Demineralized water and distilled water are both highly purified forms of water, but they are not identical. The difference lies in the specific purification methods used to create them, which results in distinct purity profiles.
How Distillation Purifies Water
Distillation is a physical separation process that mimics the natural hydrologic cycle on a small scale. The process begins by heating source water to its boiling point, converting the liquid water into steam. This phase change leaves behind virtually all non-volatile substances, including dissolved inorganic minerals, salts, heavy metals, and large organic molecules.
The purified steam travels into a separate chamber where it cools and condenses back into liquid water. This method effectively kills and separates microorganisms like bacteria and viruses from the final product. While highly effective against solids and biological contaminants, distillation may not remove volatile organic compounds (VOCs) that possess a boiling point lower than that of water, as these can vaporize along with the water and re-contaminate the condensate.
How Demineralization Purifies Water
Demineralization, often called deionization (DI), relies on a chemical process to remove dissolved mineral ions and salts. This purification is achieved by passing water through specialized synthetic resins known as ion exchangers. The system utilizes two main types of resin: one for positively charged ions and one for negatively charged ions.
In the cation exchange step, positively charged mineral ions like calcium, magnesium, and sodium are captured by the resin and exchanged for hydrogen ions (H+). Next, the water passes through an anion exchange resin, which captures negatively charged ions such as chloride, sulfate, and nitrate, swapping them for hydroxide ions (OH-). The resultant hydrogen and hydroxide ions combine to form a pure water molecule (H2O), leaving the water free of dissolved salts.
This process is highly efficient at removing ions, which are the primary cause of electrical conductivity in water. Reverse osmosis (RO), which uses high pressure to push water through a semi-permeable membrane, is frequently used as a pre-treatment step before the final ion-exchange polishing.
Key Differences in Purity Profiles and Uses
Distilled water is characterized by its overall purity, having removed solids, microbes, and nearly all non-volatile contaminants through boiling and condensation. This makes it a preferred choice for medical applications, such as preparing pharmaceuticals, and sensitive laboratory reagents where a broad range of purity is required.
Demineralized water, conversely, is defined by its low electrical conductivity, achieved by the removal of charged ions. While it excels in ionic purity, it may still contain non-ionic contaminants, organic molecules, viruses, or bacteria. Consequently, demineralized water is essential for industrial applications like electronics manufacturing and use in high-pressure boilers, where conductive minerals can cause corrosion or scaling.
The method of production also affects cost, with demineralization generally being more energy-efficient and cost-effective for producing large volumes of water. Ultimately, the selection depends on the specific purity requirement: distillation provides broad-spectrum purity, while demineralization delivers the benefit of ultra-low ionic content.