Deionization is a process used to produce highly purified water by removing nearly all dissolved mineral salts and charged molecules, known as ions. These ions, such as calcium, sodium, chloride, and sulfate, are naturally present in water sources and are the primary cause of electrical conductivity. The deionization (DI) process strips water of these electrically charged impurities, yielding a final product that is neutral and non-conductive. Water purity is often measured by its electrical resistivity, with ultra-pure grades exceeding 18 megaohms-centimeter (\(\text{M}\Omega \cdot \text{cm}\)). This measurement confirms the extremely low concentration of ions remaining in the solution.
The Chemistry Behind Deionization
The fundamental principle behind deionization is ion exchange, a chemical process that relies on specialized synthetic resins. Water flows through a column containing tiny, porous polymer beads that have specific electrically charged sites permanently fixed to their structure. These beads, which make up the ion exchange resin, are pre-charged with non-contaminant ions ready to be swapped out.
There are typically two types of resin used: cation exchange resin and anion exchange resin. Cation resin beads possess negatively charged sites that attract and capture positively charged contaminant ions (cations), like calcium (\(\text{Ca}^{2+}\)) or magnesium (\(\text{Mg}^{2+}\)). In exchange for these unwanted cations, the resin releases a hydrogen ion (\(\text{H}^{+}\)) into the water.
The water then passes through the anion resin, which contains positively charged sites. This resin attracts negatively charged contaminant ions (anions), such as chloride (\(\text{Cl}^{-}\)) or sulfate (\(\text{SO}_{4}^{2-}\)). For every anion captured, the resin releases a hydroxide ion (\(\text{OH}^{-}\)) into the water. The hydrogen ions and hydroxide ions released by the two resins immediately combine to form a molecule of pure water (\(\text{H}_{2}\text{O}\)), completing the purification process.
What Deionization Removes (and What it Leaves Behind)
The deionization process is highly effective at removing total dissolved solids (TDS) that exist as charged particles. This includes virtually all inorganic mineral salts, such as hardness minerals like calcium and magnesium, which cause scale buildup. Other common contaminants removed are heavy metals (lead and copper) and dissolved salts (sodium chloride and potassium).
Deionization, however, is a process focused purely on charged particles and does not function as a comprehensive filtration or sterilization method. It does not reliably remove uncharged contaminants, meaning biological impurities like bacteria, viruses, and protozoa can pass through the resin bed largely unaffected.
Large organic molecules and non-polar substances, such as certain pesticides, oils, and pyrogens, are also left behind by the ion exchange process. Furthermore, dissolved gases, notably carbon dioxide (\(\text{CO}_{2}\)), are not effectively removed and can react with the water to form carbonic acid, slightly lowering the water’s purity. For this reason, deionization is often paired with other purification methods, like reverse osmosis or carbon filtration, to achieve ultra-pure water free of both ions and non-ionic contaminants.
Common Uses of Deionized Water
The absence of ions in deionized water makes it an excellent solvent and rinsing agent for specialized applications where trace minerals would cause problems. In laboratory settings, DI water is routinely used for reagent preparation, glassware rinsing, and in sensitive analytical instruments. The low conductivity of the water is important for these processes, ensuring accuracy and consistency by preventing interference with chemical reactions and test results.
Industrial manufacturing relies heavily on deionized water, especially in the electronics and semiconductor industries. Circuit boards and microchips are washed with DI water to remove residues. This is necessary because mineral deposits left by regular water would be electrically conductive, potentially causing short circuits or device failure.
The automotive sector uses deionized water in lead-acid batteries and engine cooling systems. The ions in tap water can cause corrosion and scale buildup that shortens the lifespan of these components.
Pharmaceutical and cosmetic production also requires deionized water as a base for formulations. Using water free of reactive ions ensures the final product remains chemically stable and does not suffer from unwanted reactions or precipitation of ingredients. DI water is also used for cleaning delicate surfaces, such as windows and glassware, because its lack of minerals prevents the spots and streaks that occur when tap water evaporates.