Determining the acetic acid content of vinegar by titration

Vinegar is a familiar item in Australian kitchens, used for salad dressings, pickling, and household cleaning. The active ingredient, acetic acid, is regulated by Food Standards Australia New Zealand (FSANZ), which requires a minimum of 4% acetic acid for any product labelled as vinegar. Knowing the precise concentration is useful for quality control, food science coursework, and consumer awareness.

Acid-base titration is the classical technique for this purpose. By reacting a measured volume of vinegar with a standardised sodium hydroxide solution, the amount of acetic acid present can be calculated with high accuracy. The procedure is part of the VCE Chemistry syllabus taught in Melbourne and the HSC Chemistry syllabus in Sydney, making it one of the most familiar laboratory experiments for secondary students across the country.

The chemistry behind the reaction

Acetic acid (CH₃COOH) is a weak monoprotic acid, meaning each molecule donates a single proton during neutralisation. Sodium hydroxide (NaOH) is a strong base that dissociates completely in water. The neutralisation reaction proceeds in a 1:1 molar ratio:

CH₃COOH + NaOH → CH₃COONa + H₂O

Because the endpoint does not produce a sharp pH jump in a weak acid-strong base titration, an indicator that changes colour in the slightly basic region is required. Phenolphthalein, which shifts from colourless to pale pink between pH 8.2 and 10.0, is the standard choice for this determination and is included in most Australian school titration kits.

Equipment and reagents required

A 50 mL burette, a 10 mL or 25 mL pipette with a safety filler, a 250 mL conical flask, a wash bottle of deionised water, and a retort stand with a burette clamp are essential. A white tile placed beneath the flask improves visibility of the colour change near the endpoint.

For reagents, analytical-grade sodium hydroxide pellets, a primary standard such as potassium hydrogen phthalate (KHP), phenolphthalein indicator solution (1% in ethanol), and the vinegar sample are needed. These chemicals are readily available from Australian suppliers such as Chem-Supply or through university laboratories in Brisbane and Perth. The vinegar can be any commercial brand purchased from a local Coles or Woolworths.

Preparing and standardising the sodium hydroxide solution

Sodium hydroxide is hygroscopic and absorbs carbon dioxide from the atmosphere, so its solutions cannot be weighed directly for accurate work. A stock solution of approximately 0.1 M NaOH is prepared by dissolving about 4 g of pellets in 1 L of freshly boiled and cooled deionised water. Boiling removes dissolved CO₂, which would otherwise react with NaOH to form sodium carbonate and lower the effective concentration.

The base is then standardised against potassium hydrogen phthalate. About 0.5 g of KHP is weighed accurately, dissolved in 50 mL of deionised water, and titrated with the NaOH using phenolphthalein. The exact molarity of the base is calculated from the mass of KHP and the titre volume, giving a precise value for subsequent vinegar analyses.

Performing the titration on vinegar

Most commercial vinegars sold in Australia contain between 4% and 8% acetic acid, with common brands at around 5%. A measured aliquot, typically 10 mL, is pipetted into a 250 mL conical flask and diluted with about 50 mL of deionised water. Two or three drops of phenolphthalein are added, producing a colourless solution.

The standardised NaOH is dispensed from the burette while swirling the flask continuously. As the endpoint nears, the pink colour from each drop takes longer to fade. A single drop producing a persistent pale pink that lasts at least 30 seconds marks the endpoint. The titre is recorded, and the procedure is repeated until at least two concordant results, within 0.1 mL of each other, are obtained. Triplicate determinations are recommended for reliable data.

Calculating acetic acid concentration

Because the stoichiometry is 1:1, the moles of NaOH used equal the moles of acetic acid in the aliquot. Multiplying by the molar mass of acetic acid (60.05 g/mol) gives the mass present, and dividing by the volume pipetted yields the concentration in g/L. Converting to a weight/volume percentage involves multiplying by 0.1.

For example, if 10.0 mL of vinegar requires 17.5 mL of 0.105 M NaOH:

moles NaOH = 0.0175 × 0.105 = 0.001838 mol mass CH₃COOH = 0.001838 × 60.05 = 0.1103 g concentration = 0.1103 / 0.010 = 11.03 g/L = 1.10% w/v

For a retail vinegar labelled at 5%, the measured value should fall within the tolerance permitted under FSANZ labelling requirements.

Common sources of error and practical tips

Carbon dioxide absorbed into the NaOH solution reduces its effective strength, so restandardisation every two to three weeks is advisable. Overshooting the endpoint is a common student error; slowing the addition near the equivalence point prevents this. Unfiltered or naturally brewed vinegars may separate on standing and should be mixed thoroughly before sampling.

Rinsing the burette with a small amount of titrant before filling, and rinsing the pipette with the vinegar after a pre-rinze, both improve precision. Automatic burettes or potentiometric pH detection can further reduce operator bias in university research settings. The method remains one of the simplest and most reliable ways to confirm the acetic acid content of commercial vinegar, providing students and researchers with a hands-on introduction to quantitative analytical chemistry that combines practical skill with real-world relevance to Australian food standards.