Introduction
The characteristic smell of vinegar comes from the inclusion of ethanoic acid (CH3COOH), also known as acetic acid. However, unless pure distilled vinegar is purchased, with a guaranteed and stated composition, there are other acids present in vinegar. These include:
- citric acid - HOOCCh1C(OH)(COOH)Ch1COOH),
- malic acid (2-hydroxybutanedioic acid) - HOOCCH(OH)Ch1COOH
- lactic acid (2-hydroxypropanoic acid) - CH3CH(OH)COOH
- tartaric acid – (CHOH)2(COOH)2
For the purposes of this investigation the most volatile acid – ethanoic acid, is taken to be a measure of the acidity. An attempt will be made to purify the vinegar so that it is mainly the ethanoic acid that is contributing the acidic content, but this cannot be guaranteed.
Vinegars chosen will be malt vinegar (A), red wine vinegar (B) and cider vinegar (C).
Stage 1 – removing the colour from the vinegar
Materials required
100cm3 of Vinegar samples A to C.
3 volumetric flasks, labelled A to C (250 cm3)
3 conical flasks, labelled A to C (250 cm3)
Activated charcoal
Metal spatula
Glass stirring rod
Filtration funnel
Filter paper
Method
Place vinegar samples A in volumetric flask A and add 1 spatula measure of activated charcoal. Stir well using clean glass rod. Filter resulting mixture through filter paper and funnel into conical flask labelled A.
Repeat using vinegar sample B with flasks labelled B; Likewise sample C.
Note that the filtration process may need to be repeated if colour remains in the vinegar. If charcoal is not fully removed from mixture then suction filtration may be required.
Stage 2 – distilling the vinegar
Due to the presence of other acids and additive to the vinegar the mixture needs to be distilled to purify the mixture to ethanoic acid and water.
Materials required
Distillation column and separating funnel
Pear shaped flask
Conical Flasks A to C from stage 1
Anti bumping granules
Bunsen burner
Heatproof mat
Clamp and stand
Goggles
Ice bath
Universal indicator paper
Pippette
Method
Add filtered vinegar sample A to a pear shaped flask and set up distillation column above Bunsen, using clamp and stand. Ensure all gaps are well sealed.
Wash conical flask A with distilled water and use for collection of distillate
Arrange column so that distillate drips into conical flask A, placed in ice bath.
Gently heat mixture in pear shaped flask until distillate begins to collect in conical flask. Test to see when all volatile acid has been distilled by placing drops of distillate onto universal indicator and checking pH colour. When pH registers over 7 cease heating.
Repeat using vinegar samples B and C using clean apparatus.
Stage 3 – determining the concentration of ethanoic acid
Option A - chromatography
Materials
Filter paper with pencil line drawn 2cm from bottom edge and pencil spots marked and labelled at 2 cm intervals along it
Samples of ethanoic acid of known concentration eg 0.01 mol/dm3, 0.1 mol/dm3 and 0.05 mol/dm3
Glass beaker
Vinegar samples A to C
Glass pipette
Water
tweezers
Bromocresol green indicator solution
Household plant spray bottle
goggles
Method
Place spots of known concentration ethanoic acid on each of the labelled pencil spots on the filter paper. Place spots of vinegar samples A to C on remaining spots.
Place chromatogram in beaker with water up to 1 cm, ie below pencil line.
Leave until water has spread ¾ up filter paper. Remove with tweezers and leave to dry.
When dry spray with bromocresol green solution from household plant spray bottle until paper is damp but not sopping. Dry chromatogram.
Compare colour of indicator of samples A to C to known concentrations of CH3COOH, the one closest in colour would represent the actual concentration.
This method would not be chosen in the final experiment, as there is a lot of subjectivity to comparing the colours. Also the technique of spraying the chromatogram is not easy to master.
Option B – titration using sodium hydroxide (NaOH).
Equations and scientific calculation methodology
Ethanoic acid (CH3COOH) is a weak acid , which only dissociates partially in solution with water. As an acid CH3COOH donates protons, which are accepted by water in solution to form the hydroxyonium ion:
CH3COOH (aq) + h10(l) ↔ CH3COO-(aq) = H30+(aq)
Sodium hydroxide is a strong base, so it dissociates fully in water:
NaOH(aq) {+ h10 }↔ Na+(aq) + OH-(aq) {+ h10}
Note, that the water is effectively unchanged by the NaOH as the strong base dissociating causes an excess of OH-, which would shift the normal water dissociation equilibrium h10 ↔ H+ + OH- to the left.
When titrated 1 mol of CH3COOH would react with 1 mol of NaOH in a neutralisation reaction to form the salt and water:
NaOH (aq) + CH3COOH(aq) ↔ CH3COONa (aq) + h1O(l)
Therefore if a known amount of a known concentration of NaOH is used to titrate a known volume of vinegar, the concentration of ethanoic acid in the vinegar could be calculated as shown in the example below:
Molarity of solution x volume of solution = number of mols used.
