The sulfite content of white wine is determined via precipitation with strontium. The wine has 0.2 M NaOH added until the pH is above 9, then 0.6 M Sr(NO3)2 is added to precipitate SrSO3. The precipitate is filtered, washed and dried.
Many white wines have "contains sulfites" on the label and here we measure the amount of sulfite in a small sample of pinot grigio. The sulfites are used as a preservative and to prevent oxidation.
The Ksp of SrSO3 is significantly smaller than Sr(OH)2 so at a pH of 9 or 10, the precipitate should nearly completely be SrSO3 or some unknown precipitate. The reason base must be added is because at the low or neutral pH the sulfite is mostly in the form of H2SO3 (H2O/SO2) or HSO3-. The second pKa of sulfurous acid is around 7.2, so at a pH over 9 you are looking at about 100:1 ratio of sulfite to bisulfite.

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