Measuring the amount of vitamin C in fruit drinks

Have you ever poured a tall glass of a fruit drink, and wondered just how much vitamin C it contains? Well, wonder no longer, with this titration experiment students discover how much, or little, vitamin C their drink contains.  

This experiment should take 20 minutes.

Equipment

Apparatus

• Eye protection
• Student worksheet
• Microscale titration apparatus (see our apparatus and techniques for microscale chemistry guidance)
• Pipette (glass), 1 cm³
• Pipette (glass), 2 cm³
• Pipette filler
• Beaker, 25 cm³
• Measuring cylinder, 5 cm³
• Beaker (for filling titration apparatus), 10 cm³

Chemicals

• Sodium thiosulfate
• Potassium iodate
• Potassium iodide 
• Starch solution (freshly made)
• Sulfuric acid, 1 mol dm³
• Sample(s) of fruit juice

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• Sulfuric acid, 1 mol dm^–3 is a skin/eye irritant (see CLEAPSS Hazcard HC098a).
• Sodium thiosulfate, 0.010 mol dm^–3, potassium iodate, 0.001 mol dm^–3 and potassium iodide, 0.005 mol dm^–3 solutions are of low hazard, as are the starch solution and fruit juices (see CLEAPSS Hazcards HC095a, HC080, HC047b).

Procedure

1. Set up the microscale titration apparatus.
2. Fill the apparatus with sodium thiosulfate solution.
3. Using the glass pipette, add 2 cm³ of potassium iodate solution to the beaker.
4. Measure, using the measuring cylinder, 3 cm³ of potassium iodide solution, then add this to the beaker. (Note: the potassium iodide solution is added in slight excess.)
5. Add three drops of sulfuric acid. A yellow-brown colour appears due to iodine.
6. Add a few drops of starch solution. A deep blue-black colour forms.
7. Using the glass pipette, add 1 cm³ of the fruit drink to the beaker and swirl gently.
8. Titrate the remaining iodine in the beaker against the sodium thiosulfate solution. (The beaker can be swirled very gently to mix the chemicals. Alternatively, the tip of a plastic pipette can be used as a mini stirring rod.) The disappearance of the deep blue-black colour marks the end-point.
9. Do a duplicate titration and check the agreement between the two titres. If it is acceptable, take the mean value of the two titres and use it for your calculations.

Observations

The titre volume should be in the range 0.5–1 cm³, the disappearance of the blue-black colour marking the end-point.

This experiment offers possibilities for assessing students’ abilities in following instructions and/or processing results.

A survey of a range of fruit drinks (and maybe other products containing vitamin C) could form the basis of a class project or as an activity for a school or college chemistry club.

Note

The reaction to generate the iodine is based on using an accurately known volume of the potassium iodate solution (the concentration of which is accurately known).

The potassium iodide solution and the sulphuric acid are added in slight excess, and thus the concentrations of these solutions is not critical.

Instead of generating the iodine in situ, it is possible to use standard iodine solution in this procedure.

This would need to be diluted to give an aliquot containing 7.2 x 10^–6 moles of iodine for each determination.