Reverse osmosis (RO) lowers the pH of water, making it slightly more acidic. RO is a purification process that uses pressure to force water through a semi-permeable membrane, filtering out nearly all dissolved solids and contaminants. This purification removes the natural substances that normally keep water’s pH stable, leading to a drop in acidity.
Defining pH and Dissolved Solids in Water
The pH value measures the concentration of hydrogen ions (\(\text{H}^+\)) in a solution, determining its acidity or alkalinity on a logarithmic scale from 0 to 14. A pH of 7 is neutral, with values below 7 being acidic and values above 7 being alkaline. Because the scale is logarithmic, a change of one unit represents a tenfold change in acidity.
Natural water sources, such as tap water, contain Total Dissolved Solids (TDS), including minerals like calcium, magnesium, and bicarbonates. These dissolved minerals provide a natural resistance to pH changes, known as buffering capacity. This capacity, primarily due to bicarbonates and carbonates, neutralizes small amounts of acid or base to keep the water’s pH stable, typically between 6.5 and 8.5.
How Reverse Osmosis Alters Water Composition
Reverse osmosis filtration applies pressure to push water across a very fine, semi-permeable membrane. This membrane blocks almost all larger molecules and ions, effectively removing up to 99% of TDS, including contaminants like lead and arsenic, and mineral ions.
The RO process removes mineral salts, such as bicarbonates, which provided the water’s buffering capacity. The resulting demineralized water is “soft” and has lost its ability to resist pH changes. The removal of these stabilizing ions makes the post-RO water highly susceptible to becoming acidic.
The Chemistry Behind Lowered pH
When RO water is exposed to air, its pH drops quickly due to a specific chemical reaction. The water readily absorbs carbon dioxide (\(\text{CO}_2\)) from the atmosphere. Because the water lacks dissolved minerals to buffer the change, the absorbed \(\text{CO}_2\) reacts with the water (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)).
The formation of carbonic acid introduces free hydrogen ions (\(\text{H}^+\)), increasing acidity and lowering the pH. While freshly filtered RO water may initially have a near-neutral pH of 7, exposure to air can cause it to drop to an acidic range, typically between 5.0 and 6.0. This acidic nature results from the absorption of atmospheric \(\text{CO}_2\) by the unbuffered water, not the RO system itself.
Post-Filtration pH Stabilization
The slightly acidic pH of RO water is generally safe for consumption, but it can cause corrosion in metal plumbing systems or be unsuitable for sensitive applications like aquariums. Therefore, post-filtration treatment is often used to stabilize the pH.
The most common method is installing a remineralization filter after the RO membrane. These cartridges contain alkaline minerals, such as calcite or corosex media, which dissolve slowly into the purified water. This reintroduces minerals like calcium and magnesium, restoring the water’s buffering capacity and raising the pH to a neutral or slightly alkaline level, often between 6.5 and 8.5.
Another method is aeration, which strips excess dissolved carbon dioxide from the water. This allows the pH to naturally rise as the acid-forming compound is vented.