✓Why DM Water Has a pH Less Than 7, Even Without any Ions?

 ✓Why DM Water Has a pH Less Than 7, Even Without any Ions?

✓Deionized (DM) water, despite being purified and free from ions, often shows a pH of less than 7. This phenomenon can seem puzzling since pure water is expected to be neutral, with a pH of exactly 7. However, the reality of water chemistry and environmental factors sheds light on why DM water tends to be slightly acidic.

The Nature of DM Water:

DM water undergoes extensive purification through ion exchange, removing dissolved salts, minerals, and ions, theoretically leaving behind H₂O molecules. In a perfect setting, this water would have a pH of 7. However, DM water is highly reactive and prone to absorbing carbon dioxide (CO₂) from the atmosphere, which changes its pH.

CO₂ Absorption and Carbonic Acid Formation:

When DM water is exposed to air, CO₂ readily dissolves into the water, reacting with it to form carbonic acid (H₂CO₃).

CO₂ + H₂O. ----> H₂CO₃

Carbonic acid, a weak acid, dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻)

H₂CO₃ ----> H⁺ + HCO₃⁻

The release of hydrogen ions lowers the pH, causing the water to become slightly acidic, typically around pH 5.5-6.5.

The Absence of Buffering Ions:

In natural water, ions like bicarbonate, calcium, and magnesium act as buffers, helping stabilize the pH. DM water, however, lacks these ions, making it highly susceptible to pH changes from even small amounts of CO₂. This explains why, even though no significant ions are present in the water, the pH can still drop below 7.

Practical Implications:

For industrial processes, especially in power plants or chemical manufacturing, controlling the pH of DM water is crucial. Acidic water can cause corrosion in metal piping and systems, leading to operational inefficiencies. Therefore, pH monitoring controlling via Morpholine or ammonia dosing and CO₂ exclusion measures (such as nitrogen blanketing) are often employed to maintain DM water's integrity.

In conclusion, the slightly acidic nature of DM water is a result of its interaction with atmospheric carbon dioxide and the absence of buffering ions. While purified, DM water remains chemically reactive, explaining its pH behavior in real-world conditions.