Elimination of non-equilibrium effects in separate solutions

Liquid membranes are usually soaked in an analyte solution, which is high in activity, overnight before measurements so that aU exchangeable ions in the membranes are replaced with the analyte ion for a Nemstian response. This process is called conditioning. When a membrane conditioned in an analyte solution is in contact with the separate solution of an 

The non-equilibrium effects on the selectivity determination can be eliminated by measuring the bi-ionic potential in the following cell (49, 50)  

Importantly, the unbiased selectivity coefficients are thermodynamically meaningful. When an interfering ion, such as tetraethylammonium ion for cation-selective electrodes, does not bind to an ionophore, the selectivity coefficient for an ion of interest against such an interfering ion can be used to determine the formation constant of the ion-ionophore complexes (9). For example, when a neutral ionophore forms complexes with an ion, i, and does not bind to an interfering ion, j, the formation constant is given as 

Formation constants can also be determined by measuring the potential of the following 

Whereas the potential is transient because of the gradual mixing of the membrane components, the initial potential is directly related to the formation constants without requirement of a reference ion. Assuming the same ionophore and ion as equation (7.4.4), the formation constant is given by 

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