What it shows
In a clock reaction nothing seems to happen for a measured time and then the colour changes suddenly. The delay depends on how much vitamin C is present, so the time can be used to measure concentration.
The science
Hydrogen peroxide slowly oxidises iodide ions to iodine. As soon as iodine forms, vitamin C (ascorbic acid) reduces it back to colourless iodide, so no colour appears. When all the vitamin C is used up, the iodine that forms next is free to react with starch and gives a sudden blue-black colour. More vitamin C means a longer wait. Warmer mixtures react faster, so the clock runs faster, as in all rate studies.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 500 mg or 1000 mg vitamin C tablet, crushed
- Tincture of iodine (2 per cent) from a pharmacy
- Hydrogen peroxide 3 per cent from a pharmacy, 100 ml
- Starch paste: half a teaspoon of corn flour boiled in 100 ml water and cooled
- Five clear glasses and a syringe or dropper for measuring
- A stopwatch and white paper
- Warm and cold water for the temperature test
Safety
For this project: An adult must supervise. Tincture of iodine is poisonous if swallowed and stains skin and clothes. Hydrogen peroxide can irritate eyes. Wear gloves and goggles, do not taste anything and keep away from food. Pour waste down the drain with plenty of water only if the adult agrees.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Dissolve one crushed vitamin C tablet in 100 ml of water to make solution A. Stir until it is clear.
Mix 10 ml of tincture of iodine with 50 ml of water in a second glass. Add solution A drop by drop until the brown colour just disappears. The colourless mixture is solution B and it holds iodide.
In glass 1 place 10 ml of solution B, 5 ml of starch paste and 2 ml of solution A. This extra vitamin C is what the clock must use up. Add water to make 30 ml.
Add 10 ml of hydrogen peroxide, start the stopwatch at once and swirl.
Stop the watch when the whole liquid turns blue-black and write the time. If it takes more than ten minutes, use less extra solution A next time; if the colour is instant, use more.
Repeat with 4 ml, 6 ml and 8 ml of extra solution A in the other glasses, adding water to keep the total at 30 ml, to see how the time changes.
Plot a graph of the time against the volume or concentration of vitamin C and see whether it is a straight line.
Repeat one run with the glasses stood in warm water for five minutes first, and note how much shorter the time is.
What you should see
A graph showing that the time taken to turn blue is longer when more vitamin C is present, and shorter in a warmer mixture. The sudden colour change shows that the vitamin C has run out.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why is there no colour at first?
Vitamin C changes any iodine formed back into colourless iodide.
02Why does the colour appear suddenly?
When the vitamin C is finished, iodine builds up and reacts with starch at once.
03What is the role of starch?
It is an indicator that turns blue-black when iodine is present.
04How can this be used to find vitamin C in juice?
By comparing the time taken with that of known amounts of vitamin C in a graph.
Take it further
Use the graph of known amounts to estimate the vitamin C in a glass of orange juice.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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