Finding the Acetic Acid Content of Vinegar

Food labels in many countries state that vinegar contains a certain percentage of acetic acid, usually around 4–8%. Titration lets you check. It’s one of the most satisfying school practicals: a kitchen product, a clear colour change, and a result you can compare directly with the bottle.

Background

Vinegar is a dilute solution of ethanoic acid (commonly called acetic acid), CH₃COOH, made by bacteria oxidising the ethanol in wine, cider or other alcoholic liquids. Ethanoic acid is a weak acid, but it reacts completely with sodium hydroxide in a titration:

CH₃COOH + NaOH → CH₃COONa + H₂O

The reaction ratio is 1 : 1.

Because this is a weak acid–strong base titration, the equivalence point is above pH 7 (about pH 8.7). Phenolphthalein is the right indicator; methyl orange would change far too early. See how to choose an indicator.

Why dilute the vinegar?

Typical vinegar is about 0.8–0.9 mol/dm³ ethanoic acid. Titrating 25 cm³ of that directly with 0.100 mol/dm³ NaOH would need over 200 cm³ of alkali, more than four full burettes. Diluting the vinegar ten times first gives a titre of around 20–22 cm³, which is ideal: large enough for a small percentage uncertainty, small enough to fit in one burette.

Equipment and chemicals

• White vinegar (coloured vinegars such as malt vinegar make the end point harder to see)
• 25.00 cm³ volumetric pipette and filler
• 250.0 cm³ volumetric flask with stopper
• 50.00 cm³ burette, clamp and stand
• Conical flasks, white tile, funnel, wash bottle with distilled water
• Standardised 0.100 mol/dm³ sodium hydroxide solution
• Phenolphthalein indicator
• Eye protection

Method

Part 1: dilute the vinegar

1. Pipette 25.00 cm³ of vinegar into a clean 250.0 cm³ volumetric flask.
2. Add distilled water until the level is just below the mark, then add the final drops with a dropping pipette until the bottom of the meniscus sits on the line.
3. Stopper and invert the flask 10–15 times to mix thoroughly. An unmixed flask is a common source of error.

This is a tenfold dilution.

Remember this factor: you’ll need it in the calculation later.

Part 2: titrate

1. Rinse the burette with NaOH solution, then fill it. Clear air bubbles from the tip and remove the funnel.
2. Rinse the pipette with the diluted vinegar, then pipette 25.00 cm³ into a conical flask.
3. Add 2–3 drops of phenolphthalein.
4. Do a rough titration: add NaOH fairly quickly, swirling, until the solution turns pink.
5. Do accurate titrations: add NaOH quickly to about 2 cm³ before the rough end point, then dropwise, swirling after each drop, until the first permanent pale pink.
6. Repeat until you have two or more concordant titres (within 0.10 cm³).

Full technique tips are in acid–base titration.

Sample results

	Rough	1	2	3
Final reading (cm³)	21.40	21.05	42.00	21.85
Initial reading (cm³)	0.00	0.25	21.05	1.00
Titre (cm³)	21.40	20.80	20.95	20.85

Concordant titres: 20.80, 20.85 (and 20.95 is within 0.15 of the lowest; some teachers accept it, others don’t). Using the two closest: mean = 20.83 cm³.

Calculation

Step 1. Moles of NaOH n(NaOH) = 0.100 × 0.02083 = 2.083 × 10⁻³ mol

Step 2. Moles of ethanoic acid in 25.00 cm³ of diluted vinegar Ratio 1 : 1, so n(CH₃COOH) = 2.083 × 10⁻³ mol

Step 3. Concentration of diluted vinegar c = 2.083 × 10⁻³ ÷ 0.02500 = 0.0833 mol/dm³

Step 4. Concentration of original vinegar It was diluted tenfold, so c(original) = 0.0833 × 10 = 0.833 mol/dm³

Step 5. Mass concentration M(CH₃COOH) = 60.05 g/mol Mass concentration = 0.833 × 60.05 = 50.0 g/dm³

Step 6. Percentage 50.0 g per 1000 cm³ = 5.00 g per 100 cm³ = 5.00% (w/v)

If the label gives a mass-by-mass percentage, divide by the density of vinegar (about 1.005–1.01 g/cm³): 5.00 ÷ 1.006 ≈ 4.97% (w/w). Either way, a label claim of “5% acidity” is confirmed.

More practice with dilution steps is in titration calculations.

Standardising the sodium hydroxide

Sodium hydroxide solutions slowly absorb carbon dioxide from the air, which lowers their concentration. For accurate work, standardise the NaOH first against a primary standard such as potassium hydrogenphthalate (KHP, C₈H₅KO₄, M = 204.22 g/mol), which is pure, stable and non-hygroscopic:

1. Weigh about 0.4 g of dried KHP accurately into a flask and dissolve it in distilled water.
2. Titrate with the NaOH using phenolphthalein.
3. n(NaOH) = n(KHP) = mass ÷ 204.22; c(NaOH) = n ÷ titre volume.

Evaluation

Sources of error

• Incomplete mixing in the volumetric flask: portions have different concentrations.
• Colour of the vinegar can mask the faint pink end point. Use white (spirit) vinegar.
• Carbon dioxide absorbed by NaOH lowers its concentration; standardise just before use.
• Overshooting the end point makes the titre, and so the calculated acid content, too high.
• Reading errors at ±0.05 cm³ per reading.

Percentage uncertainty

• Burette: ±0.10 cm³ on 20.83 cm³ = 0.48%
• Pipette (25.00 cm³, ±0.06 cm³), used twice: 0.24% each
• Volumetric flask (250.0 cm³, ±0.3 cm³): 0.12%
• Total ≈ 0.48 + 0.24 + 0.24 + 0.12 = about 1.1%

So the result is 5.00 ± 0.05% (w/v), comfortably consistent with a 5% label.

Extension ideas

1. Compare brands and types of vinegar (white, cider, rice, balsamic after dilution and possibly decolourising).
2. Use a pH meter to plot the full titration curve and find the pKa of ethanoic acid from the half-equivalence point. See plotting a pH curve.
3. Investigate “cleaning vinegar”, which is often more concentrated (8–10%).
4. Compare with lemon juice, which contains citric acid (triprotic; adjust the ratio).

Why labels say “acidity”

Many vinegar labels say “5% acidity” rather than “5% ethanoic acid”. That’s because food regulations usually define vinegar strength by total titratable acid, expressed as if it were all ethanoic acid, exactly as this titration measures it. Vinegar also contains small amounts of other acids and flavour compounds, especially in cider, wine and balsamic vinegars, but ethanoic acid dominates. Many countries set a legal minimum, typically around 4% for vinegar sold as food, so this practical mirrors a real consumer protection test.

Safety

• Wear eye protection throughout.
• 0.100 mol/dm³ sodium hydroxide is an irritant; wash off splashes promptly.
• Phenolphthalein solution contains ethanol and is flammable; keep away from flames.
• Don’t drink the vinegar once it’s been in lab glassware.

Key takeaways

• Dilute vinegar tenfold so the titre fits in one burette.
• Titrate with standardised NaOH using phenolphthalein (weak acid–strong base).
• Calculate moles in the diluted sample, multiply by the dilution factor, then convert to g/dm³ and percent.
• Typical white vinegar is about 0.8–0.9 mol/dm³, or about 5% ethanoic acid.
• Careful mixing, standardisation and dropwise additions give results within about 1%.

Once you’ve mastered this method, the same approach works for any food acid: lemon juice, wine, fruit squash or soft drinks. Only the reaction ratio and the indicator choice need rethinking for each new sample, which makes this a great springboard for independent investigations.