The pH of tap water measures its acidity or alkalinity, determined by the concentration of hydrogen ions (H+) present in the solution. The pH scale runs logarithmically from 0 to 14, where 7 is neutral. Values below 7 indicate increasing acidity, and values above 7 indicate increasing alkalinity. For public water systems, the recommended target range is typically between 6.5 and 8.5, ensuring the water is slightly alkaline to protect infrastructure and public health.
Why Maintaining Water pH is Essential
Controlling the pH of drinking water primarily protects the public water distribution system from physical degradation. Water that is too acidic (pH below 6.5) becomes corrosive, actively dissolving the metals that make up pipes and fixtures. This corrosion causes infrastructure damage, leading to expensive repairs and premature pipe replacement throughout the system.
A health concern arises when low pH water corrodes plumbing materials and leaches toxic heavy metals, such as lead and copper, into the drinking supply. Acidity mobilizes these metals from solder, brass fixtures, and lead service lines, contaminating the water delivered to homes. Regulating the pH to a slightly alkaline level helps form a protective mineral coating, or scale, on the interior of the pipes, which minimizes metal leaching.
The effectiveness of disinfectants used in water treatment is highly dependent on maintaining a specific pH range. Chlorine, the most common disinfectant, works most efficiently in a slightly acidic to neutral range, ideally between 6.5 and 7.5. If the pH rises too high, the form of chlorine available for disinfection changes, significantly reducing its ability to eliminate harmful pathogens.
The aesthetic quality of the water is also influenced by pH levels. Strongly acidic water can impart a metallic or sour taste, while highly alkaline water may taste bitter or feel slick. Extreme pH levels can also cause discoloration, such as blue-green staining from dissolved copper, or interfere with the lathering of soap.
Factors That Determine Local pH Levels
The natural pH of local water is determined by its source and the geology of the area it passes through. Surface water (lakes and rivers) often has a more variable pH because atmospheric carbon dioxide dissolves to form weak carbonic acid. Groundwater (wells and aquifers) tends to have a higher, more stable pH because it interacts with mineral-rich bedrock.
Water that flows through regions with limestone or other carbonate materials picks up calcium and magnesium, which act as buffers to increase alkalinity. Conversely, water sourced from areas with granite or inert rock types will be softer and more prone to acidity. This geological interaction explains why pH levels can vary considerably between different cities or adjacent communities drawing water from different sources.
Water treatment facilities actively adjust the pH to meet safety and infrastructure targets before distribution. When source water is acidic, alkaline chemicals are added to raise the pH and stabilize the water. Common chemicals used include lime, soda ash (sodium carbonate), or caustic soda (sodium hydroxide).
The treatment process often introduces fluctuations, as disinfectants like chlorine can slightly lower the pH. Operators must continuously monitor and adjust the water chemistry to counteract these effects and maintain the optimal pH for corrosion control. Seasonal changes also affect source water, with heavy rainfall and snowmelt runoff potentially introducing organic matter that temporarily lowers pH levels.
Practical Steps for Testing and Adjustment
Consumers can measure the pH of their tap water at home using several accessible methods. The simplest and most inexpensive options are pH test strips or liquid reagent kits, which involve matching a color change to a provided chart. While these provide a general idea of the pH range, they are not highly precise and may be less reliable for water with low mineral content.
For more accurate measurements, a digital pH meter, which uses a glass electrode to measure hydrogen ion activity, is the preferred tool. These meters offer a precise numerical reading but require regular calibration using standardized buffer solutions to ensure accuracy. Proper storage of the probe in a solution is necessary to prevent inaccurate readings over time.
Home adjustment of tap water pH is typically only necessary for specific applications, such as managing a home aquarium, brewing beer, or if testing reveals levels outside the 6.5 to 8.5 range. If water is acidic (low pH), a homeowner can install a neutralizing filter that releases calcium carbonate (calcite) into the water. This process slowly dissolves the mineral media, raising the pH and making the water less corrosive.
If the water is too alkaline (high pH), whole-house solutions are more limited. A common method to lower the pH for drinking water is using a reverse osmosis (RO) filtration system. While RO is primarily used for contaminant removal, the resulting purified water often has a slightly lower, more neutral pH. For extremely high pH levels that cause scaling, specialized acid injection systems can be used, though these systems are complex and require professional installation and maintenance.