Deionized water is made by running tap water, spring water, or distilled water through an electrically charged resin. Usually, a mixed ion exchange bed with both positive and negative charged resins is used.
Cations and anions in the water exchange with H
+ and OH
- in the resins, producing H
2O (water).
Deionized water is reactive, so its properties start to change as soon as it is exposed to air.
Deionized water has a pH of 7 when it is delivered, but as soon as it comes into contact with carbon dioxide from the air, the dissolved CO
2 reacts to produce H
+ and HCO
3-, driving the pH closer to 5.6.
Deionization does not remove molecular species (e.g., sugar) or uncharged organic particles (most bacteria, viruses).
Distilled Versus Deionized Water in the Lab
Assuming the source water was tap or spring water, distilled water is pure enough for nearly all lab applications. It is used for:
- solvent to prepare solution
- analytical blank
- calibration standard
- cleaning glassware
- equipment sterilization
- making high purity water
The purity of deionized water depends on the source water. Deionized water is used when a soft solvent is needed. It is used for:
- cooling applications where it's important to avoid depositing minerals
- microbiology autoclaves
- many chemistry experiments involving ionic compounds
- washing glassware, especially the final rinse
- solvent preparation
- analytical blanks
- calibration standards
- in batteries
As you can see, in some situations either distilled or deionized water is fine to use. Because it is corrosive, deionized water is not used in situations involving long term contact with metals.
SUBSTITUTING DISTILLED AND DEIONIZED WATER
You don't generally want to substitute one type of water for the other, but if you have deionized water made from distilled water that has been sitting out exposed to air, it becomes ordinary distilled water. It's fine to use this type of leftover deionized water in place of distilled water. Unless you're certain it won't affect the outcome, do not substitute one type of water for another for any application that specifies which type to use
Drinking Distilled and Deionized Water
Although some people like to
drink distilled water, it's really not the best choice for potable water because it lacks minerals found in spring and tap water that improve the flavor of water and confer health benefits.
While it's okay to drink distilled water, you should not drink deionized water. In addition to not supplying minerals, deionized water is corrosive and can cause damage to tooth enamel and soft tissues. Also, deionization does not remove pathogens, so DI water may not protect against infectious diseases. However, you can drink distilled, deionized water after the water has been exposed to air for a while.