Deionized water (DI water), also known as demineralized water, is highly purified water with mineral ions removed. These ions are electrically charged atoms or molecules, such as calcium, sodium, chloride, and magnesium, naturally present in tap water. Removing these charged particles is necessary for applications where mineral residue could cause interference or damage. Industries and consumers rely on DI water for processes requiring high purity, making knowledge of its sources and grades important.
How Deionized Water is Produced
The purification process focuses on ion exchange, using specialized resin beads to capture and remove dissolved mineral ions. Source water, often pre-treated with a reverse osmosis (RO) filter, flows over a mixture of cation and anion resin beads. The cation resin exchanges positively charged ions like calcium for hydrogen ions (\(\text{H}^+\)).
The anion resin simultaneously exchanges negatively charged ions such as sulfate or chloride for hydroxyl ions (\(\text{OH}^-\)). The released hydrogen and hydroxyl ions combine to form pure water (\(\text{H}_2\text{O}\)). Using RO as a pre-filter significantly reduces total dissolved solids, extending the working life of the costly ion-exchange resins.
Deionized water differs fundamentally from distilled water, which is purified by boiling and collecting the resulting steam. Distillation removes a broad range of impurities, including ions, organics, and microbes, but the process is energy-intensive and slow. Deionization is a more rapid and cost-effective method focused specifically on removing ionic contaminants, though it may leave behind uncharged particles like bacteria or certain organic molecules.
Standard Retail and Consumer Sources
For consumers needing small quantities, deionized water is reliably found at automotive supply stores, where its primary application is vehicle maintenance. It is stocked for use in lead-acid batteries and for mixing with engine coolants, preventing scale buildup and corrosion caused by tap water minerals. This consumer-grade water is generally suitable for household appliances requiring mineral-free water.
Big-box stores and major grocery chains also carry DI water, typically in one-gallon plastic jugs, often shelved near less expensive distilled water. This retail-grade product is demineralized to a purity level adequate for preventing scale in household appliances. Common uses include steam irons, humidifiers, and Continuous Positive Airway Pressure (CPAP) machines, though this purity does not meet stringent laboratory standards.
The price for consumer-grade deionized water is usually higher than for distilled water due to the specialized ion-exchange process. Consumer DI water can cost several times more per gallon. Consumers should check the label to ensure the product is explicitly labeled “deionized” or “demineralized” water when buying from general retailers, avoiding the purchase of purified or distilled water instead.
Specialty and High-Purity Suppliers
Customers requiring higher purity levels or large volumes must turn to specialty vendors, chemical suppliers, or water treatment companies. These sources offer certified grades, measured by electrical resistivity, which indicates ionic purity. For instance, Type I water, the highest purity with a resistivity of 18.2 megaohm-cm, is necessary for sensitive laboratory work and electronics manufacturing.
Specialized suppliers provide water in bulk containers, such as 55-gallon drums, 220-gallon totes, or Intermediate Bulk Containers (IBCs), for industrial processes. Reputable suppliers provide a Certificate of Analysis (CoA) with each batch. This documentation confirms the water meets a specific purity standard for applications like pharmaceutical manufacturing or quality control testing.
An alternative for high-volume users is installing a point-of-use DI water system, often using portable exchange tanks swapped out when the resin is depleted. This approach allows businesses or individuals to generate high-purity water on demand. The choice between purchasing certified grades and generating it on-site depends on the required volume, purity level, and frequency of use.