Determination of vitamin C in a supplement
Vitamin C is one of the vitamins listed in B.5: Vitamins. You will need to discuss its structure and explain why it is water-soluble as well as discuss the causes and effects of a lack of vitamin C in the diet. The picture taken from Man-of-War by Stephen Biesty (Dorling-Kindersley, NY, 1993) graphically shows the effects of scurvy on the gums and teeth! The recommended daily allowance is about 60-70 mg and this can be obtained easily by eating fresh fruit and vegetables. However many people do also take supplementary vitamin pills (perhaps because they erroneously think that it will help to ward off the common cold - see the page which includes the section on Linus Pauling and the ad hominen fallacy).
Because it is easily oxidised (it is a good antioxidant) the percentage composition of viatmin C in a sample can be determined easily by titrating against an oxidising agent such as DCPIP (2,6-dichlorophenol-indophenol) or iodine. This experiment uses iodine. In order to make it a little more challenging the concentration of the iodine solution given to the student is unknown so the first step for them is to standardise the iodine solution. This practical is suitable for both SL and HL students. SL students will need more guidance to obtain the correct redox equations so these should probably be given. Because the calculation is not straightforward as it involves several redox reactions you could use this practical to assess Data collection and processing (DCP) and Conclusion and evaluation (CE). For this reason I have removed the calculation section from the downloadable worksheet to be given to students but have left it on the version for you below.
Most teachers will probably use this practical if they are teaching Option B, or if their students want to study some aspect of vitamin C for their Extended Essay or Individual Scientific Investigation. It could actually be used to cover the Mandatory laboratory componentTopic 1.3. Use of the experimental method of titration to calculate the concentration of a solution by reference to a standard solution.
You (or your technician) will need to prepare three separate solutions.
Iodine solution. This should be approximately 0.050 mol dm-3 and can be made by 12.8 g of iodine and 40 g of potassium iodide in water and making up to one litre.
0.100 mol dm-3 sodium thiosulfate solution.
1% starch solution. Mix 1 g of starch into a paste with a little water then add 100 cm3 of boiling water.
Use a 1 g vitamin C tablet.
DETERMINATION OF VITAMIN C CONTENT
To determine the percentage by mass of vitamin C in a vitamin tablet.
Vitamin C is present in many fruits and vegetables. It is necessary part of the diet as it is required to form the protein, collagen. A lack of vitamin C can cause lesions in the skin – a condition known as scurvy. To ensure a sufficient intake of vitamin C some people take vitamin supplements.
Vitamin C has the formula C6H8O6 and can be oxidised to form C6H6O6. The amount of vitamin C present in a vitamin tablet can therefore be determined by titrating a known amount of the tablet with an oxidising agent. In this experiment iodine, I2(aq) is used as the oxidising agent. The iodine provided is of unknown concentration so that the first step of the experiment is to standardise the iodine solution using standard sodium thiosulfate solution, Na2S2O3(aq).
ENVIRONMENTAL CARE: The small amounts used in this experiment do not contain any seriously harmful substances to the environment once diluted and can be disposed of safely down the sink.
SAFETY: Avoid skin contact with the iodine solution and be sure to wear safety glasses to avoid getting any iodine in your eyes.
(a)To standardise the iodine solution.
Pipette 10.0 cm3 of the 0.100 mol dm-3 sodium thiosulfate solution provided into a conical flask and add a few drops of freshly prepared starch solution. Titrate with the iodine solution provided until one drop causes the blue-black colour to remain permanently. Repeat the procedure to obtain two accurate results.
(b)To determine the percentage of vitamin C.
Record the mass of a vitamin C tablet and then dissolve it in approximately 50 cm3 of distilled water. Transfer the solution and all the washings into a 100 cm3 volumetric flask and make up to the mark with distilled water. Pipette a 10.0 cm3 aliquot of this solution into a conical flask and add a few drops of freshly prepared starch solution. Titrate with the solution of iodine until one drop causes the blue-black colour to remain permanently. Repeat the procedure to obtain two accurate results
Record your results in a suitable format. From your results determine the concentration of the iodine solution used and hence the percentage of vitamin C present in the tablet.
The equation for the reaction of sodium thiosufate with iodine is:
I2(aq) + 2Na2S2O3(aq) → 2NaI(aq) + Na2S4O6
1. Determine the amount (in mol) of sodium thiosulfate in 10.0 cm3 of 0.100 mol dm-3
2. Determine the amount of iodine (in mol) in the average volume of I2(aq) used in the titration
3. Determine the concentration (in mol dm-3) of the iodine solution.
The half-equation for the reduction of iodine is:
I2(aq) + 2e– → 2I–(aq)
and the half-equation for the oxidation of vitamin C is:
C6H8O6(aq) → C6H6O6(aq) + 2H+(aq) + 2e–
4. Give the overall equation for the reaction of vitamin C with iodine.
5. Calculate the amount of iodine (in mol) in the average volume used to react with the vitamin
6. Calculate the amount of vitamin C in the 10.0 cm3 aliquot of vitamin C solution
7. Calculate the amount of vitamin C present in the vitamin C tablet.
8. Calculate the percentage by mass of vitamin C in the vitamin C tablet.
Determination of vitamin C content
Here is a list of suggestions for Science Investigations. Please add/amend/extend the list as you see fit. This is essentially a brainstorm...
