Vitamin C (ascorbic acid, C₆H₈O₆) is an essential nutrient and a reducing agent that is readily oxidised. In this investigation, you will use iodometric titration to measure the vitamin C content of fruit juice after exposure to two continuous variables: temperature and metal ion concentration. Both investigations use the same titration method, making this a versatile and highly scorable IB Chemistry IA topic.
This practical is suitable for IB Diploma Chemistry and makes an excellent Internal Assessment (IA) topic.
Background Theory
The content you’ve provided is generally correct regarding the reaction of ascorbic acid with iodine in an iodometric titration. However, there is a small correction needed in the chemical equation. The correct reaction is as follows:
C₆H₈O₆ + I₂ → C₆H₆O₆ + 2HI
Ascorbic acid (C₆H₈O₆) is oxidized to dehydroascorbic acid (C₆H₆O₆) by iodine (I₂). The explanation about the iodometric titration and the endpoint detection using starch is also accurate. The description clearly outlines how the iodine reacts with ascorbic acid first, followed by the starch indicator change at the endpoint
Vitamin C degrades rapidly when heated or when exposed to certain metal ions. Copper (Cu²⁺) and iron (Fe²⁺/Fe³⁺) act as catalysts for the oxidation of ascorbic acid by dissolved oxygen. This makes both temperature and metal ion concentration excellent continuous independent variables for an IA.
Investigation A: Effect of Temperature on Vitamin C Content
Variables
- Independent variable (IV): Temperature at which juice is heated (°C) — e.g. 20, 40, 60, 80, 100 °C
- Dependent variable (DV): Volume of iodine solution required to reach endpoint (cm³), converted to vitamin C concentration (mg per 100 cm³)
- Controlled variables (CV): Volume of juice used, concentration and volume of starch indicator, iodine solution concentration, heating time, same juice brand and batch
Method
- Prepare five water baths at 20, 40, 60, 80, and 100 °C.
- Measure 25 cm³ of fresh orange juice into each of five beakers. Heat each sample at its target temperature for exactly 10 minutes. Allow to cool to room temperature before titrating.
- Transfer each cooled juice sample to a conical flask. Add 10 drops of starch indicator solution.
- Fill a burette with standardised iodine solution (0.00500 mol dm⁻³). Record the initial burette reading.
- Titrate slowly, swirling continuously, until the endpoint — a permanent blue-black colour that persists for 30 seconds.
- Record the final burette reading. Calculate the titre volume.
- Repeat each temperature at least three times and calculate a mean titre.
Investigation B: Effect of Metal Ion Concentration on Vitamin C Content
Variables
- Independent variable (IV): Concentration of copper(II) sulfate solution added to juice (mol dm⁻³) — e.g. 0, 0.001, 0.005, 0.010, 0.050, 0.100 mol dm⁻³
- Dependent variable (DV): Volume of iodine solution at endpoint, converted to vitamin C concentration
- Controlled variables (CV): Volume of juice, volume of CuSO₄ solution added, exposure time, temperature, same juice batch
Method
- Prepare a stock solution of copper(II) sulfate (CuSO₄, 0.100 mol dm⁻³) and dilute to create the required concentrations using serial dilution.
- Add 5 cm³ of each CuSO₄ solution to 20 cm³ of fresh orange juice in separate conical flasks. Include a control (0 mol dm⁻³ — 5 cm³ distilled water instead).
- Leave each sample at room temperature for exactly 30 minutes.
- Add 10 drops of starch indicator and titrate with standardised iodine solution as described in Investigation A.
- Repeat at least three times per concentration and calculate mean titres.
Equipment (Both Investigations)
- Fresh orange juice (same brand and batch throughout)
- Standardised iodine solution, I₂ (0.00500 mol dm⁻³)
- Starch indicator solution (1%)
- Copper(II) sulfate solution, CuSO₄ (Investigation B)
- 50 cm³ burette
- 25 cm³ pipette
- Conical flasks
- Water baths at required temperatures (Investigation A)
- Thermometer (±0.5 °C)
- Stopwatch
Safety
⚠️ Iodine solution is an irritant — avoid skin contact and wear gloves. Copper(II) sulfate is harmful if ingested and toxic to aquatic organisms. Dispose of all solutions into the appropriate waste disposal bottles provided.
Calculations
1. Moles of I₂ used = concentration × volume (in dm³)
2. Since 1 mol ascorbic acid reacts with 1 mol I₂: moles of vitamin C = moles of I₂
3. Mass of vitamin C (mg) = moles × 176.12 g mol⁻¹ × 1000
4. Express as mg per 100 cm³ of juice for easy comparison with label values.
Results Tables
Investigation A — Temperature
| Temperature (°C) | Titre 1 (cm³) | Titre 2 (cm³) | Titre 3 (cm³) | Mean titre (cm³) | Vitamin C (mg/100 cm³) |
|---|---|---|---|---|---|
| 20 | |||||
| 40 | |||||
| 60 | |||||
| 80 | |||||
| 100 |
Investigation B — Metal Ion Concentration
| [CuSO₄] (mol dm⁻³) | Titre 1 (cm³) | Titre 2 (cm³) | Titre 3 (cm³) | Mean titre (cm³) | Vitamin C (mg/100 cm³) |
|---|---|---|---|---|---|
| 0 | |||||
| 0.001 | |||||
| 0.005 | |||||
| 0.010 | |||||
| 0.050 | |||||
| 0.100 |
Analysis
For each investigation, plot vitamin C concentration (mg/100 cm³) on the y-axis against the continuous IV on the x-axis. For Investigation A, describe any threshold temperature above which degradation accelerates. For Investigation B, determine whether the relationship between [Cu²⁺] and vitamin C loss is linear or curved — a curved relationship suggests enzyme-like saturation kinetics of the catalytic oxidation.
Discussion Points
- Why does vitamin C degrade more rapidly at higher temperatures? Consider both direct thermal oxidation and the role of dissolved oxygen.
- Why do Cu²⁺ and Fe²⁺/Fe³⁺ ions catalyse the oxidation of ascorbic acid? What is their role in the redox cycle?
- Why does the titre decrease as vitamin C degrades — what does a smaller titre indicate?
- How does your measured vitamin C compare to the value on the juice label? Suggest reasons for any discrepancy.
- Why is it important to titrate immediately after cooling, rather than leaving the juice to stand?
IA Guidance
Choose one of the two investigations above as your IA focus. Investigation B (metal ions) tends to produce a cleaner continuous dataset and a more novel research question. To score highly:
- Research Design: Justify your choice of concentration range or temperature range with reference to pilot data. Explain the chemistry of the titration endpoint precisely. For Investigation B, explain why a control (0 mol dm⁻³) is essential.
- Data Analysis: Plot with error bars. For Investigation B, consider plotting ln(vitamin C remaining) vs [Cu²⁺] to test for a linear relationship indicative of first-order kinetics.
- Conclusion: Compare your vitamin C values to the juice label and to published degradation studies. State a clear conclusion linking IV to DV with reference to redox chemistry.
- Evaluation: Discuss sources of error such as oxidation during preparation, variability between juice batches, and the assumption that starch endpoint is consistent across all samples.
Practical Science 