To determine the total sulphur dioxide content in a brand of white wine. Theory
Hydrogensulphite is often added to white wine for its preservation. Only a little amount of free SO2 exist in white wine. All the available SO2 (including those from hydrogensulphite) are first converted to SO32- as illustrated by the following equations. SO2(aq) + 2OH-(aq) SO32-(aq) + H2O(l)
HSO3-(aq) + OH-(aq) SO32-(aq) + H2O(l)
Addition of acid to the subsequent mixture resulting the liberation of SO2 in which its concentration by titrating with a known concentration of I2(aq) using freshly prepared starch solution as the indicator.
SO32-(aq) + 2H+(aq) SO2(aq) + H2O(l)
SO2(aq) + I2(aq) + 2H2O(l) 2HI(aq) + H2SO4(aq)
Amount of available SO2 = amount of I2 used for titration
Reagents and apparatus
The volume of wine present in the bottle stated in the label was recorded. 2.
25 cm3 of white wine was transferred into a 250 cm3 conical flask. 3.
About 12 cm3 of 1 M sodium hydroxide was added and the resulting mixture was allowed to stand for about 14 minutes.
About 10 cm3 of dilute sulphuric acid and a few drops of freshly prepared starch solution were added to the flask.
The mixture was titrated with 0.0050 M iodine solution.
The titre required for producing the first faint and permanent blue colour was taken.
Steps 2 to 6 were repeated for obtaining 2 consistent titres. Results and Calculations
Brand name of white wine: ___________
Volume of wine as stated in label: __________
[I2(aq)] = 0.0050 M
Final burette reading /cm
Initial burette reading /cm
Volume of I2(aq) added / cm3
mean titre /cm3
The calculations for determination of SO2 content in the white wine were tabulated as follows: