Core practical 2: vitamin C content of food and drink

Core practical 2: vitamin C content of food and drinkSpec 1.14

DCPIP (dichlorophenolindophenol) is a blue dye. Vitamin C (ascorbic acid) is a reducing agent: it reduces DCPIP to a colourless compound. The more vitamin C a solution contains, the less of it is needed to decolourise the same volume of DCPIP.

1. Make standard solutions of ascorbic acid of known concentration (for example 1.0 mg cm⁻³ and dilutions of it).
2. Place a fixed volume of DCPIP solution (for example 1 cm³) in a test tube or small beaker.
3. Add the first standard drop by drop from a graduated pipette or burette, shaking the tube after each drop, until the blue colour just disappears. Record the volume added.
4. Repeat with each standard and plot a calibration curve of volume needed against concentration (or use the standard which decolourises 1 cm³ DCPIP to find its equivalent).
5. Extract juice from a solid food by crushing it in water and filtering, then treat the unknown in exactly the same way. Repeat each measurement at least three times and calculate a mean.
6. Use the volume needed and the calibration to find the vitamin C concentration of the unknown.

Variables in the DCPIP method

Type	Variable
Independent	Type of food or drink, or its treatment (for example fresh or boiled)
Dependent	Volume of sample needed to decolourise DCPIP, which gives the vitamin C concentration
Controlled	Volume and concentration of DCPIP, temperature, rate of adding drops, same measuring equipment, fresh solutions each time, time between preparing and testing

Risks: DCPIP can stain and irritate skin and eyes, so wear eye protection and gloves and wipe spillages. Do not eat or drink any food used in the laboratory.

Limitations: vitamin C is destroyed by heat, light and air (oxidation), so keep solutions cool and fresh and test promptly. Other reducing substances in a food may also decolourise DCPIP. The end point is judged by eye, which is more difficult in coloured juices, so dilute the juice or use a paler food, and use the same observer.

Calculating vitamin C concentration

It takes 0.8 cm³ of a 1.0 mg cm⁻³ ascorbic acid standard to decolourise 1 cm³ of DCPIP. It takes 1.6 cm³ of fruit juice to decolourise 1 cm³ of the same DCPIP. Calculate the vitamin C concentration of the juice, and the percentage change if boiling the juice raises the volume needed to 2.0 cm³.

1. Mass of vitamin C that decolourises 1 cm³ DCPIP = 0.8 cm³ × 1.0 mg cm⁻³ = 0.8 mg
2. Concentration of juice = 0.8 mg ÷ 1.6 cm³ = 0.5 mg cm⁻³
3. Boiled juice = 0.8 mg ÷ 2.0 cm³ = 0.4 mg cm⁻³
4. Percentage change = (0.4 − 0.5) ÷ 0.5 × 100 = −20 %

Answer: 0.5 mg cm⁻³; boiling reduced the concentration by 20 %.

Maths skill:

Percentage change = (new value − original value) ÷ original value × 100. A negative answer is a decrease. Show the working and include units for concentrations.

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Quick check

1. Name the four stages in the course of events leading to atherosclerosis.

   Show answer

   Endothelial dysfunction, inflammatory response, plaque formation and raised blood pressure.

2. What does thrombin do?

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   It is an enzyme that catalyses the conversion of soluble fibrinogen to insoluble fibrin.

3. Give two ways smoking raises the risk of CVD.

   Show answer

   Any two of: carbon monoxide reduces oxygen carried by haemoglobin; nicotine raises heart rate and blood pressure; chemicals damage the endothelium; platelets are stickier.

4. How do antioxidants reduce the risk of CVD?

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   They neutralise free radicals, so less LDL is oxidised and there is less endothelial damage and foam cell formation.

5. In Core practical 2, what colour change shows vitamin C?

   Show answer

   Blue DCPIP turns colourless.