Electrodialysis is an electrochemical process in which ion transfer separates salt from water. It is effective only for substances that can be ionized: for example, salt (NaCl) becomes, in solution, a mixture of Na+ and Cl− ions. (Silica, on the other hand, does not ionize and hence is not removed by electrodialysis. It could, however, be removed by reverse osmosis.) When electrodes, connected to a suitable direct current supply, are immersed in a salt solution, current will flow, carried by the ions. The ions with a positive charge are attracted towards the negative cathode and are called cations. Negatively charged anions flow towards the positive anode. In electrodialysis, filters or membranes selectively impervious to cations or anions are placed alternately between the electrodes (Figure 35). Cation filters permit the flow of anions but act as a barrier to positively charged cations. Conversely, anions are held back by the anion filter while cations pass through. In certain compartments of the tank, ions will collect as their flow is checked by an appropriate filter. Cells of increasing salt concentration thus alternate with cells of salt depletion. Water that is sufficiently desalinated is extracted from the appropriate compartments. Electrodialysis is only generally used with brackish waters as it is uneconomic for sea water desalination. It is used together with ion exchange and activated carbon to produce ultra-pure water for the electronics and pharmaceutical industries.