Table 3, Chemical Properties

1. pH – The pH of 0.1N sulfurous acid is 1.5.
2. Sulfur dioxide can function as either an oxidizing agent or as a reducing agent. However, sulfur dioxide is used most frequently as a reducing agent. Some examples of the reactions for sulfur dioxide are as follows.

1. As an oxidizing agent:

Zn + 2SO_{2 _}-> ZnS₂O₄

2H₂ + SO₂ -> 2H₂O + 3S

2. As a reducing agent:

SO₂ + Cl₂ + 2H₂O -> 2HCl + H₂SO₄

5SO₂ + 2ClO₂ + 6H₂O -> 2HCl + 5H₂SO₄

SO₂ + H₂O + H₂O₂ -> H₂SO₄ + H₂O

3SO₂ + 2H₂CrO₄ -> Cr₂(SO₄)₃ + 2H₂O*

3. As a neutralizing agent:

SO₂ + NaOH Na₂SO₃ + H₂O

*Two of the more important reactions involving sulfur dioxide, in the area of water and wastewater treatment, are listed above. The quantity of sulfur dioxide required for complete reduction may very depending on the environment in which the reaction is taking place. However, the standard amount of sulfur dioxide required for each reaction is listed below:

Pounds of SO2 Required to Treat (1) Pound of Reacting Species

Cl₂ 0.9 Stoichiometric

H₂CrO₄ 2.0 Complete Reduction