Water is rarely found in its pure state, containing dissolved solids and gases that make it a good electrical conductor. Purified water is necessary for modern industry and scientific research; one common form is DI water. This acronym stands for Deionized water, created by a process engineered to strip water of its electrically charged impurities. Understanding how deionized water is made helps recognize why it is necessary in sensitive applications.
Defining Deionized Water
Deionized water is defined by the absence of charged mineral salts, known as ions, which include common substances like calcium, sodium, chloride, and sulfate. The removal of these ions is measured by the water’s electrical conductivity, as dissolved ions are the primary conductors of electricity. Extremely pure deionized water exhibits very low conductivity, often measured as high electrical resistivity, reaching up to 18.2 megohm-centimeters (MΩ·cm) at 25°C. This low conductivity confirms the successful removal of ionic contaminants.
The core principle is that only charged particles are targeted for removal, leaving neutral molecules like organic compounds or bacteria potentially unaffected. This focus on inorganic ions distinguishes deionized water from other purification types. The resulting water has a high chemical reactivity because it actively attempts to dissolve any ions it encounters to return to a state of equilibrium.
How Deionization Works
The production of deionized water relies on a chemical exchange process using synthetic ion-exchange resins. These resins are tiny plastic beads formulated to attract and swap charged particles in the water. Two primary types of resins are used: cation resin and anion resin, often packaged together in a mixed-bed deionizer.
As water passes through the cation resin, positively charged ions (cations) like calcium or magnesium are captured, releasing a hydrogen ion (H+) in exchange. Next, the water flows over the anion resin, which captures negatively charged ions (anions) like chloride or nitrate, releasing a hydroxyl ion (OH-). The released hydrogen and hydroxyl ions then combine to form a neutral water molecule (H+ + OH- = H2O), effectively replacing the contaminants with pure water.
Comparing DI Water to Other Purified Types
Deionized water is one of several purified types, each defined by the impurities it removes. Standard tap water contains a full spectrum of contaminants, including ions, suspended particulates, and microorganisms. Reverse Osmosis (RO) water uses high pressure to force water through a semipermeable membrane, removing a large percentage of dissolved solids and larger molecules, achieving 90 to 99% purity. RO is often used as a pre-treatment step because it removes the bulk of impurities before the deionization stage.
Distilled water is created by boiling water and condensing the steam back into a liquid, leaving behind mineral salts and most non-volatile contaminants. While distillation effectively removes ions and kills bacteria, it may not remove volatile organic compounds (VOCs) that have a lower boiling point than water. Deionized water’s distinction is its selective removal of ions; it achieves ultra-high purity in conductivity but may still contain bacteria or uncharged organic materials that other methods would eliminate.
Where DI Water is Used
The lack of conductive ions makes deionized water indispensable across numerous high-tech and scientific fields. In electronics manufacturing, such as microchips and circuit boards, DI water is used for rinsing components. Using regular water would leave behind conductive mineral deposits that cause short circuits and failure.
Laboratories rely on deionized water for preparing chemical solutions and rinsing glassware to ensure experiments are not contaminated by trace minerals. The automotive industry uses DI water in lead-acid batteries and engine cooling systems, as minerals in tap water cause scale buildup and corrosion, reducing efficiency and shortening component life. Pharmaceutical companies require ultra-pure water for mixing medications and sterilization processes to guarantee product stability and prevent unwanted chemical reactions.