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Energy and the Environment
Junction potentials bias measurements of ion exchange membrane permselectivity Ryan Scott Kingsbury, Sophie Flotron, Shan Zhu, Douglas Call, and Orlando Coronell Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018
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Environmental Science & Technology
Junction potentials bias measurements of ion exchange membrane permselectivity Ryan S. Kingsbury,a Sophie Flotron,a Shan Zhu,a Douglas F. Call,b Orlando Coronella,* a
Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA b Department of Civil, Construction, and Environmental Engineering, College of Engineering, North Carolina State University, Raleigh, NC 27695, USA *Corresponding author [tel: 1-919-966-9010; fax: +1-919-966-7911; e-mail:
[email protected]]
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Abstract
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Ion exchange membranes (IEMs) are versatile materials relevant to a variety of water and waste
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treatment, energy production, and industrial separation processes. The defining characteristic of
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IEMs is their ability to selectively allow positive or negative ions to permeate, which is referred
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to as the permselectivity. Measured values of permselectivity that equal unity (corresponding to a
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perfectly-selective membrane) or exceed unity (theoretically impossible) have been reported for
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cation exchange membranes (CEMs). Such non-physical results call into question our ability to
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correctly measure this crucial membrane property. Since weighing errors, temperature, and
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measurement uncertainty have been shown to not explain these anomalous permselectivity
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results, we hypothesized that a possible explanation are junction potentials that occur at the tips
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of reference electrodes. In this work, we tested this hypothesis by comparing permselectivity
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values obtained from bare Ag/AgCl wire electrodes (which have no junction) to values obtained
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from single-junction reference electrodes containing two different electrolytes. We show that
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permselectivity values obtained using reference electrodes with junctions were greater than unity
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for CEMs. By contrast, electrodes without junctions always produced permselectivities lower
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than unity. Electrodes with junctions also resulted in artificially low permselectivity values for
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AEMs compared to electrodes without junctions. Thus, we conclude that junctions in reference
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electrodes introduce two biases into results in the IEM literature: (i) permselectivity values larger
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than unity for CEMs, and (ii) lower permselectivity values for AEMs compared to those for
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CEMs. These biases can be avoided by using electrodes without a junction.
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Keywords: permselectivity; junction potential; membrane potential; ion exchange membrane; reference electrode; electrochemistry
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Introduction
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Ion exchange membranes (IEMs) are versatile materials used in desalination, waste treatment,
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energy production, and industrial separation.1–8 The IEM-based technologies used in these
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processes help mitigate human impacts to the environment in several ways. For example,
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diffusion dialysis (DD) enables recovery of acids and bases from industrial wastewaters,9
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electrodialysis (ED) can be used to produce drinking water from brackish groundwater with low
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energy input and high water recovery,9–11 and reverse electrodialysis (RED) is an emerging
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technology for clean energy production and storage9,12 that could satisfy the energy demand of
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many coastal communities.13 As such, understanding IEM performance is essential for the
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continued development of many technologies relevant to environmental protection.
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The defining characteristic of IEMs is their permselectivity, which refers to their ability to
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selectively allow ions of opposite charge to the membrane (counter-ions) to permeate.4
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Permselectivity ranges from 0 to 1, where 1 indicates perfect selectivity for counter-ions.
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Measured values of permselectivity that equal or slightly exceed unity (e.g., permselectivity =
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1.00-1.04) have been reported for cation exchange membranes (CEMs) by several research
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groups (see Table S1).14–16 These results do not make physical sense, and call into question our
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ability to correctly measure this crucial membrane property. Moreover, the fact that CEM
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permselectivity measurements appear biased suggests that measurements of anion exchange
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membrane (AEM) permselectivity may also be inaccurate.
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Ji et al.16 determined that weighing errors, temperature, or measurement uncertainty in the
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membrane potential cannot explain these non-physical values of permselectivity. Having ruled
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out these factors, we hypothesize that a possible explanation for the artificially-high values of
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permselectivity for CEMs are junction potentials arising at the tips of reference electrodes used
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to measure membrane potential. Junction potentials occur due to differences in ion mobility and,
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in some cases, ionic selectivity of the separator (e.g., the tip of a reference electrode)17–19
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wherever there is an interface between two electrolyte solutions of different concentration (e.g.,
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the electrode filling solution and the bulk electrolyte).
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Accordingly, towards determining how to correctly measure membrane permselectivity, our
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objective was to determine whether junction potentials affect permselectivity measurements of
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CEMs and AEMs. We tested our hypothesis by comparing permselectivity values obtained using
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bare Ag/AgCl wire electrodes (which have no junction) with values obtained from single-
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junction reference electrodes filled with either NaCl or KCl solutions. We show that measuring
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permselectivity using reference electrodes with junctions produces values of permselectivity
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greater than unity for CEMs, and values that are artificially low for AEMs. These biases can be
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avoided by using electrodes without a junction.
