Reverse osmosis (RO) is a purification method that forces water through a semi-permeable membrane to remove contaminants, dissolved minerals, and salts. This process yields highly purified water that is low in total dissolved solids (TDS). A frequent question concerns the water’s potential of hydrogen (pH), which measures its acidity or alkalinity. Understanding the pH of RO water is important for the longevity of plumbing systems and for individuals concerned about water quality.
The Typical pH Range of Reverse Osmosis Water
Reverse osmosis water, when tested immediately after filtration, is often slightly acidic, typically falling into a pH range of 5.0 to 6.5. While pure water theoretically has a neutral pH of 7.0, the nature of the RO process makes this reading uncommon in a household setting. The pH of RO water is consistently below 7.0. Most municipal drinking water falls within the Environmental Protection Agency (EPA) recommended range of 6.5 to 8.5, highlighting the slight acidity of RO water in comparison. This lower pH is a direct consequence of the filtration’s effectiveness in stripping the water of nearly everything but H2O molecules.
Why Reverse Osmosis Affects Water pH
The shift toward acidity in RO water is a chemical reaction involving the atmosphere and the water’s lack of mineral content. Standard tap water contains mineral ions, such as calcium and magnesium, which act as natural buffers to stabilize the water’s pH. The reverse osmosis membrane effectively removes these buffering agents, leaving the water demineralized. This lack of buffering capacity means the water cannot resist changes to its pH level.
When the purified water is exposed to air, it readily absorbs atmospheric carbon dioxide (CO2). This absorbed CO2 reacts with water (H2O) to form carbonic acid (H2CO3). The presence of carbonic acid introduces hydrogen ions into the water, which lowers the pH and makes the water slightly acidic. This process can cause the pH of freshly filtered RO water to drop from a neutral 7.0 down to 5.0 or 5.5.
Safety and Consumption Considerations
The slightly acidic nature of reverse osmosis water often raises concerns about its safety for consumption, but the acidity level is generally not a health risk. The human stomach is an acidic environment, typically operating at a pH between 1.5 and 3.5 to aid digestion. When mildly acidic RO water is consumed, it has a negligible effect on the body’s systemic pH. The body’s pH is tightly regulated by the kidneys and lungs to remain slightly alkaline, around 7.4. The acidity of RO water is far less than that of common beverages like coffee, tea, or fruit juice.
The primary consideration with RO water is its lack of mineral content, not its mild acidity. Since the RO process removes nearly all dissolved solids, it also removes minerals like calcium and magnesium, which are recognized sources of dietary intake. For healthy individuals with a balanced diet, this mineral deficiency is not usually problematic. However, long-term reliance on demineralized water may require greater dependence on food sources for these essential nutrients.
Concerns about acidic water causing corrosion are more relevant to the plumbing system than to human health. Water with a pH below 6.5 can be corrosive, potentially leaching metals like copper and lead from pipes into the drinking water. While this is a concern for the integrity of the plumbing, the health risk comes from the leached heavy metals, not the water’s acidity itself.
Practical Methods for pH Correction
Consumers who wish to counteract the slight acidity of RO water often seek to increase its pH through remineralization. The most common method for home systems is the installation of a post-filter remineralization cartridge. These cartridges are typically added as a final stage in the filtration system, positioned after the RO membrane.
These filters contain media such as calcium carbonate, often in the form of calcite, which slowly dissolves into the purified water. As the water passes through, it picks up these minerals, which raise the pH and restore the water’s buffering capacity. This process moves the water’s pH closer to a neutral or slightly alkaline range of 6.5 to 8.5. Other methods include adding mineral drops directly to the drinking water or blending a small portion of untreated tap water back into the final RO product.