Deionized water (DI water), often referred to as demineralized water, is water purified to remove almost all dissolved mineral ions. These charged particles, such as sodium, calcium, chloride, and sulfate, are the primary impurities that make water conductive. The goal of deionization is to achieve extremely low conductivity, which is necessary for sensitive applications in laboratories, electronics manufacturing, and automotive maintenance. Achieving this purity requires specialized methods, ranging from chemical exchange processes to physical separation techniques. This article explores the primary methods used to create deionized water and the necessary steps to ensure its quality.
Defining Deionization Versus Distillation
Deionization and distillation represent two distinct approaches to water purification, targeting different classes of contaminants. Distillation is a physical process involving boiling water and collecting the condensed steam, which effectively removes non-volatile impurities like minerals, salts, and large organic molecules. While the resulting water is generally pure and free of bacteria, it may still contain dissolved gases or volatile organic compounds.
Deionization, in contrast, is a chemical process designed specifically to remove charged ionic impurities. This method is highly efficient at removing dissolved salts, often achieving a purity level where the water’s electrical resistance approaches its theoretical maximum. However, deionized water does not reliably remove uncharged contaminants such as bacteria, viruses, or non-ionic organic compounds. For this reason, deionization is often used as a final “polishing” step after other purification methods have removed these non-ionic substances.
Creating Deionized Water Using Ion Exchange Resins
The most common and effective method for producing high-purity deionized water relies on a chemical process called ion exchange. This technique uses specialized synthetic resin beads housed in a cartridge or column that selectively remove ionic contaminants. The resins are small, porous polymer beads that have charged functional groups built into their structure.
Water is passed through two main types of resin: cation exchange resin and anion exchange resin. The cation resin is charged with hydrogen ions (\(H^+\)) and attracts positively charged ions (cations) like calcium (\(Ca^{2+}\)) or sodium (\(Na^+\)), exchanging them for \(H^+\). The water then flows through the anion resin, which is charged with hydroxide ions (\(OH^-\)). This resin attracts negatively charged ions (anions), such as chloride (\(Cl^-\)) or sulfate (\(SO_4^{2-}\)), exchanging them for \(OH^-\).
The key to the deionization process is the recombination of the exchanged ions. The hydrogen ions released by the cation resin immediately combine with the hydroxide ions released by the anion resin to form a new, pure water molecule (\(H^+ + OH^- \rightarrow H_2O\)). This continuous exchange and recombination effectively removes nearly all ionic impurities from the water stream. For applications requiring the highest possible purity, a mixed-bed resin system is used, where cation and anion resins are thoroughly mixed in a single vessel to maximize efficiency.
DIY deionization setups often use small, refillable cartridges filled with these mixed-bed resins. The resin’s capacity is finite, meaning it will eventually become saturated with captured ions. Saturated resins must be either regenerated using strong acids and bases or replaced entirely. Because the resin’s effectiveness depends on the ionic load, a pre-treatment step is often recommended.
Using Reverse Osmosis and Thermal Distillation
Reverse osmosis (RO) and thermal distillation are physical processes frequently used to prepare water for deionization or to achieve high purity. Reverse osmosis systems force water under pressure through a semi-permeable membrane with extremely small pores. This membrane acts as a physical barrier, blocking contaminants larger than water molecules, including most dissolved inorganic solids, bacteria, and viruses.
An RO system typically removes 90% to 99% of total dissolved solids, making the water significantly purer than tap water. However, RO water is not truly deionized because a small percentage of ions can still pass through the membrane, and the process does not reliably remove dissolved gases. Many high-purity systems, often called RO/DI systems, use the RO membrane first to remove the bulk of contaminants, followed by a final DI resin cartridge to polish the water and remove remaining ions.
Thermal distillation relies on a phase change, converting liquid water into steam and back into liquid, leaving non-volatile contaminants in the boiling vessel. The process is highly effective at removing minerals, salts, heavy metals, and microorganisms. The primary drawback of distillation is its high energy consumption and slow rate of production compared to other methods.
A limitation of distillation is that volatile organic compounds with boiling points near or below that of water can vaporize and condense with the purified steam. Despite this, distillation is an excellent method for creating water that is both low in mineral content and biologically pure. Using a distilled or RO pre-treatment stage significantly extends the working life of the final deionization resins.
Quality Control and Maintaining Water Purity
After deionized water is produced, its quality must be verified, and its purity must be actively maintained, as pure water is highly reactive. The most accurate way to measure the quality of deionized water is by measuring its electrical resistivity, which is the inverse of conductivity. Since dissolved ions conduct electricity, the absence of ions results in high resistivity, with a value of \(18.2\ megohm-centimeters\ (M\Omega\cdot cm)\) at \(25^\circ C\) representing the theoretical maximum purity.
While a Total Dissolved Solids (TDS) meter can give a quick reading in parts per million (ppm), it is a less precise measure than resistivity for high-purity water. Freshly produced deionized water should have a conductivity measurement of less than \(10\ \mu S/cm\). Monitoring this value ensures that the ion exchange resins are still actively performing the purification process.
Maintaining the purity of deionized water requires careful storage because its lack of ions makes it an aggressive solvent that will absorb contaminants from its environment. Deionized water rapidly absorbs carbon dioxide (\(CO_2\)) from the air, which lowers its resistivity by forming carbonic acid. Therefore, it should be stored in airtight containers made from inert materials like glass or high-density polyethylene (HDPE), which are less likely to leach ions into the water.