The pH of Sulfur Dioxide in Water: Understanding the Acidity and Balancing the Solution

The pH value of pure water is 7, which is considered neutral. However, when sulfur dioxide (SO2) is dissolved in water, it forms an acidic solution due to the formation of sulfurous acid (H2SO3). The pH of an aqueous solution of SO2 is less than 7, making it acidic.

Understanding the Acidity of Sulfur Dioxide in Water

When sulfur dioxide (SO2) is dissolved in water, it reacts with water molecules to form sulfurous acid (H2SO3):

SO2 + H2O ⇌ H2SO3

Sulfurous acid is a weak acid, which means it only partially dissociates in water, releasing hydrogen ions (H+) and sulfite ions (SO3²-):

H2SO3 ⇌ H+ + HSO3-

The presence of these hydrogen ions (H+) in the solution lowers the pH, making the solution acidic. The pH of an aqueous solution of SO2 is typically in the range of 3 to 5, depending on the concentration of the dissolved SO2.

Qualitative Assessment of Acidity

The pH of an aqueous solution of SO2 can be compared qualitatively with that of pure water by observing the color change of litmus paper. If the litmus paper turns red, the solution is acidic, indicating a pH value less than 7. Conversely, if the litmus paper remains blue, the solution is basic, indicating a pH value greater than 7.

Factors Affecting the pH

Contaminants or substances present in an aqueous solution of SO2 can affect the pH value. For example, the presence of strong acids or bases can lower or raise the pH value, respectively. Additionally, the presence of other acidic or basic gases, such as carbon dioxide (CO2) or ammonia (NH3), can also affect the pH value of the solution.

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Balancing the pH of Sulfur Dioxide in Water

PH of sulphur dioxide in waterImage source: wikimedia

To balance the pH value of an aqueous solution of SO2, one can add a base, such as sodium hydroxide (NaOH), to neutralize the acidity. The reaction between the base and the sulfurous acid produces a salt and water:

H2SO3 + 2NaOH → Na2SO3 + 2H2O

However, it is important to note that adding a base can also increase the pH value, making the solution basic. Therefore, it is crucial to add the base slowly and monitor the pH value to ensure it remains within the desired range.

Aeration and pH Adjustment

Aerating an aqueous solution of SO2 by bubbling air through it can increase the pH value due to the removal of carbon dioxide (CO2) from the solution. CO2 is an acidic gas that can lower the pH value of a solution when dissolved in water. By removing CO2 from the solution, the pH value can increase, making the solution less acidic.

Testing and Adjusting pH at Home

At home, one can test the pH value of an aqueous solution of SO2 using litmus paper or a pH meter. If the pH value is too low, one can add a base, such as baking soda (sodium bicarbonate), to neutralize the acidity. However, it is important to note that adding a base can also increase the pH value, making the solution basic. Therefore, it is crucial to add the base slowly and monitor the pH value to ensure it remains within the desired range.

Conclusion

The pH value of an aqueous solution of SO2 is less than 7, making it acidic. The pH value can be affected by the presence of contaminants or substances, such as strong acids or bases, and can be balanced by adding a base. At home, one can test the pH value using litmus paper or a pH meter and balance it by adding a base, such as baking soda.

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References:
EPA: pH Scale
BYJUS: Comparison of pH Values
YouTube: Explaining pH of SO2 in Water
Chemistry Stack Exchange: pH of Aqueous SO2
Wikipedia: Sulfur Dioxide