Distilled water is water purified by boiling it into steam and then condensing it back into a liquid, a process that effectively removes minerals, salts, and most contaminants. This results in nearly pure H₂O, making it suitable for sensitive applications like medical devices and laboratory work. However, this purity makes the water susceptible to rapid degradation once the container is opened and exposed to the surrounding air. The primary concern is how quickly the water loses its purified state, which directly impacts its usable life.
The Science of Purity Loss
The purity of distilled water means it is highly reactive and readily absorbs substances from its environment. The most immediate chemical change is the absorption of carbon dioxide (CO₂) from the air, which dissolves into the water to form carbonic acid. This reaction causes the water’s pH level to drop from a neutral 7 toward an acidic range, sometimes reaching a pH between 5.5 and 6.5 within 24 hours of exposure. This slight acidification is a significant compromise for high-purity applications, as it increases the water’s conductivity and corrosive potential for certain materials.
A second major mechanism of purity loss is microbial contamination, which occurs through exposure to airborne bacteria, mold spores, and dust particles. Although distilled water lacks the nutrients to support robust microbial growth initially, contaminants can be introduced from the air, unsterile containers, or even contact with hands near the opening. If the water is then stored in conditions that promote growth, such as warmth or bright light, bacteria and algae can begin to thrive. Studies indicate that a significant percentage of opened distilled water samples can show signs of bacterial growth within a month.
Optimal Storage Practices
To maximize the usable life of opened distilled water, storage practices must minimize contact with air and inhibit microbial growth. The choice of container is important; glass or high-density polyethylene (HDPE) containers are preferable because they are non-reactive and less likely to leach chemicals into the water compared to low-grade plastics. Regardless of the material, the container must be clean and always sealed tightly with an airtight lid immediately after use to limit the absorption of airborne contaminants and CO₂.
Temperature control is another factor, as storing the water in a cool, dark environment helps to inhibit the growth of microorganisms. The ideal temperature range for storage is typically between 59°F and 77°F (15°C to 25°C), and the container should be kept away from direct sunlight, which can encourage algae growth. Proper handling is also important, meaning users should avoid touching the container opening or pouring spout to prevent the transfer of bacteria from the skin.
When stored using these optimal practices, opened distilled water can maintain acceptable quality for general household use for an extended period. A concrete timeline for high-purity applications is about 30 days, provided the container remains airtight and cool. For less sensitive purposes, the water can often be used for several months as long as there are no visible signs of spoilage.
Application-Specific Guidelines
For applications requiring the highest degree of purity, such as Continuous Positive Airway Pressure (CPAP) machines, medical nebulizers, and laboratory work, the risk of using degraded water is high. In these scenarios, the presence of even small amounts of introduced bacteria or the slight acidity from CO₂ absorption can compromise health or damage delicate equipment. For medical and high-purity uses, it is often recommended to discard opened water after 7 to 14 days, even with good storage, to mitigate these risks.
For low-purity applications, the water’s shelf life is considerably longer because the main concern is preventing mineral buildup, not microbial load. Devices like steam irons, car batteries, and general household cleaning tasks can tolerate the minor chemical changes that occur after opening. If stored correctly, the water can be used for these purposes for up to six months or longer, assuming it remains visually clear.
Regardless of the intended use, the water should be discarded immediately if any visual or olfactory signs of contamination are present. These signs include cloudiness, visible sediment, or any unusual odor.