Sulfur Dioxide Species Determination

Dissolved sulfur dioxide (SO$_2$) in water exists in equilibrium with sulfurous acid (H$_2$SO$_3$) and its dissociation ions, bisulfite (HSO$_3$^–$) and sulfite (SO$_3$^{2–}$). The specific species dominating depends on the solution's pH relative to the acid dissociation constants (pKa) of sulfurous acid.

Sulfurous Acid Equilibria

The relevant equilibria and approximate pKa values are:

1. SO$_2$ (dissolved) + H$_2$O <=> H$_2$SO$_3$ (Sulfurous acid)
2. H$_2$SO$_3$ <=> H$^+$ + HSO$_3$^–$ (Bisulfite ion) pKa1 ≈ 1.85
3. HSO$_3$^–$ <=> H$^+$ + SO$_3$^{2–}$ (Sulfite ion) pKa2 ≈ 7.2

Note: SO$_2 \cdot$ H$_2$O represents the hydrated form, essentially H$_2$SO$_3$. The species H$_2$SO$_3$ is dominant when pH < pKa1.

pH Analysis

The rainwater sample has a pH of 8.45. We compare this pH to the pKa values:

• Comparing pH (8.45) to pKa2 (7.2): Since 8.45 > 7.2, the equilibrium shifts towards the deprotonated form, SO$_3$^{2–}$.
• Comparing pH (8.45) to pKa1 (1.85): Since 8.45 > 1.85, the HSO$_3$^–$ form would be deprotonated further.

Dominant Species Identification

Based on the pH analysis:

• At pH = 8.45, the pH is significantly higher than both pKa1 (1.85) and pKa2 (7.2).
• When pH > pKa2, the species SO$_3$^{2–}$ is the most abundant.
• When pKa1 < pH < pKa2, the species HSO$_3$^–$ is the most abundant.
• When pH < pKa1, the species H$_2$SO$_3$ (or SO$_2 \cdot$ H$_2$O) is the most abundant.

Therefore, at pH 8.45, the dominant species of dissolved sulfur dioxide is the sulfite ion (SO$_3$^{2–}$).