Deionized (DI) water is purified water used in specific applications where dissolved mineral salts (ions) could cause interference or damage. Ions are abundant in standard tap water. Understanding DI water’s unique characteristics and securing a quality supply are important steps for users. This article covers what deionized water is, its common uses, and how to purchase and maintain its purity.
What is Deionized Water and How is it Different
Deionized water is water from which almost all mineral ions have been removed through ion exchange. This process uses specialized resin beds to remove positively charged ions (like sodium and calcium) and negatively charged ions (like chloride and sulfate). The resulting water has a very low level of Total Dissolved Solids (TDS), often measuring less than 1 part per million (ppm), making it a poor electrical conductor.
This purification method differs significantly from other common water types. Standard filtered water removes some particulate matter and contaminants but leaves most of the dissolved mineral ions intact. Distilled water, created by boiling water into steam and then condensing it back into a liquid, removes minerals, viruses, and bacteria, but the process is generally slower and more energy-intensive than deionization. Deionization specifically targets ion removal, which is why it is often preferred in applications sensitive to mineral content.
Common Applications for Deionized Water
The absence of dissolved minerals makes DI water suitable for tasks where mineral deposits, or “scaling,” would be detrimental. In the automotive sector, DI water is used to top off lead-acid batteries, as minerals in tap water shorten battery life. It is also used in cooling systems to prevent mineral buildup that reduces heat transfer efficiency.
Within the home, DI water prolongs the life of appliances that generate steam or mist, including humidifiers, steam irons, and CPAP machines. It prevents mineral residue accumulation on heating elements and internal components. For specialized cleaning, like rinsing electronics or washing cars and windows, DI water ensures a spot-free finish since no minerals are left behind when the water evaporates.
In laboratory and industrial settings, high purity DI water is necessary for preparing chemical solutions, rinsing glassware, and manufacturing electronics. In microchip production, trace amounts of conductive ions interfere with delicate circuitry. The lack of mineral content allows for reliable, repeatable results in experiments and prevents unwanted chemical reactions in pharmaceutical and cosmetic production.
Where to Purchase Deionized Water
Deionized water is widely available, with purchasing options varying based on the volume and grade of purity required. For small volumes, typically one-gallon jugs, common physical retail locations are the most convenient source. These include major grocery stores and supermarkets, often found near bottled water or cleaning supplies. Hardware and home improvement stores, as well as automotive supply shops, also stock DI water, marketing it for battery and cooling system maintenance. The purity level in these retail options is usually sufficient for household use but may not meet stringent laboratory requirements.
For larger quantities or higher purity grades, online vendors and specialized suppliers are a better option. Large e-commerce platforms offer various sizes, including 5-gallon containers. Specialized chemical or laboratory supply companies offer bulk containers, such as 55-gallon drums or 275-gallon totes, and provide specific purity specifications (e.g., ASTM Type I or Type II) suitable for industrial or scientific applications.
Quality and Storage Considerations
Maintaining the quality of deionized water after purchase requires careful storage to prevent re-contamination. The water’s highly purified state makes it chemically “aggressive,” meaning it readily seeks to dissolve and absorb substances it contacts. Specifically, deionized water will quickly absorb carbon dioxide (CO2) from the air, which then forms carbonic acid, reducing the water’s pH and increasing its conductivity.
For this reason, it is important to ensure the water is stored in a tightly sealed container, preferably the one it was purchased in, to minimize exposure to atmospheric gases. If the container is opened frequently, the purity of the remaining water will degrade over time. A simple way to check the water’s quality is by using a handheld Total Dissolved Solids (TDS) meter, which measures the electrical conductivity of the water and gives an immediate reading of its ion content in parts per million. A reading of 0 ppm to 5 ppm is acceptable for most high-purity applications.