Vintage titrations: sulfur dioxide concentrations in wine

Wine contains different concentrations of sulfur dioxide, and it’s your job to discover which has the most. 

Equipment

Materials per group

• White wine, 120 cm³ 
• Iodine solution (stabilized with potassium iodide), 50 cm³ of 0.01 mol dm^–3 
• H₂SO₄ (Corrosive to skin and eyes), 20 cm³ of 2.5 mol dm^–3 
• Sodium hydroxide, 25 cm³ of 1 mol dm^–3 
• 2% starch solution, 7 cm³ 
• Deionised water

Equipment per group

• Burette, 50 cm³ 
• Pipette, 25 cm³
• Measuring cylinders, 10 cm³ and 25 cm³ 
• Conical flasks, 250 cm³ 
• White tile
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Wear eye protection. 
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Sodium hydroxide is corrosive to skin and eyes, see CLEAPSS Hazcard HC091a. 

Commentary

This analysis is based on the familiar titration with iodine, using starch as an indicator. In trialling, some students needed help to devise an appropriate method; others coped easily. 

Procedure

Free SO₂ 

50 cm³ of wine is pipetted into a 250 cm³ conical flask and ca 5 cm³ of sulphuric acid and 2–3 cm³ of starch solution added.

The solution is titrated with 0.01 mol dm³ I₂ solution. The end-point is taken to be the appearance of a dark blue colour, which persists for about 2 minutes. 

In the interests of economy, one very careful titration should be sufficient.

Total (free and combined) SO₂

25 cm³ of 1 mol dm^–3 NaOH is placed in a 250 cm³ conical flask, using a measuring cylinder. 50 cm³ of wine is pipetted into this flask. The flask is shaken and left to stand for 15 minutes, then 10 cm³ of sulphuric acid and 2–3 cm³ of starch solution are added. The solution is titrated with 0.01 mol dm³ I₂ solution as above.

Calculation

The amounts of free and combined SO₂ can be calculated as mol dm^–3 and as mg dm^–3 (parts per million or ppm). The legal limit for total SO₂ varies from one country to another; 250 ppm is a commonly accepted value.

Although there is no legal limit on the amount of free SO₂, levels from 20 to 40 ppm safeguard the wine without affecting its taste. If the level is below 10 ppm in a white wine, it is in danger of going bad.

Extension 

The method is not usually recommended for red wines because the colour masks the end-point. However, it can normally be seen without too much difficulty if the mixture in the flask is compared with a sample of the original wine. 

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.