Also known as Ion Exchange, Demineralization, Deionized or D.I. Water
Deionization is a method used most often by laboratories to produce purified water on-demand and is able to purify water to a maximum resistivity of 18.2 megohm/cm at 25șC. A deionization system usually consists of from one to four cylindrical cartridges hooked up to plumbing and hanging on a wall near a sink. While it doesn't produce absolutely pure water, it is convenient and quick, and may be sufficient for many applications. It is an excellent system for removing dissolved solids and gases, although it has a generally poor rating for other impurities.
It works by exchanging hydrogen ions for cationic and hydroxyl ions for anionic contaminants in the feedwater. The deionization resins are tiny spherical plastic beads through which the feedwater passes. After a while the impurities replace all of the hydrogen and hydroxyl groups in the resin, and it has to be replaced or regenerated.
This system has some disadvantages. It requires the assistance of another system to produce absolutely pure water. Small fragments of the ion exchange resin are washed out of the system during operation, and stagnant water in the cartridges can actually encourage the growth of bacteria. It does not remove all of the dissolved organics from the feedwater, and these can foul the ion exchange resin. It needs to be combined with other purification technologies to achieve the level of purity required for research.