The terms distilled water and deionized water are often used interchangeably, yet they refer to two fundamentally different types of purified water. Both are produced to remove impurities, but their distinct purification methods target different classes of contaminants, resulting in unique chemical profiles. Understanding these processes reveals why they are not equivalent and why one may be suitable for a specific application while the other is not.
Understanding Distilled Water
Distilled water (DW) is produced through a process that mimics the natural hydrologic cycle. Raw water is heated to a boil, converting it into steam. This steam rises, leaving behind virtually all non-volatile impurities that do not vaporize at water’s boiling point, such as dissolved mineral salts and heavy metals.
The purified steam is collected and cooled in a separate chamber, causing it to condense back into liquid water. The high heat kills most biological contaminants, resulting in water highly free of microorganisms and inorganic solids. However, distillation is not effective at removing volatile organic compounds (VOCs), like certain pesticides or solvents, which can vaporize along with the water and re-contaminate the final product.
Understanding Deionized Water
Deionized water (DIW) is created using a chemical process called ion exchange, which specifically targets dissolved mineral salts and other electrically charged particles, known as ions. Water is passed through resin beds containing positively and negatively charged beads (cation and anion resins). These resin beads attract contaminant ions, such as calcium and sodium, and exchange them for hydrogen and hydroxyl ions, which then combine to form pure water.
The effectiveness of this process is measured by the water’s electrical conductivity, which is extremely low because the conductive ions have been removed. Deionization does not use heat and therefore does not effectively remove non-ionic contaminants, including many organic compounds, bacteria, and viruses.
Key Differences in Purity and Contaminants
The fundamental distinction lies in the type of impurity each method removes, resulting in different final purity profiles. Deionization is highly effective at removing ions (dissolved salts and minerals) that contribute to electrical conductivity, often achieving a higher level of ionic purity.
Distilled water achieves purity by excluding non-volatile compounds through a physical phase change, removing a broad range of inorganic contaminants and eliminating microbes. However, it often retains small amounts of volatile organic compounds (VOCs). Conversely, deionized water, while ionically pure, frequently contains residual non-ionic organic molecules and microbes, as the ion exchange process does not remove them.
Therefore, distilled water is often preferred when the concern is non-volatile inorganic solids and biological material. Deionized water is superior when the primary concern is the presence of charged ions that could interfere with sensitive chemical processes. The two methods are occasionally combined, often following reverse osmosis, to create an ultra-pure product addressing both ionic and non-ionic contaminants.
Specific Uses for Each Type
Distilled water is favored in situations where the presence of mineral scale and biological contaminants must be avoided. This makes it the standard choice for household appliances like steam irons, humidifiers, and CPAP machines, where mineral buildup could cause damage or reduce efficiency.
Deionized water is indispensable in industrial and laboratory environments where the presence of even trace amounts of ions can be detrimental. It is the preferred rinse agent in the manufacturing of semiconductors and electronics, as ions can interfere with sensitive circuitry. In chemical and pharmaceutical laboratories, DIW is used for preparing reagents, diluting samples, and in experiments where ionic interference must be minimized.