Electrochemical separations - Electrodialysis

Electrodialysis is a technique for extracting ions from a liquid or pasty medium, while remaining in the liquid phase and at constant temperature: under the influence of an electric field, ions from one solution are transferred to another solution separated from the first by a membrane.

Electrodialysis is commonly used to demineralize liquids initially containing mineral ions in aqueous or non-aqueous solution, at concentrations of the order of 0.05 to 1 mol/L: seawater, brackish water, sugar juices, wastewater from electroplating baths. Alongside these industrial applications, however, there are a number of analytical applications, which began as early as 1903 with the purification of lyophilic colloids, and can involve much more dilute solutions (a few milligrams per liter), falling into the category of membrane-assisted analysis.

Electrodialysis can operate in a temperature range from 0 to 80 ˚C; however, the choice of the best temperature currently depends essentially on the performance of the membranes used. In addition, the temperature rise must not cause degradation of the biological solutions. In addition, the medium must not be very viscous, so that it can be pumped through the small channels (0.25 to 20 mm) of electrodialysis equipment.

Nevertheless, thanks to its easy-to-access operating conditions, electrodialysis allows you to choose the constant temperature best suited to the substances contained in the solutions, to modify or not the ambient environment, and to modulate the separation time.

Initially, electrodialysis was not an analytical method, since it was not a means of detection: it was and remains a separation method prior to the qualitative or quantitative measurement of a species. However, as theoretical knowledge has improved and equipment has been perfected, electrodialysis can be used to determine the quantity of an ion, but cannot be used to identify it.

With this in mind, it can be used for operations in which the notion of energy efficiency or cost-effective production is not the primary consideration.

By taking advantage of the ability to produce a solution with a very low ion content, to form a concentrate containing the ions from the diluate and to react the ions formed, electrodialysis can be used as a means of preanalytical separation in three different ways:

• or to rid a solution of traces so as to retain only the single noble product, which can be assayed by a conventional method or used as an assay reagent;

• or to concentrate one or more traces in the concentrate compartment, so as to obtain a concentration that can be measured by a conventional assay...