of the equihbrium responses of hquid membrane ISEs in mixed ion solutions was proposed (41). The model is based on phase boundary potentials under an equilibrium exchange of an analyte and an interfering co-ion at the membrane/sample solution interface. With ionophore-based membranes, the ion-exchange process is followed by complexation of the ions with an ionophore, where free ionophore was assumed to be always present in excess to simplify the model. The charge of the ions was not fixed so that their effect on the potentiometric responses can be addressed by the model. Under equilibrium conditions, the model demonstrated that the Nikolsky-Eisemnan equation is valid only for ions with the same charge (zj = Zj). The selectivity coefficient, however, can still be used in the new model to quantify the potentiometric responses in the mixed ion solution. For example, the potentiometric responses to a monovalent cation in the presence of a divalent cation are given as... 

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