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DCPIP is a redox indicator whose oxidised blue form can be reduced by vitamin C. In the fixed-DCPIP-volume method used here, measure the volume of a reference vitamin C solution needed to reach the specified decolourisation endpoint. The same DCPIP amount then requires approximately the same vitamin C amount when an unknown sample is tested under comparable conditions.
Video by Professor SNAB. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Standardise a fixed DCPIP endpoint using a vitamin C reference
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: In the fixed-DCPIP-volume arrangement, which sample needs less added volume?
The more concentrated vitamin C sample. The endpoint requires a comparable reducing amount.
After watching 2: Why can food colour limit the measurement?
It can obscure the indicator endpoint. A visible endpoint depends on the optical background as well as chemistry.
What you'll be able to do
DCPIP is a redox indicator whose oxidised blue form can be reduced by vitamin C. In the fixed-DCPIP-volume method used here, measure the volume of a reference vitamin C solution needed to reach the specified decolourisation endpoint. The same DCPIP amount then requires approximately the same vitamin C amount when an unknown sample is tested under comparable conditions.
A more concentrated vitamin C sample therefore needs a smaller added volume in this arrangement. If a method instead fixes sample volume and varies DCPIP, the volume relationship differs. Describe the actual setup before using an equation rather than memorising “larger volume means more vitamin C” without a defined method.
Use a fresh appropriate reference, consistent DCPIP volume and concentration, controlled mixing and a clear endpoint rule. Prepare or filter extracts as specified and record their dilution. Use repeats and an appropriate blank/control to assess background colour and endpoint judgment. Food pigments or turbidity may hide a blue-to-colourless change.
Other reducing substances can affect the assay, and acidity or storage may change the response. Treat reference and unknowns comparably. Vitamin C can be lost during preparation or storage, so define the time, temperature and handling being compared rather than attributing every difference to the food’s original composition.
For the stated same-DCPIP-threshold model, c(reference)V(reference) ≈ c(unknown)V(unknown). Use consistent volume and concentration units. Multiply a measured diluted concentration by the total dilution factor to estimate the original sample, and distinguish mg cm⁻³ from mg per 100 cm³.
The calculation estimates vitamin C under the assay assumptions. A precise mean titre does not remove interference from another reductant or an obscured endpoint. State the method’s specificity, reference quality and preparation limits before comparing a label claim or ranking foods. Vitamin C content alone is not a complete measure of nutritional benefit.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: That unknown was diluted fourfold before testing. Find the original concentration in mg cm⁻³ and mg per 100 cm³.
0.160 mg cm⁻³, equivalent to 16.0 mg per 100 cm³.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.