The process of creating distilled water is straightforward, relying on a simple physical separation method. Water is heated until it turns into steam, leaving behind nearly all dissolved minerals, salts, and non-volatile impurities. The resulting pure steam is collected and cooled, condensing back into liquid water highly free of contaminants. This process is extremely effective at removing minerals like calcium and magnesium. However, not all commercially available distilled water is identical, as differences emerge in final purity levels and post-production handling.
Purity Standards and Grades
The perception that all distilled water is the same is dismissed when considering the rigorous purity standards governing various industries. Purity is scientifically quantified, often by measuring the water’s electrical conductivity or its Total Dissolved Solids (TDS). Since pure water is a poor conductor of electricity, a lower conductivity reading indicates a higher degree of purity, reflecting the absence of conductive ions. For instance, typical consumer-grade distilled water often has a TDS of less than 1 part per million (PPM) and a conductivity below 2 microsiemens per centimeter (\(\mu\)S/cm).
Specialized industries, such as pharmaceuticals and high-tech laboratories, require water that meets far stricter certification protocols. The United States Pharmacopeia (USP) sets standards for grades like Purified Water, which must meet precise limits for conductivity and Total Organic Carbon (TOC). These high-specification grades are subjected to consistent testing and validation, ensuring that even minute traces of organic compounds or specific ions remain below established thresholds. The difference between consumer water and certified USP grade lies in the stringent quality control and documentation required for specialized applications.
Post-Distillation Handling and Re-Contamination
Even if two batches of water start with the same purity level immediately after distillation, they can become different during storage and distribution. Distilled water, due to its mineral-free nature, is highly reactive and readily seeks to dissolve substances it contacts. This characteristic makes it vulnerable to re-contamination once exposed to the atmosphere or packaging materials.
A common form of re-contamination is the absorption of atmospheric carbon dioxide (\(\text{CO}_2\)). When exposed to air, the water quickly absorbs \(\text{CO}_2\), which reacts to form a small amount of carbonic acid. This reaction reduces the water’s pH, making it mildly acidic, and simultaneously increases its electrical conductivity. The increase in conductivity means the water is no longer at its initial peak purity.
The material used for packaging can also introduce trace contaminants over time through leaching. Plastic storage containers can release minute amounts of organic compounds into the ultrapure water, especially with prolonged storage or exposure to temperature fluctuations. This effect is why pharmaceutical-grade waters often require specific containers and continuous monitoring, as minor leaching can compromise the water’s integrity for sensitive applications.
Practical Implications for Consumer Use
For the average consumer, the differences between commercial distilled water products are often negligible for most household uses. Applications like steam irons, automotive batteries, and non-critical humidifiers primarily require the absence of minerals to prevent scale buildup. This standard is achieved by any store-bought distilled water, as trace amounts of \(\text{CO}_2\) or plastic leaching do not impact the water’s ability to prevent mineral deposits.
However, higher purity is important for medical devices and other critical technologies. Continuous Positive Airway Pressure (CPAP) machines, for example, rely on distilled water in their humidifiers to prevent mineral scale accumulation that could damage the heating element or water chamber. While generic distilled water is usually sufficient, individuals with sensitive medical equipment may prefer water certified to specific standards. The final purity and subsequent handling determine whether the product is suitable for a simple steam iron or a specialized laboratory procedure.