Take a look into redox titrations using grape juice’s older cousin
Wine contains different concentrations of sulfur dioxide, and it’s your job to discover which has the most.
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.
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.
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.
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.
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.
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.
- Experiment | PDF, Size 0.32 mb
- Experiment | PDF, Size 0.27 mb
The resources were originally published in the book In Search of More Solutions.