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Theoretical Background
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The apparent membrane permselectivity (, dimensionless) is given by4,20
,
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=
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where (V) is the potential across the IEM, , (V) is the potential of an ideally-
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selective membrane, and (dimensionless) are the solution-phase transport numbers of co-
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ions and counter-ions, respectively, and the term “apparent” signifies that the permselectivity
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calculated in this way does not include the effects of water transport by osmosis and electro-
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osmosis.4 , is given by the Nernst equation21,22
,
(1)
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ln %.' "
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Δ, = !
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where ) is the charge on the counter-ion, * is the activity of the salt (NaCl in this case) in the
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respective solutions, and subscripts indicate the NaCl concentration in the respective solutions.
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is typically measured using two reference electrodes placed on opposite sides of a
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membrane separating 0.5 M and 0.1 M NaCl solutions.15,16,20,21,23,24 Most measurements employ
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single- or double-junction Ag/AgCl reference electrodes containing a filling solution separated
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from the bulk salt solution by a porous frit.14–16,20,24 This measurement apparatus is represented
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by the shorthand electrochemical cell notation given in Figure 1. Under these measurement
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conditions, , is ‒37.8 mV for a CEM (see Supporting Information), where is
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defined as the potential of the concentrated side of the cell with respect to the dilute side.
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Therefore, a measurement error of just ±0.4 mV in would cause 1% error in the
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permselectivity calculation.
%.(
,
(2)
90 91 92 93 94 95 96 97
Figure 1. Shorthand electrochemical notation for the cell commonly used to measure apparent membrane permselectivity. In the figure, “/” indicates a phase boundary while “,” indicates a boundary between components in the same phase, after Bard & Faulkner.17 The cell reflects the use of single-junction Ag/AgCl reference electrodes. Subscripts “C” and “D” refer to the sides of the membrane containing the more concentrated and dilute salt solutions, respectively.
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As shown in Figure 1, the measured potential (+,- ) comprises several additional potentials
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in addition to the membrane potential . First, there may be a difference in the potential of
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the two reference electrodes due to differences in the respective filling solution concentrations.
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This “offset potential” (Δ-. , V) can be expressed as
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Δ-. = -.,/ -.,0 ,
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where -. refers to the potential between the Ag/AgCl wire of the electrode and the
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corresponding filling solution, and the subscripts 1 and 2 indicate the electrodes immersed in the
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more concentrated and dilute electrolyte solutions, respectively. Δ-. can be measured directly
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by recording the potential difference between both electrodes placed in the same salt solution,
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and is zero for ideal reference electrodes.
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Second, when using single- or double-junction reference electrodes, there will be a potential due
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to the junction formed between the filling solution and the bulk solution in the cell, provided that
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their concentrations are different. In general, this junction potential consists of two components:
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1) the liquid junction potential arising from differences in ion mobility17,18,25 (see Figure 2a) and
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2) a “tip potential” caused by the ionic selectivity of negatively-charged porous glass frits.19
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Because the bulk salt concentrations on either side of the membrane are different, the junction
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potentials on the two sides will also be different. Thus, we define the difference in junction
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potential between the two sides (Δ3 , V) as
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Δ3 = 3,/ 3,0 ,
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where 3,/ and 3,0 are the junction potentials of the electrodes immersed in the concentrated and
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dilute electrolyte solutions, respectively (here 0.5 M and 0.1 M NaCl). We cannot calculate Δ3 a
(3)
(4)
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priori because there is no way to quantitatively estimate the difference in tip potentials.
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However, Mousavi et al.19 showed that the tip potential is reduced at higher ionic strengths (>0.1
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M) due to charge screening. Considering that the ionic strengths of the solutions typically used
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for permselectivity measurement (0.5 M and 0.1 M NaCl) are relatively high, we will estimate
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Δ3 based only on the liquid junction potential, recognizing that this calculated value will
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represent a lower bound of the true junction potential. Accordingly, 3 is approximated as the
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liquid junction potential, which is given by17
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3 =
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where 7 (8.314 J.mol-1.K-1) is the ideal gas constant, 8 (K) is the temperature, 9 (96,485 C.mol-
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1
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summation is performed over all ions (:) in solution, and the limits of integration represent the
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two liquid phases (fill and bulk solutions). In this study, we define phase 1 as the external
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solution and phase 2 as the electrode filling solution. By assuming linear activity gradients
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through the junction, Equation (5) can be solved analytically to give an activity-corrected form
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of the well-known Henderson equation.26
"
∑ 5
!
6 ln *
,
(5)
) is the Faraday constant, ) (dimensionless) is ion charge, * (dimensionless) is ion activity, the
;< ;= ∑ [ ?@AB ? @]
