When people use copper vessels or have copper plumbing, a frequent question arises about whether the metal changes the water’s chemical nature. Specifically, many wonder if storing water in copper will make it more alkaline. This query is rooted in traditional practices and the known chemical reactivity of copper. The following analysis explains the science of how copper interacts with water, focusing on the resulting effect on its acid-base balance and the practical context of drinking water safety.
Defining pH and Alkalinity
To understand the effect of copper, it is helpful to first distinguish between two related but distinct terms: pH and alkalinity. The pH scale measures the concentration of hydrogen ions (\(\text{H}^+\)) in a solution, determining its acidity or basicity on a logarithmic scale from 0 to 14. A value below 7.0 indicates acidity, 7.0 is neutral, and a value above 7.0 is considered basic or alkaline.
Alkalinity, on the other hand, is not a measure of the current pH level but rather the water’s capacity to neutralize acids, often called its buffering capacity. This capacity is primarily determined by the concentration of dissolved compounds like bicarbonates, carbonates, and hydroxides. Water with high alkalinity is highly resistant to changes in its pH, meaning a large amount of acid would be needed to lower its pH value significantly.
How Copper Interacts with Water
The interaction between solid copper metal and water is a process of corrosion, which is an electrochemical reaction that releases copper into the water. This process is highly dependent on the presence of an oxidizing agent, with dissolved oxygen (\(\text{O}_2\)) being the primary driver in drinking water systems. Metallic copper (\(\text{Cu}^0\)) is oxidized at the metal surface, typically forming cupric ions (\(\text{Cu}^{2+}\)) that leach into the liquid.
The overall corrosion process consumes dissolved oxygen and metallic copper, allowing copper ions to enter the water. This reaction is significantly accelerated if the water is slightly acidic (lower pH), because the acid helps to dissolve the naturally forming protective copper oxide layer on the metal surface. High levels of carbonic acid, formed from dissolved carbon dioxide, can also strip this protective layer, exposing the metal to further corrosion. The resulting copper concentration in the water is a direct result of this corrosion mechanism and the water’s specific chemistry.
The Actual Effect on Water pH Levels
When copper ions are released into the water, they initiate reactions that can slightly influence the water’s pH. The resulting cupric ions (\(\text{Cu}^{2+}\)) react with the water’s available anions, such as carbonates and hydroxides, to form copper corrosion products. In a typical drinking water environment, the reaction often leads to the formation of copper hydroxide (\(\text{Cu}(\text{OH})_2\)) or copper carbonate (\(\text{CuCO}_3\)) compounds.
The formation of copper hydroxide involves the consumption of hydrogen ions (\(\text{H}^+\)) or the release of hydroxide ions (\(\text{OH}^-\)) from the water molecules. This shift causes the \(\text{pH}\) to rise, resulting in a slight alkalinizing effect on the water. However, this increase is usually minor and temporary because the water’s natural alkalinity acts as a buffer against drastic pH changes. The water’s buffering capacity, determined by its mineral content, is the main factor controlling the final \(\text{pH}\), typically rendering the copper’s influence negligible in the long term.
Copper Intake and Drinking Water Standards
Copper enters drinking water predominantly through the corrosion of household plumbing, not necessarily from copper storage vessels. Water that sits for several hours in copper pipes, such as overnight, allows more time for the leaching process to occur, leading to higher morning concentrations. While copper is an essential nutrient for human health, excessive intake can cause gastrointestinal distress and potentially lead to liver or kidney damage in sensitive individuals.
Regulatory bodies have established guidelines to ensure the safety of drinking water relative to copper concentration. The U.S. Environmental Protection Agency (EPA) has set an Action Level for copper at \(1.3\) milligrams per liter (\(\text{mg/L}\)). This is a trigger point that requires water systems to implement corrosion control measures if exceeded in more than \(10\%\) of customer samples. The World Health Organization (\(\text{WHO}\)) recommends a guideline value of \(2.0 \text{ mg/L}\) for copper in drinking water. These standards focus on limiting the amount of copper ions released, regardless of the slight \(\text{pH}\) change that may accompany their presence.