Some industries will mix liquids by cascading one liquid into another, assuming mixing will be complete. These drops may prevent complete mixing and need to be investigated.
Recent research has shed light on this phenomenon: The fast-moving shiny drops that skid across the surface of soapy water. Detergents play a crucial role – but how does the concentration of detergent affect the drops? What about ‘non-ionic’ detergents? What about a surface of anionic detergents and drops of cationic ones?
Challenges: Making drop-producing apparatus, Researching the chemistry of detergents, Assessing the numbers of drops
Thiosulphate/acid reaction (‘disappearing cross’)
Chemical engineers wishing to check if turbidity is a useful surrogate measurement for whether a reaction has reached completion.
How does the reaction respond to changes in acid concentration (it is first order for thiosulphate concentration but varies from first to zero order with acid).
How can we assess the amount of oil contamination on beaches?
Various groups will want to assess oil contamination, and a simple quick method would be appreciated.
The method used solvent extraction. Sand samples were laced with oil and the efficiency of recovery was measured. The sensitivity of the method was also estimated.
Synthesis and isolation of propanoic acid from propan-1-ol.
Chemical suppliers who want to investigate production of this chemical.
Trialling different methods.
Making biodiesel from waste vegetable oil.
Motoring enthusiasts who want to make their own environmentally-friendly and cheap fuel.
Using methanol and sodium hydroxide it is possible to make diesel fuel from old chip fat.
Analysing the gases remaining after burning a candle in a gas jar.
Motor vehicle engineers who want to check the composition of exhaust gases.
CO2 is easy; it is readily absorbed with NaOH.
O2 is harder; an oxygen probe is very useful but otherwise oxygen could be absorbed by, for example, the slow reaction of steel wool.
CO is challenging; it will be absorbed by CuCl solution but it is in very low concentrations.
Suppliers or independent analysts want to assess the caffeine content of tea products. Chemical suppliers want to provide pure caffeine to clients.
This method usually involves solvent extraction with a chlorinated alkane (e.g. dichloromethane or 1,1,1,-trichloroethane). With care it is possible to recycle the solvent and avoid health and environmental damage.
- Investigation of weak acid strengths/synthesis of new acids
- Rate expression for tin/iodine reaction – kinetics of a reaction
- Nobili’s rings – strange colours seen during electrolysis.
- A transient red colour – the chemistry of iron/sulphate complexes.
- Liesegang rings – stripey effects in gels.
- Polarimetry to investigate concentrations of sugars. Suppliers of sugar syrups who want to assess their products.
- Flame photometry to determine concentrations of aqueous ions. Analysis of mineral water samples to check manufacturers' claims.
- Is the level of chloramines found in swimming pool water affected by temperature? Swimming pool operators concerned about eye irritation amongst their customers.
- Effectiveness of two major washing powders on five common household stains. Consumer testing organisation who wants to test commercial claims.
- Factors affecting the decomposition of biodegradable plastics. Environmental agencies testing claims of environmental-friendliness from plastics manufacturers.
- Studies of infrared spectroscopy: nature and applications. Applications include breath-testing for alcohol and testing vehicle exhaust emissions.
- NaCl and potential alternatives to NaCl as a roadway de-icer. Highways agency investigating de-icers which cause less environmental damage - but at what cost?
- Synthesis and analysis of hexane-3,4-diol.
- Calcium carbonate content of seashells.
- Characterisation of New Polyethylenes Prepared by Single-Site Catalysis.
- An Investigation Into The Uses Of Environmentally Friendly Additives/Alternatives To Petrol.
- A study into the colour of ketchup.
- Comparison By Spectral Characterisation Of Myoglobin In Beef, Pork and Turkey.
- Spectrophotometric Determination of Trace Amounts of Iron in Plant Samples.
- The effect of disulphide reduction on the structure, and activity of Yeast Alcohol Dehydrogenase.
- Investigation into the effect of pH on the mechanism of glycation of phenylalanine as a model of the glycation of the B chain of insulin.
- Analysis of Vitamin C in Orange Juice through a Chemiluminescent Clock Reaction.
- Investigating the Effect of Different Halide Ions on the Stern Volmer Constant.
- Developing and Validating an Assay for Tissue Thiols and the Analysis of the Effects of Aging and Taurine on Tissue Thiols.
- Relation between the Slow Relaxation Time and Foaming of Selected ICI Surfactants.
- Purification of ibuprofen from commercial tablets.
- Building a breathalyser.g
- Titration of various commercial vinegar samples.
- Titration of vitamin C content of commercial fruit juices.
- Investigation in the synthesis of acetylsalicylic acid (aspirin).
- ↑ abcdefghijklmnopqrstSuggested IB Extended Essay projects
- ↑ abcdIn search of more solutions - Royal Society of Chemistry resource book