The pH of filtered water is not a fixed value but depends entirely on the specific filtration method used. The term pH measures the acidity or alkalinity of water, determined by the concentration of hydrogen ions. The scale ranges from 0 to 14, where 7.0 is perfectly neutral. Values below 7.0 are acidic, and values above 7.0 are alkaline, or basic. When water passes through a filter, the removal or addition of dissolved substances directly influences this chemical balance.
Understanding pH and Municipal Water
Municipal water originates from a source where its chemistry is carefully managed. The U.S. Environmental Protection Agency (EPA) recommends that public water systems maintain a pH range between 6.5 and 8.5, which is considered safe for human consumption and plumbing. Tap water typically falls within this range, often around 7.5, making it slightly alkaline.
This relative stability in municipal water is due to its natural buffering capacity, which resists changes in pH. The buffering comes from dissolved minerals, primarily bicarbonates and carbonates of calcium and magnesium (known as water hardness). These alkaline compounds neutralize small amounts of acids or bases, keeping the pH from fluctuating easily. The presence of these mineral buffers sets the stage for how filtration processes will alter the water’s final pH.
The Impact of Different Filtration Systems
Different filtration technologies alter the water’s chemistry in distinct ways, leading to a wide range of possible pH outcomes. Filters that rely on adsorption, such as activated carbon, tend to have the smallest impact on pH. Activated carbon works by trapping contaminants and chlorine on its porous surface but does not remove the dissolved minerals responsible for buffering. Consequently, water passing through a carbon filter or a standard pitcher filter usually maintains a pH very close to the original tap water, remaining neutral or slightly basic.
In contrast, filtration methods that remove nearly all dissolved solids, like reverse osmosis (RO), cause the most significant pH change. RO systems use a semipermeable membrane to strip out ions, including buffering minerals like calcium and magnesium. This de-mineralization process leaves the water without its natural defense against acidity. The purified water then readily absorbs carbon dioxide from the air, which forms a weak carbonic acid. The result is that RO water is typically slightly acidic, with a pH often ranging between 5.0 and 6.5.
Similarly, distillation involves boiling water and collecting the condensed steam, which leaves all minerals behind. Like RO water, this de-mineralized product absorbs atmospheric carbon dioxide and becomes slightly acidic, usually registering between pH 5.5 and 6.0.
Ion exchange systems, commonly used for water softening or alkalizing, swap ions to modify the water’s chemistry. Traditional water softeners exchange hardness ions (calcium and magnesium) for sodium ions, which subtly increases alkalinity. Specialized alkalizing or re-mineralization filters are often added as a final stage to RO systems to raise the low pH. These post-filters introduce beneficial minerals, such as calcium carbonate, back into the water to restore buffering capacity and raise the pH to a neutral or slightly alkaline range, often between 7.0 and 8.5.
Why Water pH Matters
The pH of filtered water carries practical implications, particularly concerning the plumbing in a home. Water with a low pH is considered corrosive, meaning it can dissolve metals from pipes and fixtures. Acidic water, such as that produced by RO or distillation, can leach metals like copper and lead from household plumbing components. This is the primary practical concern for homeowners using these filtration methods, as it can introduce harmful contaminants into the drinking water.
While the low pH of filtered water is a concern for plumbing, it rarely poses a direct health risk to the consumer. The human body is highly effective at regulating the pH of the blood and tissues, making the slight acidity of filtered water negligible. However, low-pH water can affect the taste, often being described as flat or slightly sour due to the lack of minerals that provide a more palatable flavor. Homeowners concerned about acidity can use simple tools like pH test strips or digital meters to check water quality.