Making Assessment Criteria
Experiment
Analysis of Sulphur Dioxide Content in Wine
Introduction
In the presence of atmospheric oxygen, the alcohol content of wine can be
converted to ethanoic acid making the wines sour and unpalatable. Even small
amount of air, over a period of time, can adversely affect wines. The problem can be
minimized by the introduction of a suitable reductant which will preferentially react
with the oxygen. One such reductant is sulphur dioxide but because this substance is,
itself, toxic and pungent in odour, limits are set on the amount of 'free' sulphur
dioxide allowed in wine. Most of the preservative present in wine is 'fixed' in the
form of sodium hydrogensulphate(IV) (sodium bisulphite). Although this can act as a
source of sulphur dioxide, the actual amount of free sulphur dioxide is quite low.
In this experiment, the amount of total available SO2 in the wine, irrespective of
its actual form in the sample is determined. The method involves first the conversion

of all SO2 into SO32 ion. Acidification of the solution then liberates all SO2,
+
SO2
3 (aq) + 2H (aq)  SO2 (aq) + H2O(l))
which is then titrated with iodine solution according to the equation
SO
SO22(aq)
(aq) II22(aq)
(aq)  22H
H22O(
O((l)
)) 
 22HI(aq)
HI(aq) H
H22SO
SO44(aq)
(aq)
Chemicals
White wine (non-sparkling or non-carbonated), 1 M NaOH, 2 M H2SO4, 0.0050 M I2,
starch solution (freshly prepared)
Apparatus
Titration apparatus, measuring cylinder, teat pipette
Procedures
1.
2.
3.
4.
5.
6.
7.
Determine, from the label, the volume of wine in a bottle.
Using a pipette, transfer 25 cm3 of white wine into a 250 cm3 conical flask.
Add about 12 cm3 of 1 M sodium hydroxide solution and allow to stand for
about 15 minutes.
Add about 10 cm3 of 2 M sulphuric(VI) acid to the mixture and then a few drops
of starch indicator. Quickly titrate the mixture with 0.0050 M iodine solution.
(Why should this titration be done quickly?)
Record the actual molarity of the iodine solution.
Record the titre required to produce the first faint but permanent blue colour.
Repeat the procedure to get two additional concordant titres.
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Results and discussion
1.
Mean titre / cm3
No. of moles of I2 in the titre
No. of moles of SO2 in the original aliquot
No. of moles of SO2 in the bottle of wine
Mass of SO2 in the bottle of wine / mg
2.
3.
4.
5.

Compare your results with the limit of 450 mg dm 3 stated in the Hong Kong
Preservatives in Food Regulations. Comment upon the comparison.
What is the most questionable assumption you have made in arriving at this
figure?
By the use of oxidation numbers, show that the reaction during the titration was
a redox reaction.
Suggest an experiment to determine the efficiency of sodium
hydrogen-sulphate(IV) as a wine preservative. If time allows, conduct the
experiment and comment on your results.
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