An encyclopedic resource on the ion exchange process, deionized water properties, and why DI water — not distilled — has become the purity standard for laboratories, electronics, and pharmaceutical manufacturing.
The foundation of modern laboratory and industrial purity.
Deionized water (DI water, DIW or de-ionized water) is water that has had almost all of its mineral ions removed, such as cations like sodium, calcium, iron, and copper, and anions such as chloride and sulfate.
Because the majority of water impurities are dissolved salts, deionization produces high-purity water that is generally similar to distilled water, but produced faster and with significantly lower energy requirements. It is measured by its electrical resistivity — pure water is a poor conductor of electricity, so higher resistivity indicates higher purity.
For most scientific and industrial applications, deionized water is not just a preference but a requirement to ensure the integrity of experiments and the longevity of sensitive equipment.
Understanding the molecular exchange that creates ultrapure water.
Uses resins to remove positively charged ions (cations) like calcium, magnesium, and sodium, replacing them with hydrogen (H+) ions.
Uses resins to remove negatively charged ions (anions) like chloride, sulfate, and bicarbonate, replacing them with hydroxyl (OH-) ions.
The gold standard. Combines both resin types in one vessel to achieve the highest possible purity levels, up to 18.2 MΩ·cm resistivity.
Comparing purification methods is critical for selecting the right system. Read more about RO vs. DI vs. Distillation on the ASTM Water Solutions blog.
Most laboratories follow the ASTM D1193 standard to classify water quality.
| Parameter | Type II (Pure) | Type I (Ultrapure) | Type III (General) |
|---|---|---|---|
| Resistivity (MΩ·cm) | > 1.0 | 18.2 | > 4.0 |
| TOC (ppb) | < 50 | < 10 | < 200 |
| Sodium (ppb) | < 5 | < 1 | < 10 |
| Chloride (ppb) | < 5 | < 1 | < 10 |
*Specifications based on ASTM D1193 standards for Reagent Grade Water.
From the cleanroom to the laboratory, DI water is the essential baseline.
Used for reagent preparation, HPLC, mass spectrometry, and cell culture where even trace ions can interfere with sensitive results.
Learn about Type II Water →Washing silicon wafers requires ultrapure water to prevent microscopic mineral deposits that could cause circuit failures.
Essential for manufacturing non-sterile drugs, cleaning validation, and as a feed source for Water for Injection (WFI) systems.
Used in high-pressure boilers to prevent scale buildup, in electroplating, and for spot-free rinsing in automotive manufacturing.
Expert answers to the most frequently asked questions about DI water.
Distillation removes impurities by boiling water and condensing the steam. Deionization uses ion-exchange resins to chemically remove dissolved minerals. DI water is usually purer (higher resistivity) and the process is more efficient for large-scale production.
Purity is most commonly measured by electrical resistivity (MΩ·cm) or conductivity (µS/cm). Ultrapure water has a resistivity of 18.2 MΩ·cm. You can use an inline conductivity meter or a handheld TDS meter for general assessments.
While not toxic, deionized water is not recommended for drinking. It lacks the essential minerals (electrolytes) your body needs and has a very flat taste. It is designed for laboratory and industrial use, not human consumption.
DI water is highly reactive and will leach ions from its container over time. For high-purity needs, it should be stored in high-density polyethylene (HDPE) or fluoropolymer containers and used as quickly as possible. Read our guide on storage and handling for more details.
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For high-compliance laboratory and industrial applications, we recommend sourcing from a verified ASTM-standard supplier.
Shop ASTM Water SolutionsDeionizedWater.org is an independent educational hub dedicated to the science of water purification. Based in the heart of the Maryland biotech corridor, we provide technical guides, standard summaries, and expert insights for the scientific community.
Our mission is to democratize high-level purification knowledge, helping laboratories and industries achieve consistent, reliable results through better understanding of their most critical reagent: water.