You can drink tap water, but it’s not suitable for most laboratory tests, preparing solutions, calibrating equipment, or cleaning glassware. For the lab, you want purified water. Common purification methods include reverse osmosis (RO), distillation, and deionization. Distillation and deionization are similar in that both processes remove ionic impurities, but distilled water and deionized water (DI) are not the same and are not interchangeable for many lab purposes. Let’s take a look at how distillation and deionization work, the difference between them, when you should use each type of water, and when it’s okay to substitute one for the other.
Distilled water is a type of demineralized water that is purified using distillation. The source water for distillation could be tap water, but spring water is used most often. Usually, the water is boiled and the steam is collected and condensed to yield distilled water. Most minerals and certain other impurities are left behind, but the purity of the source water is important because some impurities (e.g., volatile organics, mercury) will vaporize along with the water. Distillation removes salts and particulates.
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 H2O (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 CO2 reacts to produce H+ and HCO3-, driving the pH closer to 5.6. Deionization does not remove molecular species (e.g., sugar) or uncharged organic particles (most bacteria, viruses). It is used in microelectronics, printed circuit boards, instrument manufacture, pharmacy, washing liquids, etc. The purity of deionized water can exceed the purity of distilled water.
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. 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.
