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Ripper titration can be used to determine free $\ce{SO2}$ in wine by titration with iodine.

The method requires sulphuric acid to be added to wine before titration. Why is this the case?

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There are 3 forms:

  • sulfur dioxide (molecular) $$\ce{SO2}$$ ,
  • Bisulphite $$\ce{HSO3-}$$
  • Sulphite $$\ce{SO3^2-}$$
  • (Additionally, bisulphite can become oxidized with water to form sulphuric acid)

These species exist in equilibrium, and shifts in pH with shift the concentration of each species.

The addition of sulphuric acid lowers pH and favors the molecular form. The molecular form can react with iodine and water. When there is no longer any of the molecular form to react with the iodine, the end point its reached and the color changes.

$$\ce{SO2 + I2 + 2H2O → H2SO4 + 2HI}$$

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