Measuring vitamin C with DCPIP is a favourite 9700 Paper 5 planning question. Here whole lemons are stored for up to 10 days, a juice extract is made on each test day, and the juice is run from a burette into 5.0 cm³ of DCPIP until the blue colour just disappears. Standard ascorbic acid solutions calibrate the method. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Biology 9700 Paper 5, Question 1. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9700/51 October/November 2023, Question 1 (ascorbic acid in ripening apples, measured with DCPIP from a burette) · official past papers
Practise next: 9700/53 October/November 2023, Question 1 (the same vitamin C planning question) · official past papers
Practise next: 9700/52 February/March 2023, Question 1 (effect of storage time on potatoes, with a calibration curve) · official past papers
Examiner reports used: 9700 March 2023, November 2023 (Papers 51 and 53) and March 2024.
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What a full-mark diagram shows
The independent variable
Store whole fruit from one batch under stated conditions (20 °C, dark) and test after at least five stated times, such as 0, 2, 4, 6, 8 and 10 days.
The extract
Remove peel and seeds, weigh a set mass, blend with a stated volume of distilled water, filter through filter paper. Name the apparatus.
The measurement
A stated volume of DCPIP in a conical flask; juice from a burette drop by drop, swirling; record the volume at the end point, read at the bottom of the meniscus.
Reliability and safety
Three replicates and a mean for each storage time; standards made by proportional dilution to calibrate; hazard, risk and precaution.
Habits the examiner reports keep asking for
Store the whole fruit, not pieces or juice, and state the conditions rather than “the same temperature”.
Get the end point the right way round. Juice into DCPIP: the blue just disappears. DCPIP into juice: the blue first remains.
Measure a volume, not drops or time, and read the bottom of the meniscus at eye level.
Write “mean” and “distilled water”, not “average” and “water”, with a mean for each storage time.
“Low risk” scores nothing. DCPIP is an irritant: gloves and eye protection. Cut away from your hand.