Deionized water, often referred to as DI water or demineralized water, is a type of purified water that has undergone a process to remove nearly all of its mineral ions. These ions are electrically charged atoms or molecules, such as positively charged calcium and sodium, or negatively charged chloride and sulfate, which are naturally present in water sources. The purification process specifically targets and eliminates these dissolved salts and minerals, leaving behind water that is chemically pure in terms of its ionic content.
The Chemical Basis of Deionized Water
The core chemical goal of creating deionized water is to remove the charged impurities that enable water to conduct electricity. Ions act as mobile charge carriers, allowing an electrical current to pass through the water. Tap water, which contains a variety of dissolved mineral ions, is an excellent electrical conductor.
By removing these ionic substances, the water’s electrical resistance approaches its theoretical maximum. The purity of deionized water is often measured by its resistivity, which can reach up to 18.2 megaohm-centimeters (MΩ·cm) at 25°C. This high resistivity confirms the successful removal of the vast majority of ions, making the resulting water ideal for sensitive applications where electrical interference from dissolved salts must be avoided.
Production Through Ion Exchange
The production of deionized water relies primarily on a chemical purification technique called ion exchange. This method uses specialized plastic polymer materials known as ion exchange resins, which are contained in tanks or cartridges. The resins are designed to chemically swap their own harmless ions for the unwanted contaminant ions in the source water.
Water is passed through two types of resins: cation and anion resins. The cation exchange resin is typically loaded with hydrogen ions (H+), which it trades for positively charged mineral ions like calcium or sodium. The water then flows through the anion exchange resin, which is loaded with hydroxyl ions (OH-) and exchanges them for negatively charged ions like chloride or sulfate.
When the hydrogen ions and hydroxyl ions are released into the water, they combine to form pure water (H2O). This continuous process effectively removes dissolved salts by replacing them with the components of water itself. Once the resins have exhausted their capacity to exchange ions, they must be chemically regenerated with strong acids and bases to restore their functionality.
Distinguishing Deionized, Distilled, and Tap Water
These three common types of water differ significantly in their purification methods and the types of impurities they remove. Tap water is municipal water that contains dissolved inorganic ions, organic matter, and often trace amounts of chlorine or other disinfectants. While safe for drinking, its impurity profile is too high for many industrial and scientific uses.
Distilled water is produced by boiling water and condensing the resulting steam, a physical process that leaves behind non-volatile contaminants like salts and heavy metals. This method also effectively removes bacteria and viruses, resulting in water that is both demineralized and typically sterile. However, distillation is less effective at removing volatile organic compounds that can evaporate with the steam.
Deionized water is exceptionally effective at removing charged inorganic ions through chemical exchange, achieving extreme ionic purity. The deionization process, however, is not designed to remove non-ionic contaminants such as uncharged organic molecules, bacteria, or viruses. Therefore, deionized water is considered chemically pure but not necessarily sterile or free of biological pathogens.
Common Uses and Consumption Safety
Deionized water is indispensable across numerous industries because of its unique lack of mineral ions. Its high purity makes it the standard for rinsing sensitive equipment in laboratories and for cleaning components in electronics manufacturing, such as semiconductors. The absence of dissolved minerals prevents the formation of scale or residue that could interfere with chemical reactions or damage machinery.
It is also used in the automotive sector for car batteries and cooling systems, where mineral deposits from tap water would cause corrosion and inefficiency. Pharmaceutical and cosmetic production rely on deionized water as a solvent and ingredient to ensure the final product is uncontaminated and pure.
While not immediately toxic, drinking deionized water is not recommended for regular consumption, as it lacks the beneficial minerals like calcium and magnesium found in tap water. The water’s highly pure, ion-hungry nature means it actively seeks to dissolve and absorb ions from anything it contacts, including trace minerals from the body’s tissues. This effect can potentially lead to the leaching of electrolytes from the body over time. Furthermore, deionization does not eliminate all non-ionic impurities, meaning the water could still contain bacteria or other organic compounds if not treated by additional methods.