The phrase “Alkaline Reverse Osmosis (RO) water” is commonly used, but it combines a purification process (RO) with a separate water property (alkalinity). Reverse osmosis is a technology designed for water purity, while alkalinity measures the water’s pH level and its ability to resist pH changes. This combination often leads to confusion, as many assume the filtration process itself creates the alkaline characteristic. Understanding this relationship requires separating the mechanics of the RO system from the chemical state of the water it produces.
Defining Reverse Osmosis Filtration and Water Alkalinity
Reverse Osmosis is a method of water purification that uses pressure to force water molecules through a semi-permeable membrane. This membrane blocks nearly all dissolved solids, contaminants, and large molecules, effectively removing impurities like heavy metals, salts, and chemicals. The process yields exceptionally clean water, often referred to as demineralized due to its very low Total Dissolved Solids (TDS) count.
Water alkalinity is a measure of the water’s pH on a scale of 0 to 14, indicating how acidic or basic it is. Water above a pH of 7.0 is alkaline, below 7.0 is acidic, and 7.0 is neutral. This alkaline property is chemically determined by buffering minerals, primarily calcium, magnesium, and potassium. These mineral compounds neutralize acids, allowing the water to maintain a stable, higher pH level.
The Direct Answer: The pH of Water After Reverse Osmosis
The reverse osmosis process itself does not produce alkaline water; in fact, the opposite is typically true. Water fresh out of a standard RO system is generally slightly acidic, often measuring between a pH of 5.0 and 6.5. This result is a direct consequence of the membrane’s efficiency at removing nearly everything but the water molecules.
The removal of minerals like calcium and magnesium eliminates the natural buffers that stabilize the water’s pH. Without these buffering agents, the water becomes highly susceptible to changes in its chemical balance. The purified water also readily absorbs atmospheric carbon dioxide (\(\text{CO}_2\)) from the air it contacts in the storage tank or dispensing faucet.
When \(\text{CO}_2\) dissolves into the demineralized water, it forms a weak solution of carbonic acid (\(\text{H}_2\text{CO}_3\)). This chemical reaction is the primary reason the pH of RO water drops into the acidic range. The slight acidity is not a health concern, as many common beverages like coffee and orange juice are far more acidic. The pH drop illustrates that purity and alkalinity are separate and often opposing properties in water chemistry.
Creating Alkaline Water: Remineralization and Ionization
To achieve an alkaline state in RO water, manufacturers must introduce an additional step after the initial filtration. This post-treatment process is known as remineralization and is necessary to counteract the acidic nature of the pure RO water. Remineralization is typically accomplished by installing a dedicated alkaline filter cartridge as the final stage of the RO system.
These specialized filters contain beds of alkaline compounds, such as calcium carbonate, magnesium oxide, or a blend of other beneficial minerals. As the purified, slightly acidic RO water flows through the cartridge, it dissolves the minerals. This raises the pH to a slightly alkaline level, usually between 8.0 and 9.5. This process not only increases alkalinity but also restores a small amount of the dietary minerals lost during filtration.
A different method for increasing alkalinity is water ionization, which uses electrolysis. An ionizer device runs the water over electrically charged plates to separate it into two streams: one highly alkaline and one acidic. While ionization does not add minerals like a remineralization filter, the process concentrates the existing alkaline minerals, elevating the pH of the drinking stream. Ionizers often work best when installed after an RO system to ensure the source water is contaminant-free.
Comparing RO Water and Enhanced Alkaline Water
The two final products—standard RO water and enhanced alkaline RO water—offer distinct characteristics. Standard RO water is defined by its exceptional purity and its slightly acidic, mineral-free composition. Many find its taste flat or empty because the minerals that contribute to flavor have been removed.
Alkaline RO water is a two-stage product: purified first and then chemically enhanced. The reintroduction of alkaline minerals gives the water a smoother, more palatable taste that many consumers prefer over demineralized water. This improved taste is often the most noticeable benefit of the alkaline enhancement stage.
While marketing often highlights the potential health benefits of alkaline water, the most practical difference for most people is the change in flavor and the addition of dietary minerals like calcium and magnesium. The enhanced water provides the benefit of high-purity filtration combined with the sensory experience of mineralized, higher-pH water. The choice between the two often depends on whether a consumer prioritizes absolute purity and cost-effectiveness or a combination of purity, improved taste, and mineral content.