Filtering water can change its pH, which measures how acidic or alkaline the water is on a scale of 0 to 14. A pH below 7 indicates acidity, while a pH above 7 indicates alkalinity, with 7 being neutral. The extent of this change depends entirely on the specific filtration technology used. Some filters leave the pH largely unaffected, while others purposefully remove the substances that stabilize it, resulting in a measurable shift.
The Mechanism of pH Change in Water
The primary reason water’s pH can shift during filtration relates to its natural buffering capacity. This capacity is largely determined by the presence of dissolved minerals, specifically bicarbonates and carbonates, which are the main components of alkalinity. Alkalinity refers to the water’s ability to neutralize acids and resist changes to its pH level.
These buffering compounds are part of the water’s Total Dissolved Solids (TDS). When a filtration method removes a significant amount of these buffering minerals, the water becomes chemically unstable. This “de-buffered” water is then highly susceptible to external influences, such as absorbing carbon dioxide from the air, which can rapidly lower the pH. Removing or altering these compounds is the fundamental mechanism by which filtration systems affect the water’s acidity or alkalinity.
Filtration Methods with Minimal pH Impact
Certain filtration methods are designed to remove suspended solids, microorganisms, and organic chemicals without significantly disrupting the water’s mineral balance. These filters leave the natural alkalinity largely intact, meaning the final pH remains close to that of the source water. The two most common types in this category are sediment filters and activated carbon filters.
Sediment filters physically strain out large particles like sand, rust, and dirt. Since they do not remove dissolved ions, they have no lasting effect on the pH or buffering capacity. Activated carbon filters work primarily through adsorption, where chemical contaminants like chlorine and volatile organic compounds stick to the carbon’s surface. Because these filters largely ignore dissolved minerals, the long-term, stable pH of the filtered water is minimally changed.
New activated carbon filters may sometimes cause a temporary pH spike, potentially reaching 9.5 to 10.5, due to the leaching of inorganic ash residues. This effect is short-lived and dissipates as the soluble ash is rinsed out during the first few uses. After this initial period, the carbon filter treats the water with little to no permanent alteration of its pH.
Filtration Methods That Significantly Alter pH
Filtration techniques that specifically remove dissolved ions will significantly and consistently alter the water’s pH by eliminating the buffering capacity. Reverse Osmosis (RO) and distillation are two processes that produce highly purified water by removing nearly all dissolved solids, including the minerals responsible for alkalinity. By stripping away these buffering compounds, the water becomes highly reactive.
Reverse Osmosis and Distillation
The water leaving an RO system or a distiller is nearly pure. While theoretically neutral with a pH of 7, it immediately begins to absorb carbon dioxide (CO2) from the atmosphere. This dissolved CO2 reacts with water (H2O) to form a weak acid called carbonic acid (H2CO3), which rapidly lowers the water’s pH. As a result, RO and distilled water is typically slightly acidic, often measuring in the range of pH 5.0 to 6.5.
Ion Exchange Systems
Ion exchange systems, such as water softeners, also change the mineral composition, but they typically do not significantly lower the pH. These systems replace hardness ions like calcium and magnesium with sodium or potassium ions. This process can actually increase the water’s alkalinity. The sodium combines with bicarbonate, which slightly enhances the water’s buffering capacity. Softened water generally remains in a neutral to slightly alkaline pH range, typically between 6.5 and 7.8.