Letting tap water sit out is a simple, passive method that can reduce the concentration of chlorine, a common disinfectant used by municipal water systems to ensure the water supply is free of harmful microorganisms. The effectiveness of this technique depends entirely on the specific chemical used in your local water treatment. For this method to work, the water must contain volatile free chlorine, which naturally escapes the liquid phase and dissipates into the surrounding air over time. The rate at which this occurs is influenced by several factors, including the water’s temperature and the surface area exposed to the atmosphere.
How Volatile Chlorine Escapes Water
The principle behind chlorine removal by letting water sit is a chemical process called off-gassing, or volatilization. In municipal water, chlorine exists primarily as an equilibrium mixture of hypochlorous acid (HOCl) and the hypochlorite ion (OCl⁻), collectively known as free chlorine. Hypochlorous acid is the more volatile form and is the component that escapes from the water.
The chlorine compound is unstable and seeks to establish an equilibrium with the surrounding air. Since the concentration of chlorine gas in the atmosphere is essentially zero, the chlorine dissolved in the water continually moves across the water’s surface and into the air. This movement is driven by the difference in concentration, and the process continues until the concentration drops to a negligible level. For water treated only with free chlorine, a significant reduction typically occurs within 24 to 48 hours of exposure to the air.
Practical Variables Affecting Removal Time
The speed at which free chlorine off-gasses can be manipulated by altering the physical conditions of the water. One of the most effective ways to accelerate the process is by maximizing the water’s surface area. Using a wide-mouth container, such as a bowl or a bucket, exposes a larger surface to the air compared to a narrow-necked bottle, allowing more chlorine molecules to escape simultaneously.
Temperature plays a significant role in volatility, as warmer water causes molecules to move faster. Increasing the water temperature accelerates the transition of dissolved chlorine from the liquid phase to the gas phase, reducing the time required for removal. Another practical technique is aeration, which involves physically agitating the water by stirring it or pouring it repeatedly between two containers. This action creates more contact between the water and the air, driving the dissolved chlorine out of the solution and speeding up the off-gassing process.
The Difference Between Chlorine and Chloramines
The most important factor determining if simply letting water sit will work is the specific disinfectant chemical used by the water utility. While some systems use free chlorine, a growing number of municipalities now use chloramines, which are chemical compounds formed by combining chlorine with ammonia. Chloramines are a more stable disinfectant than free chlorine, providing a longer-lasting residual effect as water travels over long distances.
This increased stability is what makes the sitting method ineffective for chloramine removal. Chloramine molecules are non-volatile and significantly less prone to off-gassing compared to free chlorine. While water treated with free chlorine may become nearly chlorine-free in a day or two, chloramine-treated water can take several days to weeks for a noticeable reduction through evaporation alone. For sensitive applications, such as maintaining an aquarium or home brewing, relying on the off-gassing of chloramines is impractical. In these cases, chemical neutralizers or specialized carbon filters are required to break the strong chemical bond between the chlorine and ammonia.