Donnan Failure of Ion-Selective Electrodes with Hydrophilic High-Capacity Ion-Exchanger Membranes

Hydrophilic ion-exchanger membranes with a high ion-exchange capacity not only find numerous applications in ion separations, but also have interesting properties when used as sensing membranes of ion-selective electrodes (ISEs). As pointed out in the literature, the hydrophilic nature of these membranes may reduce biofouling of ISE sensing membranes as caused by electrically neutral, hydrophobic interferents. This work shows that hydrophilic high-capacity ion-exchanger membranes are more resistant to Donnan failure (i.e., the limitation of the upper detection limit by co-ion transfer into the sensing membrane) than both hydrophobic ionophore-doped and hydrophobic ionophore-free ion-exchanger membranes. Nernstian responses of hydrophilic anion-exchanger membranes were found for anions as large as 2.0 nm, in spite of the cross-linked nature of the anion exchanger that was used. This shows that the high resistance of hydrophilic anion-exchangers to Donnan failure caused by cations such as tetrabutylammonium is not the result of size exclusion. For typical ions, the hydrophilicity of the anion exchanger does not play a decisive factor either. Instead, the excellent resistance to Donnan failure exhibited by hydrophilic ion exchange membranes is primarily caused by the high activity of exchangeable ions in the ion exchanger phase, which disfavors partitioning of ions of opposite charge (along with target ions) from samples into these sensing membranes. The absence of Donnan failure caused by hydrophobic co-ions may be of substantial benefit for measurements in biological samples.