If 23ml of NaOH is used to titrate 20ml of vinegar and
Molarity x volume / 1000 = no of mols used
1 x 23/1000 = 0.023mols of NaOH used to neutralise 20ml vinegar
As 1 mol of NaOH is needed to neutralise 1 mol of CH3COOH then 20ml of the vinegar solution must also contain 0.023mols of CH3COOH.
Using molarity x volume /1000 = no of mols used with the data that is known:
Molarity x 20 / 1000 = 0.023
0.023 x 1000 = molarity x 20
0.023 x 1000 / 20 = molarity
= 1.15M
Therefore the vinegar is of concentration 1.15M.
However, it is known however that vinegar contains around 5% ethanoic acid in water. Therefore it can be assumed that the concentration of ethanoic acid would be closer to 0.1 mol/dm3 than 1mol/dm3. As 1mol of CH3COOH neutralises 1 mol of NaOH, it would therefore be sensible to use NaOH of 0.1 mol/dm3.
- Part i – standardisation of alkali
NaOH solution left for any length of time includes gases that have diffused in from the air. Therefore, in order to be certain that the concentration is precise the alkali should be newly mixed immediately prior to use.
Materials
Sodium hydroxide (solid)
Distilled water
Measuring cylinder
Glass stirring rod
Metal spatula
volumetric flask (1000 cm3)
Method
As NaOH has a molar mass of 40g/mol to make a 0.1 mol/dm3 solution 2.0g of NaOH(s) should be dissolved in 500ml distilled water in a volumetric flask and stirred until dissolved.
- Part ii - titration of ethanoic acid with NaOH
Materials
Burette
White tile
Clamp and stand
Funnel
Conical flask (250cm3)
Volumetric flask (250cm3)
Phenolphthalein indicator solution
Phenol red indicator
0.1M NaOH solution
Vinegar samples A to C
Method
Fill clean burette with 0.1M NaOH solution.
Place 10cm3 of vinegar sample A in conical flask with a few drops of phenolphthalein indicator.
Add NaOH to vinegar sample slowly, swirling flask to ensure mixing. Phenolphthalein will turn from colourless to pink at the equivalence point (the point at which the amount of moles of one reactant is just enough to react exactly with the measured amount of the other reactant) as there would be the same amount of NaOH and CH3COOH present in the flask, leading to a neutralisation reaction, as explained above. It will be assumed for this experiment that the end point and equivalence point are the same.
The experiment would be repeated with vinegar samples B and C.
The experiment would also be repeated using phenol red as an indicator. This would be expected to change from yellow to red at the equivalence point .
The amount of 0.1M NaOH used could be used to calculate the concentration of ethanoic acid in each of the samples, via the equation:
Molarity x volume / 1000 = no of mols used
Risk assessment and safety – chemicals
NaOH is an irritant and can cause burns. Ethanoic acid is a corrosive agent. Phenolphthalein is an irritant and can cause burns.
Goggles and protective clothing should be worn throughout.
If chemical comes into contact with the skin the area should be washed with plenty of clean running water. If a large area of skin is affected or skin shows signs of burning (eg redness / swelling) medical advice should be sought immediately.
If chemical comes into contact with the eyes wash out immediately with sterile eye wash.
If chemical is ingested wash out mouth with clean water and seek medical advice immediately. Do not induce vomiting as this may cause further damage.
Do not breathe fumes of fresh NaOH solution. Ideally experiment should take place in a fume cupboard. If fumes are inhaled move immediately to an area of fresh air. In case of difficulty in breathing medical advice should be sought.
Risk assessment and safety - equipment
Burette should be filled with a funnel and should not be above shoulder height to avoid spillage.
Goggles and protective clothing should be worn when using flames.
Long hair should be tied back and loose clothing fastened securely when using flames.
Care should be taken when assembling distillation equipment so that breakage does not occur. Particular attention should be paid when inserting one tube into another and this should not be done in palm of hand in case of slippage and possible damage to hand.
References and Bibliography
Daintith, J. (ed) 1990, A concise dictionary of chemistry, 2nd edn, Oxford University Press, Oxford.
Harwood, P. & Hughes, M. 2001, "Acids and bases" in Chemistry A2, eds. P. Harwood & M. Hughes, 2nd edn, Harper Collins, London, pp. 28-53.
Hunt, A. 2001, "Physical Chemistry" in A2 Chemistry, ed. A. Hunt, 1st edn, Hodder & Stoughton, London, pp. 6-86.
McNeil, K. 2004, COSHH Standard Assessment for Work in Chemistry Research Laboratories.University of Bristol Online. Available at: , Accessed 8th February 2006 .
The Nuffield Foundation 2003, 13th November 2003-last update, Investigating Vinegars tutorial, The Nuffield Foundation Online. Available at: Accessed, 7th February 2006.
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