What it shows
The acetic acid content of vinegar can be compared by titrating a measured volume with a standard washing soda solution until an indicator changes colour. Running three brands and a home-made dilution ranks them by strength and, with a simple calculation, gives an approximate percentage of acid to compare with the label.
The science
Sodium carbonate neutralises acetic acid, forming sodium acetate, water and carbon dioxide; one carbonate unit neutralises two acetic acid molecules. When the sodium carbonate solution is added drop by drop to vinegar, the acid is used up gradually. Phenolphthalein stays colourless in acid and turns pink once there is a slight excess of carbonate, so the first lasting pink marks the end point. The volume of soda solution needed is proportional to the acid in the sample, so brands can be ranked, and the known concentration of the soda solution allows the acid percentage to be estimated.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three vinegar brands and a 1 to 1 dilution of one of them
- Washing soda (sodium carbonate), 10.6 g dissolved in water and made up to 1 litre, or a solution prepared by the teacher
- Phenolphthalein from the school lab, or red cabbage indicator
- A 1 ml syringe or a dropper with counted drops, and a 5 ml syringe
- Small white cups or conical flasks
- A notebook and calculator
Safety
For this project: Vinegar and washing soda solution can irritate eyes; wear goggles if available and wash hands. The teacher handles phenolphthalein. Nothing is tasted.
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.
All the safety rulesStep 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.
Calibrate your dropper by counting how many drops make 1 ml using the syringe, and use the same dropper throughout.
Measure exactly 2 ml of the first vinegar into a white cup and add 10 ml of water and two drops of indicator.
Add washing soda solution drop by drop, swirling, and watch for fizzing and for a faint pink that lasts thirty seconds.
Record the number of drops, convert to millilitres, and repeat twice more for the same brand.
Repeat for the other brands and the diluted sample, using fresh cups.
Average the three results for each sample and rank the samples.
Calculate approximate acetic acid: moles of carbonate used times two gives moles of acid, times 60 g per mol gives grams of acid in 2 ml, then convert to a percentage.
Compare your percentages with the labels and discuss sources of error such as the drop size and the faintness of the end point.
What you should see
Each brand needs a reproducible volume of soda solution, the diluted sample needs almost exactly half the volume of its parent, and the calculated acid content comes out near the 4 to 5 percent usually stated on labels.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the end point?
The point where the acid has just been completely neutralised, shown by the indicator changing colour.
2Why does one carbonate neutralise two acetic acid molecules?
Carbonate can accept two hydrogen ions, one from each acid molecule.
3Why did the diluted sample need half the drops?
It contains half the acid in the same volume.
4What was the biggest source of error?
Drop size varying, and judging the faint pink end point by eye.
Take it further
Titrate lemon juice and lime juice the same way and compare their citric acid content, remembering that citric acid gives three hydrogen ions per molecule.
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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