Analysis of Sulphur Dioxide Content in Wine

To determine the amount of sulphur dioxide, an antioxidant, present in wine by using volumetric analysis.

Principle

All free SO2 molecules is first convert into SO32- by NaOH solution:

SO2 + 2OH- � SO32- + H2O

Acidification of the solution liberates all SO2:

2SO32- + 2H+ � 2SO2 + 4H2O

which is then titrated with 0.0057M iodine solution in which starch is used as end-point indicator:

SO2 + I2 + 2H2O � 2HI + H2SO4

Chemicals

white wine (carbonated), 1M NaOH, 2M H2SO4, 0.0057M I2, starch solution

Apparatus

volumetric apparatus, pipette, measuring cylinder, dropper, white tile

Procedure

1.> Find out the volume of wine from the label on the bottle.

2.> Pipette 25cm3 of white wine into a conical flask.

3.> Add about 12cm3 of 1M NaOH and stand for about 15 minutes.

4.> Add about 10cm3 of 2M H2SO4 to the mixture and then few drops of starch solution as indicator. Quickly, titrate the mixture with 0.0057M

iodine solution.

5.> Record the titre required to produce pale blue colour.

6.> Repeat steps 2-5 for 2-3 times.

Data Analysis

Trial

1st

2nd

Final reading /cm3

3.05

12.80

26.30

Initial reading /cm3

0.25

9.70

23.25

Volume of I2 added /cm3

2.80

3.10

3.05

Average volume of I2 added /cm3

3.075

Concentration of I2 solution: 0.0057 M

Volume of wine bottle: 750 cm3

Number of moles of 0.0057M I2 in the titre: (3.075)(0.0057) = 0.01753 mmol

Number of moles of SO2 in the 25cm3 sample: 0.01753 mmol

Number of moles of SO2 in the bottle of wine: 0.01753 x 30 = 0.5259 mmol

Mass of SO2 in the bottle of wine: (0.5259)(32+16×2) = 33.66 mg

SO2 content in wine: 33.66 � (750×10-3) = 44.88 mgdm-3

Precaution

1.> Open the bottle of wine just before the experiment, stopper the conical flask containing 25 cm3 wine before adding NaOH and titration should be

done quickly after the addition of H2SO4 to avoid any SO2 loss.

2.> Shake the conical flask gently during titration to reduce SO2 loss or oxidized by oxygen in air.

3.> Iodine solution need to be standardized before titration since iodine will vapourize and its concentration varies with time.

Possible Error

1.> Some of SO2 in carbonated white wine may be drawn away by CO2.

2.> Oxidation of SO2 by air during titration.

Discussion

Beer is an alcoholic drink that contains dilute ethanol (about 5%), sugar, ester and antioxidant, etc. In the presence of atmospheric oxygen, ethanol in wine can be oxidized to ethanoic acid which makes the wine sour. Even small amount of air, over a period of time, can adversely affect the wine.

~The need for wine additives~

Other than anti-oxidation, additives are added to wine to enhance its flavour and colour. The example are sugar, to reduce bitter taste of wine; ester, as flavouring agent to enhance flavour; sulphur dioxide, as antioxidant.

Some wine which is needed to store for a long time especially need to preserve. Since dilute ethanol in wine can prevent bacterial growth, anti- oxidation is thus more important in preservation of wine.

~Safety of using wine additives~

No additives may be used unless it has been extensively tested on animals. Considerable research has been carried out in order to observe whether the additives affect growth, reproduction, or prevalence of cancer. These data are then used by experts to assess the risks to humans in using the additives.

~Sulphur dioxide as antioxidant~

Sulphur dioxide, including its salts, namely, sulphites (SO32-) and bisulphites (HSO3-) can preferentially react with atmospheric oxygen into harmless SO42-, thus prevent oxidation of wine. Besides wine, sulphur dioxide and its salts are widely used to preserve dehydrated fruits and vegetables, fruit juices and syrups.

Sulphur dioxide is a poisonous gas with pungent smell. When ingested, sulphur dioxide and its salts attack the respiratory system and make the individuals who suffer from bronchitis and asthma get worse. Thus, limits are set on the amount of free sulphur dioxide allowed in wine. Hong Kong Preservation in Food Regulations states that sulphur dioxide content in wine must not exceed 450mgdm-3. The acceptable daily intake (ADI) for human of sulphur dioxide should not exceed 0.7mg per kg body weight.

In fact, most of preservatives present in wine is in form of sodium bisulphite which can acts as a source of sulphur dioxide; however, the actual amount of free sulphur dioxide is quite low. Despite this, we can still conduct an experiment to find out the amount of total available sulphur dioxide in wine.

~Analysis of sulphur dioxide content in wine~

In the experiment, all HSO3- and free SO2 molecules are converted to SO32- by sodium hydroxide solution:

HSO3- + OH- � SO32- + H2O

SO2 + 2OH- � SO32- + H2O

SO32- is then converted back to SO2 by sulphuric acid and titrates with I2 solution. It’s assumed that there is no SO2 loss to surroundings throughout the experiment. The experiment aims to determine total available SO2 in wine, despite the presence of HSO3-. The reaction between I2 and SO2 is actually a redox reaction:

SO2 + I2 + 2H2O � 2HI + H2SO4

The oxidation number of S is changed from +4 to +6 whereas that of I2 is changed from 0 to -1.

~Result~

By the experiment, SO2 content in wine is 44.88 mgdm-3 which is much lower than the limit of 450 mgdm-3 stated. However, this value is supposed to be not accurate since in this experiment, carbonated wine is used, and the carbon dioxide evolves will affect the measurement by pipette and can draw away the free sulphur dioxide in wine. So, non-carbonated white wine should be use (since colour may affect the detection of end-point). Although a bottle of wine doesn’t exceed the limit of 450 mgdm-3, the heavy drinking of alcohol can damage heart, kidney, liver and central nervous system.

Other than as antioxidant in wine, SO2 is also widely used in bleaching industry, Contact Process in manufacturing sulphuric acid and disinfectant.

Conclusion

Sulphur dioxide is used in wine as antioxidant. However, due to its toxicity and allergy to asthma patient, the limit of using sulphur dioxide must be set.