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The swelling degree can have a direct influence on the dimensional stability, permselectivity and specific electrical conductivity. The swelling degree can be evaluated by measuring the weight difference between dry and swollen membrane [130, 131]. In order to calculate the volumetric swelling, the membrane material density is needed [66]. The IEC indicates the amount of fixed charges in the IEM. It is commonly expressed as milli- equivalent (meq) of functional groups per gram or cm3 of dry membrane (though in some cases they can be expressed per gram or cm3 of swollen membrane) and it is useful to estimate the fixed charges concentration. The main technique for the measurement of IEC is titration with NaOH for negative groups and with HCl for positive groups [89, 130, 131]. In ED and related processes, very large concentration differences can be achieved in the two compartments. For this reason, salt and water diffusion through IEMs can represent an important factor for determining process performance. Salt diffusion can easily be measured using the time-lag method or a more advanced method adopting radioactive tracers [49]. Water moves across membranes by two main mechanisms: osmosis (water passage driven by an osmotic pressure difference) and electroosmosis (passage of water molecules entrained in the solvation shell of ions, thus proportional to the ions flux). In order to determine the water osmotic permeability it is possible to measure the water flux in an ED batch process by simply measuring the weight change in the reservoirs, periodically switching off the stack current to exclude the electroosmotic effect [42]. Electrochemical properties have the most important influence on the performance of IEMs. In particular, the two main electrochemical properties to be measured are permselectivity and electrical resistance. Permselectivity indicates how selective the membrane is to the passage of counter-ions. A low perm- selective membrane will allow the passage of co-ions between compartments, thus negatively affecting the separation efficiency. The fastest method to estimate membrane permselectivity is by measurement of non-Ohmic membrane potential [66, 132]. The real (measured) membrane potential in the case of a single electrolyte in solution can be written as [32]: 𝑅𝑇 𝑎, (5) ∆𝜑 =𝛼 𝑙𝑛 𝑧𝐹 𝑎, where 𝑎, and 𝑎, are the salt activities in the solutions at the two sides (right and left) of the membrane and 𝛼 is the membrane permselectivity, which can be seen as the ratio between the 18PDF Image | Electrodialysis for water desalination
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