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Junction potentials bias measurements of ion exchange membrane permselectivity

preprint
revised on 26.10.2017 and posted on 26.10.2017 by Ryan Kingsbury, Sophie Flotron, Shan Zhu, Douglas F. Call, Orlando Coronell
Ion exchange membranes (IEMs) are versatile materials relevant to a variety of water and waste treatment, energy production, and industrial separation processes. The defining characteristic of IEMs is their ability to selectively allow positive or negative ions to permeate, which is referred to as the permselectivity. Measured values of permselectivity that equal unity (corresponding to a perfectly-selective membrane) or exceed unity (theoretically impossible) have been reported for cation exchange membranes (CEMs). Such non-physical results call into question our ability to correctly measure this crucial membrane property. Since weighing errors, temperature, and measurement uncertainty have been shown to not explain these anomalous permselectivity results, we hypothesized that a possible explanation are junction potentials that occur at the tips
of reference electrodes. In this work, we tested this hypothesis by comparing permselectivity values obtained from bare Ag/AgCl wire electrodes (which have no junction) to values obtained from single-junction reference electrodes containing two different electrolytes. We show that permselectivity values obtained using reference electrodes with junctions were greater than unity for CEMs. By contrast, electrodes without junctions always produced permselectivities lower than unity. Electrodes with junctions also resulted in artificially low permselectivity values for AEMs compared to electrodes without junctions. Thus, we conclude that junctions in reference electrodes introduce two biases into results in the IEM literature: (i) permselectivity values larger than unity for CEMs, and (ii) lower permselectivity values for AEMs compared to those for CEMs. These biases can be avoided by using electrodes without a junction.

History

Topic

  • Transport Phenomena (Chem. Eng.)

Email Address of Submitting Author

ryanskingsbury@alumni.unc.edu

Email Address(es) for Other Author(s)

coronell@email.unc.edu dfcall@ncsu.edu

Institution

University of North Carolina at Chapel Hill

Country

United States

ORCID For Submitting Author

0000-0002-7168-3967

Declaration of Conflict of Interest

no conflict of interest

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