Understanding the pH Scale and Pure vs. Purified Water
The pH scale, which ranges from 0 to 14, is a fundamental concept in chemistry that measures the acidity or alkalinity of a solution. A pH of 7 is considered neutral, with lower values indicating increasing acidity and higher values indicating increasing alkalinity. In its ideal, theoretical state, water composed only of hydrogen and oxygen ($H_2O$) is neutral with a pH of 7. However, this theoretical ideal exists only in a perfectly controlled laboratory vacuum. In the real world, purified water is far more complex.
Purified water refers to water that has undergone a process to remove impurities, minerals, and contaminants. This differs from simple filtered water, which may retain many of its natural minerals. It is the removal of these minerals—specifically mineral buffers like calcium and magnesium—that is central to understanding why purified water becomes slightly acidic. Without these buffers, the water's pH becomes highly susceptible to external influences, most notably dissolved carbon dioxide ($CO_2$) from the air.
The Key Chemical Reaction That Causes Acidity
The primary reason for the slight acidity of purified water is a simple but important chemical reaction with ambient air. When purified water is exposed to the atmosphere, it readily absorbs carbon dioxide gas. The $CO_2$ then reacts with the water molecules to form a weak acid known as carbonic acid ($H_2CO_3$).
$H_2O + CO_2 \rightleftharpoons H_2CO_3$
Carbonic acid then partially dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$), which increases the concentration of hydrogen ions and, by definition, lowers the water's pH. This process is why freshly produced purified water may test close to 7, but will steadily become more acidic over time when left open to the air.
Factors That Lower Purified Water's pH
- Removal of Mineral Buffers: Purification methods like reverse osmosis and distillation strip the water of minerals like calcium and magnesium. These minerals would typically act as buffers, resisting changes in pH. Their absence makes the water's pH much more volatile.
- Absorption of Carbon Dioxide: The immediate absorption of $CO_2$ from the air into the water forms carbonic acid, which is the direct cause of the pH drop.
- Storage Materials: The type of container can also influence pH. Certain plastic containers can leach trace compounds into the water, potentially affecting its chemical properties and pH balance.
- Temperature: The pH of water is temperature-dependent. At 25°C, neutral pH is 7, but it can be slightly different at other temperatures.
Comparison: Purified Water vs. Tap Water pH
The pH of tap water varies widely depending on its source and the treatment processes used by municipal water suppliers. Tap water often has a pH between 6.5 and 8.5 due to the presence of dissolved minerals that provide buffering capacity. The contrast with purified water is stark, as detailed in the following table.
| Feature | Purified Water (RO or Distilled) | Tap Water |
|---|---|---|
| Typical pH Range | 5.5–7.0 | 6.5–8.5 |
| Buffering Capacity | Low; lacks minerals to resist pH changes | High; contains dissolved minerals that act as buffers |
| Reason for pH Variation | Absorption of atmospheric $CO_2$ and lack of minerals | Local geological sources and municipal treatment processes |
| Potential Health Concerns | None from slight acidity; corrosion of pipes is main risk | Can have varying contaminants; pH is regulated by EPA standards |
| Taste | Often described as flat or bland due to lack of minerals | Can be described as having a crisp taste due to mineral content |
Is Mildly Acidic Purified Water Safe to Drink?
For most people, the slightly acidic nature of purified water is not a health concern and is perfectly safe to consume. The resulting carbonic acid is extremely weak and is present in very low concentrations. For perspective, the acidity is far less than that of orange juice (pH 3.3–4.2) or black coffee (pH 5). The human body is well-equipped to manage and neutralize this mild acidity without issue. For a reliable external resource on drinking water quality, visit the U.S. Environmental Protection Agency website. However, there is one area where the low pH of purified water can cause problems: corrosion of plumbing.
Potential Issues of Low pH Water
- Pipe Corrosion: Water with a low pH (typically below 6.5) is corrosive, meaning it can dissolve metals from plumbing, including copper, lead, and zinc. This can lead to heavy metal contamination of the drinking water, presenting a serious health risk.
- Metallic Taste: As pipes corrode, the water may take on a metallic taste due to the leaching of metals.
- Plumbing Damage: Over time, acidic water can damage plumbing fixtures, especially in older homes with lead solder or copper pipes.
Conclusion
To answer the question, is purified water acidic, the real-world answer is yes, but only slightly and typically due to exposure to air, not the purification process itself. While theoretically neutral, the removal of stabilizing minerals by processes like reverse osmosis and distillation leaves it vulnerable to absorbing carbon dioxide, which forms a weak acid. This mild acidity is generally not harmful to human health. The primary concern is for older plumbing systems, where corrosive water could leach metals. For the majority of consumers with modern plumbing, drinking purified water is a safe practice. If you have concerns about the pH of your water, simple home testing kits can help you monitor it and address any issues.
