Reverse osmosis (RO) is a highly effective water purification method that removes nearly all contaminants, producing water that is close to pure H₂O. While this purity is beneficial, it often results in water with a low, or acidic, pH. This occurs because the RO process strips the water of its natural mineral buffers, making the water chemically unstable and susceptible to pH changes. Adjusting the pH is necessary for preventing corrosion in plumbing and appliances, improving the taste of drinking water, and ensuring suitability for specific applications like hydroponics or aquariums.
Understanding the Low pH of RO Water
The reason reverse osmosis water is often acidic is directly related to what the filtration process removes. The RO membrane removes almost all dissolved solids, including beneficial minerals such as calcium and magnesium bicarbonates. These minerals act as natural buffers, maintaining a stable pH level in the water supply.
When these buffering agents are absent, the water is left with a near-neutral pH of 7 immediately after filtration. However, the purified water is highly reactive and quickly begins to absorb carbon dioxide (CO₂) from the surrounding air or dissolved CO₂ in the source water. Once dissolved, the carbon dioxide reacts with water molecules to form carbonic acid (H₂CO₃).
This formation of carbonic acid introduces hydrogen ions, which lowers the pH, typically resulting in a range between 5.0 and 6.5. This demineralized and slightly acidic state is referred to as “hungry water” because of its tendency to absorb whatever it contacts, including trace metals from plumbing systems. The lack of minerals can also give the water a flat or slightly metallic taste, making it less desirable for drinking.
Raising pH Using Dedicated Remineralization Filters
The most convenient and effective way to consistently raise the pH of RO water is through dedicated inline remineralization, or alkaline, filters. These cartridges are installed as a final stage in the RO system, ensuring the pH adjustment is automatic and continuous. The filters contain natural, food-grade mineral media that slowly dissolves back into the purified water as it passes through.
The media typically consists of alkaline compounds like calcium carbonate (calcite) and magnesium oxide. As these compounds dissolve, they replenish the water’s mineral content, restoring the lost buffering capacity. This process raises the pH from the acidic range back toward neutral, often reaching the mildly alkaline range of 7.5 to 8.5.
The reintroduction of minerals serves a dual purpose by improving the taste and mouthfeel of the water. The added calcium and magnesium are essential electrolytes. These specialized filters provide a long-term solution requiring minimal user intervention beyond periodic cartridge replacement, typically every six to twelve months. The stability and consistency offered by these post-filters make them the preferred method for homeowners using RO water for consumption.
Simple Techniques for pH Adjustment
While dedicated inline filters offer a permanent solution, several simpler, manual techniques can be used to raise the pH of RO water in smaller batches. One physical method is aeration, which involves vigorously stirring or bubbling air through the water. This turbulence facilitates the outgassing of the dissolved carbon dioxide that formed the carbonic acid.
As the CO₂ leaves the water, the concentration of carbonic acid decreases, driving the pH upward. Aeration is a purely physical process that raises the pH without increasing the Total Alkalinity, making it useful for applications like aquariums where mineral content must be precisely controlled. Simple aeration methods include using an air stone or pouring the water repeatedly between two containers.
Chemical buffering is another option, often utilizing common food-grade additives. A small, carefully measured amount of baking soda (sodium bicarbonate) can be dissolved into the water to raise the pH. The bicarbonate acts as a buffer, neutralizing the acidity and slightly increasing the mineral content. Alternatively, specialized food-grade mineral drops containing concentrated calcium, magnesium, and potassium provide both pH adjustment and mineral enrichment.
When using any chemical additive, precision is necessary. Adding too much can result in an excessively high pH or an unpalatable taste, so begin with very small amounts and monitor the change.
Testing and Target pH Levels
Accurately measuring the pH of purified RO water is a critical step in the adjustment process. Because pure RO water lacks mineral buffers, its pH is highly sensitive and difficult to measure reliably with simple paper strips. The most accurate tools for monitoring are quality digital pH meters or liquid test kits that use color comparison charts.
The optimal target pH level depends entirely on the water’s intended use. For drinking water, the goal is a neutral to mildly alkaline range, typically between 7.0 and 8.5, to improve taste and counter corrosive tendencies. Although the Environmental Protection Agency (EPA) does not regulate drinking water pH, levels between 6.5 and 8.5 are considered safe.
For specialized uses, the targets vary. Hydroponics and general gardening often require a slightly acidic range, with optimal pH for nutrient uptake falling between 5.5 and 6.5. In the aquarium hobby, the specific needs of the aquatic life dictate the target. Regular testing ensures the chosen adjustment method consistently achieves the desired pH for the specific application.