Serial dilutions and end points judged by eye are 9700 Paper 3 regulars. By hand, make a halving series from a 1.0 % vitamin C solution with two syringes, measure 1.0 cm³ of DCPIP into a tube, then push the solution in slowly from a 5 cm³ syringe and stop the moment the blue has gone. Read the syringe before and after, repeat every concentration, titrate the fruit juice the same way, then plot your calibration curve and read the juice off it. Every attempt uses a different DCPIP batch and a different juice. Allow about 45 minutes.
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Five concentrations that halve each time, each tube made with equal volumes from the previous tube and distilled water, and tubes on the bench that really hold those concentrations. Examiners note that candidates confuse serial with proportional dilution (9700 June 2025 examiner report).
Headings with units, every volume to 0.1 cm³, volumes that are the syringe readings you took, at least two runs per concentration, correct means, 1.0 cm³ of DCPIP in every run, and end points not overshot by more than a quarter of a cm³.
Concentration on the x-axis and mean volume on the y-axis, a smooth curve or ruled lines with no extrapolation, the juice titrated twice, and its concentration read by interpolation from your own line. Reading a value off a graph needs units and sensible precision (June 2025 report).
An inverse relationship described in two stages, explained by the number of vitamin C molecules per cm³, and the meaning and drawback of a serial dilution.
The subjective end point as the main error, an improvement that really reduces it (drop by drop, white tile, one judge, a colour standard), the volume of DCPIP as the standardised variable with the method, and a low-risk assessment that names DCPIP as a stain and irritant.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.