The tap water we drink is purified water that contains minerals and ions, such as sodium, calcium, and magnesium. In manufacturing, we often hear the terms deionized and distilled for describing pure water. But what does that mean?
Distilled Water
Distillation does not involve a chemical reaction; it involves changing the state of water from liquid to vapor and then condensing it back to liquid form. Distilled water is made by boiling water, evaporating it, then re-condensing it to remove most impurities.
Water is heated to the point of evaporation and forms steam. It is then condensed and collected in a sterile container. Water’s lower boiling point leaves contaminants (including minerals) behind when it turns into steam. Water that has been distilled is mineral and contaminant free and has a pH level of around 7. In terms of conductivity, it is still possible for distilled water to have some conductivity because it can pick up charged particles from the air or other sources.
- Most bacteria are removed during the distillation process.
- While household items can be used to distill water, special equipment is needed to deionize it.
- During distillation, contaminants with the same boiling point as water are evaporated with water, and they can be carried into the condensate.
Deionized Water
Unlike distilling, deionizing water alters the properties at a molecular level. The term deionized water refers to water that has been treated to remove all ions, most commonly mineral salts that are dissolved in it. The process of deionization involves flowing water across resin beads that exchange ions for hydrogen and hydroxyl to form water.
Minerals are ions which means they have a positive or negative charge. The charge is what makes the exchange possible. Resin beads contain charged hydrogen (positive) or hydroxyl (negative). The ion/mineral gets exchanged from the water into the bead when water passes over a bead with hydrogen in it, and the hydrogen gets exchanged between the water and the bead. Hydrogen (H) and hydroxyl (OH) from the beads form H2O when they meet in water.
Common Ions in Water
Hydrogen – Positively charged
Hydroxide – Negatively charged
Calcium – Positively charged
Magnesium – Positively charged
Sodium – Positively charged
Chloride – Negatively charged
Sulfate – Negatively charged
Carbonate – Negatively charged
It is important to note that VOCs are not removed from water by deionization. One method to remove VOCs is by using activated carbon filtration. Activated carbon adsorbs contaminants onto the carbon surface when water is passed through it. The result is extremely pure water that is free from ions as well as volatile organic compounds.
With deionized water, conductivity depends on the degree of deionization. Because deionized water contains no or very few ions capable of conducting electricity, it has very low conductivity. The pH of deionized water can vary depending on the type of ion exchange resin used.
- When bacteria and viruses are present, deionization alone does not remove them.
Conclusion
Both processes remove impurities from water but in two entirely different ways. They have their advantages and limitations. In deionization, only ions and charged organic particles are removed from the water. It does not remove VOCs and bacteria. Distillation removes ions, VOCs, and bacteria but not everything is guaranteed to be removed. If there are any chemicals whose boiling point matches or is lower than that of water, they will evaporate and mix with the distilled water. It is up to the end user to decide which is better for their specific end use.