Use DCPIP to test for vitamin C
Choose the reagent, explain the method and interpret controlled observations.
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall the distinction between an element, a molecule and a cell structure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the target of the DCPIP test?
What you will learn
- Choose the reagent, explain the method and interpret controlled observations.
- Explain the conclusion from stated evidence and recognise its limits.
Choose the target and method
DCPIP tests for vitamin C. Add prepared vitamin-C sample to a measured portion of blue DCPIP using the specified procedure until the endpoint is reached. The blue colour is removed; acidity or strong sample colour can complicate endpoint observation.
The starting or negative appearance is blue; the positive observation is decolourisation of the blue dye. Record the observed change before writing a conclusion. A coloured reagent is not automatically a positive result.
Check what the result proves
A known vitamin-C solution is a positive comparison. Keep DCPIP volume and concentration fixed for this sample-into-dye method. Less sample needed for that same endpoint suggests more vitamin C per unit sample volume. If the alternative method adds DCPIP to a fixed sample volume, more dye needed instead suggests higher vitamin-C content: do not reverse the interpretation or mix the protocols. Strong acidity can leave a pink endpoint, and strong juice colour or other reducing substances can limit interpretation. A target-free water comparison checks that addition alone does not remove the blue dye. Repeat endpoint measurements and compare means; prepare equal dilutions if needed and account for them. These classroom results do not establish a clinical or nutritional recommendation.
Use a separate clean sample portion for each test and clean pipettes to avoid contamination. A positive test supports detection of the named target under these conditions; it does not measure the whole nutrient composition. A negative result means the target was not detected by this method, not proof that none exists at any concentration. Practical work needs teacher-approved procedures and supervision; these written exercises do not certify laboratory competence.
Pause and explain
A smaller sample volume decolourises the same fixed DCPIP portion. Why is claiming lower vitamin-C concentration incorrect?
Worked example
A properly controlled DCPIP test gives decolourisation of the blue dye. Interpret it.
Show the worked solution
- Identify the reagent and its named target.
- Compare with the positive and negative controls.
- Restrict the conclusion to the target detected.
Answer Vitamin C detected under the stated test conditions.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the target and positive observation of DCPIP.
Give me a hint
Link reagent to one target.
Compare my reasoning
- Identify vitamin C.
- State decolourisation of the blue dye.
Vitamin C; decolourisation of the blue dye.
Look for these in your work
- Name the tested target and the observed change.
- Use the correct procedure and comparison when interpreting the result.
A dark juice masks the DCPIP endpoint while a known vitamin-C control decolourises the dye and the water control stays blue. Can the juice be reported as vitamin-C-free? Suggest an improvement.
Give me a hint
Working controls do not remove every sample-specific interference.
Compare my reasoning
- The masked endpoint cannot be read reliably.
- A negative conclusion is therefore unsupported.
- Use a teacher-approved preparation or known dilution that makes the endpoint observable; account for dilution and repeat comparisons.
No. The juice colour obscures the endpoint; improve preparation or use a known dilution under teacher supervision and repeat.
Look for these in your work
- Name endpoint masking.
- Avoid claiming absence from an unreadable result.
Samples A and B decolourise identical DCPIP portions using 4.0 cm³ and 2.0 cm³ respectively, under matched conditions without interfering substances. Which has higher vitamin-C concentration and by what ratio?
Give me a hint
The same dye endpoint requires the same vitamin-C amount.
Compare my reasoning
- For fixed DCPIP, concentration is inversely related to required sample volume.
- Compare 4.0 ÷ 2.0 = 2.
- The smaller-volume sample B is more concentrated.
B has approximately twice A’s vitamin-C concentration under the stated assumptions.
Look for these in your work
- Keep DCPIP fixed.
- State the comparison assumptions.
Common mistakes
- Starting reagent colours and sample colours can mislead.
- Controls help validate the comparison and procedure.
What can you explain now?
A smaller sample volume decolourises the same fixed DCPIP portion. Why is claiming lower vitamin-C concentration incorrect?
Compare with the explanation
The observation must be interpreted using the correct method and controls.
Under matched conditions this suggests higher, not lower, vitamin-C concentration.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall the target, method, positive change and one control without looking.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.