Reverse osmosis (RO) is an extremely effective method for purifying water, capable of removing a wide spectrum of contaminants. This high level of filtration often results in water that registers a low pH, meaning the water is slightly acidic. This acidity stems from a specific change in water chemistry that occurs when nearly all impurities are stripped away. Understanding why RO water becomes acidic requires examining both the filtration process and the fundamental chemical reactions that govern water’s pH balance.
The Function of Reverse Osmosis Filtration
Reverse osmosis is a purification process that relies on pressure to force water molecules through a highly selective, semi-permeable membrane. This membrane has extremely small pores, acting as a barrier to dissolved particles, ions, and larger molecules. The applied pressure must overcome natural osmotic pressure to push the water against its concentration gradient, leaving contaminants behind.
This process is exceptionally efficient at reducing the concentration of Total Dissolved Solids (TDS). TDS includes inorganic salts, metals, and minerals like calcium, magnesium, sodium, and chloride. RO systems typically remove 95% to 99% of these dissolved salts and ions, producing water that is close to pure H2O.
Stripping the water of most dissolved inorganic matter fundamentally alters the water’s chemical makeup. This demineralization is beneficial for purity but sets the stage for the subsequent pH drop. The water loses the components that naturally keep its pH stable.
Defining Water Acidity and the Carbon Dioxide Link
Water acidity is measured using the pH scale, which quantifies the concentration of free hydrogen ions (H+) in a solution. A neutral pH is 7.0; values below 7.0 indicate increasing acidity due to a higher concentration of these ions. Pure water, free of dissolved substances, has a theoretical neutral pH of 7.0.
When water is exposed to the atmosphere, it immediately begins to absorb carbon dioxide (CO2) gas. This atmospheric absorption is a constant natural process. When CO2 dissolves into water (H2O), a chemical reaction occurs to form carbonic acid (H2CO3).
This reaction is represented by the formula CO2 + H2O yields H2CO3. Carbonic acid is a weak acid, but it quickly dissociates in the water. This dissociation releases hydrogen ions (H+) and bicarbonate ions (HCO3-).
The introduction of these free hydrogen ions lowers the pH, making the water acidic. Even clean rainwater, which has absorbed atmospheric CO2, typically registers a pH between 5.0 and 5.5 due to this natural carbonic acid formation. This chemical process explains why any highly purified water, including RO water, shows an acidic reading simply from contact with air.
The Critical Role of Buffering Capacity
The primary reason RO water registers a significantly lower pH than typical tap water is the complete removal of its buffering capacity. Buffering capacity refers to the water’s ability to resist changes in pH when an acid or base is introduced. This resistance is provided by dissolved alkaline minerals, predominantly carbonates and bicarbonates, which are stripped out during the RO process.
In typical tap water, these alkaline ions act as a natural defense system against the hydrogen ions released by carbonic acid. When CO2 from the air forms carbonic acid, the bicarbonate ions (HCO3-) neutralize the resulting hydrogen ions (H+). This reaction effectively consumes the acid, keeping the water’s pH stable and near neutral, often between 7.0 and 8.5.
RO water is demineralized, meaning it contains negligible amounts of these buffering agents. Because the protective minerals are absent, the slight amount of carbonic acid formed by atmospheric CO2 immediately causes a disproportionate drop in pH. The low mineral content makes the purified water extremely sensitive to any acidic input, resulting in a pH reading that can drop rapidly to 5.5 or even lower.
This lack of buffering capacity explains why RO water is uniquely acidic compared to unpurified water. The water itself is not inherently acidic when created, but its purity makes it chemically reactive and unable to counteract the slight acidity introduced by dissolved carbon dioxide. This can cause the water to become corrosive to metal plumbing and fixtures over time.
Neutralizing RO Water: Post-Filtration Adjustments
The slightly acidic nature of RO water, while not a health concern in itself, can cause practical issues like the corrosion of metal pipes, fittings, and storage tanks. For this reason, many filtration systems include a final stage to raise the pH and restore some level of buffering capacity. This process is generally referred to as remineralization or post-filtration adjustment.
The most common solution involves installing a remineralization cartridge after the main RO membrane. These cartridges contain alkaline media, typically a blend of calcium carbonate (calcite) and sometimes magnesium oxide or dolomite. As the acidic RO water flows through this media, it dissolves a small, controlled amount of these minerals.
This reintroduces beneficial minerals back into the water, serving two primary functions. The alkaline minerals raise the pH back toward a neutral or slightly alkaline range, often stabilizing it between 7.0 and 8.5. They also restore the buffering capacity, making the water less susceptible to future pH drops caused by dissolved CO2.
Other Adjustment Options
Other options for pH adjustment include adding concentrated mineral drops, which contain electrolytes like calcium and magnesium, directly to a glass or pitcher of water. For whole-house systems, specialized neutralizer tanks containing mineral media can be used to treat the entire water supply. This ensures the water is non-corrosive before it enters the plumbing.