Purified water is a broad category, defining water that meets stringent standards for low levels of chemical and biological contaminants. Deionized (DI) water, often called demineralized water, is a specific type of high-purity water created by a distinct method focused on removing charged particles. This means that while all deionized water is a form of purified water, not all purified water is deionized water, as it may have been cleaned using other techniques like distillation or reverse osmosis. The fundamental difference lies in the types of impurities targeted and the technology employed.
What Defines Purified Water?
Purified water is defined by the result of its treatment, meaning it must have a very low concentration of total dissolved solids (TDS). This standard is often measured by electrical conductivity, where a lower conductivity indicates fewer dissolved salts and minerals remaining in the water. For instance, water meeting laboratory standards for purification must often have a resistivity of at least 1 megohm-centimeter, which demonstrates a high degree of purity.
Achieving this high standard typically requires a combination of purification steps, not just a single process. Common methods include initial filtration to remove large particles, followed by reverse osmosis (RO), which uses pressure to force water through a semi-permeable membrane, removing 90–99% of contaminants. Distillation is another technique where water is boiled and the steam is condensed, leaving non-volatile contaminants behind.
The regulatory context for purified water is strict, especially in pharmaceutical and scientific applications, where it must meet specific criteria set by organizations like the United States Pharmacopeia (USP). These standards often include limits on total organic carbon (TOC), microbial counts, and endotoxins, ensuring the water is suitable for use in manufacturing medicines or conducting sensitive experiments. Deionization is frequently used as a final “polishing” step to meet the most demanding purity requirements.
Deionization: A Specific Purification Method
Deionization is a chemical process that specifically targets the removal of dissolved inorganic salts and minerals, which exist in water as electrically charged ions. Water molecules themselves are neutral, but impurities like calcium, sodium, chloride, and sulfate carry a charge. The deionization process utilizes specialized synthetic materials called ion exchange resins to chemically “trade” these charged contaminants for hydrogen (H+) and hydroxyl (OH-) ions.
The water is passed through two types of resin beds: a cation resin, which exchanges positively charged ions for H+ ions, and an anion resin, which exchanges negatively charged ions for OH- ions. These exchanged H+ and OH- ions then combine to form pure water (H2O), effectively removing the mineral content that causes high conductivity. This method is extremely efficient at reducing conductivity to near-zero levels, making the water electrically neutral and highly resistive.
Deionization does not effectively remove non-ionic contaminants. These include uncharged organic molecules, such as pesticides or petroleum byproducts, as well as microorganisms like bacteria and viruses. Because deionization relies solely on electrical charge exchange, it cannot filter or destroy these neutral or biological impurities. Therefore, it must be combined with other purification technologies for applications requiring total contaminant removal.
Key Differences and Practical Uses
Deionized water is uniquely suited for applications where the presence of charged ions would interfere with a process, which is measured by its high electrical resistivity. This type of water is used extensively in manufacturing electronics, where even trace amounts of conductive ions can damage sensitive components. It is also the standard for laboratory rinsing, cooling systems, and filling automotive batteries, where mineral buildup or scaling must be prevented.
General purified water, which often includes a broader removal of contaminants through reverse osmosis or distillation, is commonly used for drinking and in the food and beverage industry. While deionized water is technically pure of ions, its potential to still contain biological or organic contaminants means it is generally not recommended for consumption unless further treated. Therefore, the choice between the two depends entirely on the specific application: ionic purity for technical use, or broad contaminant removal for consumption and general safety.