Deionised water is a highly purified form of water in which all ions have been removed through a process called "ion exchange." Ions that the deionisation process removes include calcium, chlorides, and sodium. Deionised water can be used in the home to ensure that only the purest water is being utilised by one's family, however deionised water also has a large range of commercial and industrial applications.
Positively charged ions, called anions, are exchanged for hydrogen and hydroxyl. Most water deionizers are filled with a resin that bonds with all other types of ions and removes them from the water, leaving only the hydrogen and hydroxyl ions behind. The other ions are physically displaced by concentrated acid and caustic within the resin bed and are subsequently stripped away.
Testing Deionized Water
Due to the enormous variety of deionizers available, the level of water quality one can expect to get from a deionizer also varies. The most effective way to determine the quality of purified water is by performing a resistivity & conductivity test. Deionised water should have a resistivity of 18.2 million ohm-cm (18.2 mega-ohm) and a conductivity of 0.055 microsiemens. The amount of electrical conductivity of deionised water is determined by the amount of ionised particles present in the water. Resistivity and conductivity are simply a means of measuring this amount.
Instruments for testing the quality of deionised water can be either potable or installed in line with direct water flow. Portable testers are recommended for on-site work, however they are generally not as accurate as in line testers and should only be used where proper water deionisation is not absolutely critical.
In line testing equipment automatically monitors all water coming through the line and should be installed so that the water reaches the tester after it passes through the deionizer's filter. In line testers are more accurate due to the fact that deionised water, once it leaves the pipe, will collect carbon dioxide from the air. Carbon dioxide will effect resistivity testing results and thus may provide corrupted data.
There are four main kinds of deionizers: disposable cartridges, portable exchange tanks, automatic units, and continuous units. There are two-bed systems and mixed-bed systems, with mixed-bed being more effective in most applications.
Several industries rely heavily on purified water. Among these are health care, laboratory, pharmaceutical, cosmetics, electronics, food processing and car washing.