Natural water is not usually a chemically pure substance. It contains dissolved salts, minerals, organic compounds and gases. These substances can occur in varying concentrations depending on the source of the water. Dissolved salts exist as ions and can be removed from the water using an ion exchange process or adsorption (= binding to solid state).
Ions can carry one or more charges. Usually, they have 1,2 or 3 charges
(e.g. +, ++, +++). Moreover, ions can also consist of only a single atom (e.g. H+) or a combination of several indissolubly linked atoms, forming a molecule (e.g. CO3--).
The charge of ions in the water always must be zero. This means the number of positive and negative ions is the same.
Ion exchange is a very powerful method to remove impurities, residues and contaminants from water.
For the ion exchange, substances are used that have a surface property allowing ions to adhere very well (= so-called ion exchangers). These ion exchangers are loaded with positively charged hydrogen ions H+ and/or negatively charged hydroxide ions OH-.
These ions have a low charge (+) (-).
The higher the charge and the smaller the radius of an ion, the more the ion is bound to the ion exchanger.
If the water to be treated is now passed through an ion exchanger with positive and negatively charged ions, all positively charged cations (+) in the water are replaced with positively charged hydrogen ions (H+) and all negatively charged anions (-) in the water are replaced with negatively charged OH ions (OH-). This means that the ion exchanger repels all hydrogen and hydroxide ions and picks up the positive and negative ions from the water. The H+ and OH- ions repelled by the ion exchanger now combine to ultrapure, residue-free water outside of the ion exchanger.
This process takes place until the ion exchanger no longer can give off any H+ or OH- Ions.
Deionized water is also called fully demineralized water.
The purity of demineralized water is expressed through its conductivity.
In solutions, the current is carried by cations and anions = electrical conductivity. Cations and anions are thus the mobile charge carriers.
This conductivity is specified as a conductance value in µs/cm1.
The purer the water, the lower the conductance value. Ultrapure water, for example, has a very low conductance value of <0.1 µs/cm1.