Is Fresh Best? – Standard Procedure

1. Scope

This experiment is used to determine the vitamin C content in a food substance. In particular this SP is used to determine the vitamin C content in fresh and cookedcabbage.

This SP has been adapted from:

‘Experiment 2 Investigating the effect of cooking on thevitamin C content of cabbage’; Nuffield Advanced ChemistryFood Science Students’ book. Furse, A (Ed); 2005. Available from:

[Accessed 08/05/09]

Teacher and technician guidance available from:

2. Principle

Vitamin C is found in green vegetables, fruits, and potatoes. It is essential for a healthy diet. The chemical name for vitamin C is ascorbic acid. Ascorbic acid is a good reducing agent and therefore it is easily oxidised. Methods for the detection of vitamin C involve titrating it against a solution of an oxidising agent. There are several oxidising agents that can be used and a commonly used one is 2,6-dichlorophenol-indophenol or DCPIP. You need to standardise this against a known concentration of vitamin C. This means finding out how much DCPIP reacts with a known amount of vitamin C. You can check the end point colour by testing a small amount of vitamin C with the DCPIP and observe the disappearance of the blue colour. The end point is usually a pink colour that persists for about 15 seconds or longer. You can then use your standardised results to determine the concentration of vitamin C in fresh cabbage compared to cooked cabbage.

3. Reagents

  • De-ionised water
  • 2,6-dichlorophenol-indophenol or DCPIP standard solution - 0.04 % solution (400 mg dm–3).
  • Standard (200 mg dm–3) vitamin C solution
  • 5% phosphoric(v) acid solution
  • Fresh cabbage

4. Apparatus

  • 50 cm3burette
  • 25 cm3volumetric pipette
  • pipette filler
  • funnel
  • flat-bottomed conical flask
  • white tile
  • food liquidiser
  • 500 cm3 measuring cylinder
  • 250 cm3 glass beaker
  • Bunsen burner, heatproof mat, tripod, gauze

5. Safety

A Risk Assessment must be developed and used before doing this experiment.

Hazcard 72 – Phosphoric acid

6. Procedure

i) Standardization of indicator solution

Before the indicator is used to determine the vitamin C content of cabbage,it must be standardised – to find the number of mg of vitamin C equivalent to1 cm3 of dye solution. The indicator has been made up by dissolving 0.4 g of the dye 2,6-dichlorophenolindophenol in 200 cm3 of warm water, filtering thesolution, and making up the filtrate to 1 dm3 with pure water, to giveapproximately 0.04 % solution (400 mg dm–3).

a) Use a pipette, with safety filler, to transfer 25.0 cm3 of standard vitamin Csolution (200 mg dm–3) to a conical flask and titrate rapidly with the dyesolution from a burette.

b) As the dye is run in, the deep blue colour of the dye is discharged to give acolourless solution. The end-point is taken to be when the pink coloration, dueto the dye, persists for 10 seconds.

c) A blank titration using 25.0 cm3 of 5% phosphoric(v) acid solution must becarried out to the same end-point.

ii) Estimation of vitamin C in uncooked cabbage

a) Weigh out 50 g of cabbage and put it in a liquidiser with 250 cm3 of5.0% phosphoric(V) acid. Liquidise at high speed. Filter off the liquid into a 500 cm3 measuring cylinder, using a muslin filter.

b) Make up the volume of extract plus washings to 300 cm3.

c) Titrate 25.0 cm3 of the extract with indicator solution as before. Alltitrations should be carried out in duplicate.

iii) Estimation of vitamin C in cooked cabbage

a) Simmer 50 g of cabbage in 100 cm3 of water for 10 minutes, allowingsome of the water to evaporate. Repeat the above experiment on the cookedcabbage.

b) Determine the loss in vitamin C on cooking cabbage for 10 minutes.

c) If time allows, repeat the titration on the residual water and determine theamount of vitamin C present in solution.

If time allows, the effect of cooking time on vitamin C loss could be studied (1 min, 2 min, 3 min, 5 min, 20 min)

7. Calculations

i) Calculation of dye factor

The number of mg of vitamin C equivalent to 1 cm3 of dye (dye factor) maybe calculated by deducting the blank titre from the standardisation titre andusing the concentration of vitamin C.

Dye factor = F mg cm–3

= mg of vitamin C equivalent to 1 cm3 of dye solution

So dye factor (F)

=volume of standardised vitamin C/cm3 x concentration of vitamin C/mg dm–3

(standardisation titre/cm3 – blank titre/cm3) x 1000

ii) and iii) Calculation for uncooked and cooked cabbage

Mass of sample = 50 g

Volume of extract solution = 300 cm3

Volume of dye titre = V cm3

Work out for yourself that:

100 g of sample contain V x F x 300 x 2/25 mg of vitamin C

8. Expression of results

You need to express the concentration of vitamin C in both cooked and uncooked cabbage in mg of vitamin C per 100 g of cabbage. If time allows and you are able to determine the concentration of vitamin C in different cooking times, plot a graph of vitamin C concentration (y-axis) against cooking time (x-axis) – state the pattern in your results.