Water without minerals such as, salt etc. called de-mineralized water or demi water or de-ionized water. With low mineral content, demi water has some changed expected properties such low conductivity, low corrosive effects etc. Demi water has some specific uses in industrial applications such as, boiler, pipeline etc.
Why demi water is used:
Regular water with mineral contents has some corrosive effects to metals or conductivity issues for certain applications. Additionally, minerals deposits or residues might settle in the metal surface, which in time become hard. This is called Scaling effects.
Demi water is used extensively for all mos all of the industrial applications in which there are water use provisions. Without demi water, the expensive parts of would be of shorten life time with simple water.
Quality of Demi water:
Demi water quality and specification varies as per the use. Usually the manufacturer of the equipment dictates specification of the demi water in which it is going to be used. Even the equipment manufacturer itself provides some time provided the demi water system.
There is a international standard- American Society for Testing and Materials (ASTM) D1193-91, Standard specification for Reagent Grade Water
How demi water is prepared:
Demi water is prepared through ion exchange resin. These are special types of insoluble matrix (or support structure) normally in the form of small (0.5-1 mm diameter) beads, fabricated from an organic polymer substrate.
What ion exchange polymer does is just trap certain ions in exchange of releasing of other ions. In case of basic demi water preparation, ion exchange resin releases hydrogen, hydroxide, and traps minerals ions. For specific application and usages, the ion exchange process may vary with additional steps.