Acid-Base Titration

Rinse the burette with a little of the sodium hydroxide solution and discard it, then fill.

Run solution through the tap until no air bubble remains in the tip, and bring the level onto the scale.

Read the bottom of the meniscus, at eye level.

A bubble in the tip is the classic hidden error — it comes out during the titration and is counted as delivered volume that never reached the flask.

Pipette 25.0 ml of vinegar into a conical flask. Add about 50 ml of water and two drops of phenolphthalein.

The dilution water does not change the answer — it adds no acid and removes none, it only makes the colour change easier to see.

Swirl to mix.

Add base quickly, swirling, until the flask goes pink and stays pink. Record the volume to the nearest millilitre and discard.

This is deliberately sloppy and deliberately first.

It tells you where the endpoint is, so on every later run you can add fast to within a millilitre or two of it and slow right down — which is the only way to get a precise result without spending an hour per titration.

Repeat with a fresh 25.0 ml sample. Add fast to 2 ml below the rough figure, then drop by drop, swirling after each.

Stop at the first faint pink that persists for 30 seconds of swirling.

Repeat until three runs agree within 0.10 ml. Average those three.

A strong pink means you overshot. The endpoint is the first permanent trace of colour, not a convincing one.

Moles of base = concentration x average volume in litres. Acetic acid reacts one-to-one with hydroxide, so moles of acid = moles of base.

Multiply by the molar mass of acetic acid, 60.05 g/mol, to get the mass in your 25.0 ml sample, and convert to a percentage.

Compare with the bottle.

Expect to land close to the stated figure but not exactly on it — and the difference is a result, not a failure.

Repeat the titration using a pH meter instead of the indicator, recording pH after every 1 ml and every 0.2 ml near the jump. Plot pH against volume.

Find the steepest part of the curve — that is the equivalence point. Mark where phenolphthalein turned pink.

Expect the equivalence point to sit at a pH above 7, because neutralising a weak acid leaves its conjugate base behind, and expect the indicator's change to fall on the steep section but not exactly on the midpoint.

Choosing an indicator means choosing one whose range lies inside the vertical part of the curve. Methyl orange, which turns in acid, would be badly wrong here.

Sodium hydroxide is a genuinely bad primary standard, and this is the largest hidden error in the whole experiment. Solid NaOH absorbs water from the air, so a "weighed" mass is partly water; and it absorbs carbon dioxide, converting some hydroxide to carbonate. A solution made up by weighing is therefore approximately the concentration on the label. Real analytical work standardises it first by titrating against a substance that can be weighed honestly — potassium hydrogen phthalate is the usual choice, because it is stable, pure and not hygroscopic. Until you have done that, every percentage you calculate inherits the error in the bottle.

Why phenolphthalein and not something else. At the equivalence point of a weak acid against a strong base, the flask contains sodium acetate, and acetate is a weak base — so the solution is alkaline, typically around pH 8-9. An indicator that changes in that region reports the equivalence point closely; one that changes at pH 4 would signal long before the acid was used up. Indicator choice is not aesthetic, it is determined by the shape of the titration curve, which is why step 6 is worth the extra time.

The technique is older than the theory. Volumetric analysis grew through the eighteenth and nineteenth centuries out of practical assaying — testing the strength of bleach, of potash, of indigo dyebaths — long before anyone could explain what an indicator was doing. Karl Friedrich Mohr's mid-nineteenth-century textbook systematised the apparatus and much of the modern burette practice; the word titre comes from the assayer's language of fineness, as in the title of a coin.

What a titration cannot tell you. It measures total titratable acid, not which acid. Vinegar is mostly acetic acid, but any other acid present is counted in and reported as though it were acetic — so the answer is a well-defined quantity that is not quite "the acetic acid content". This is why food labels specify a method: the number depends on how you asked. The same limitation is a virtue elsewhere: total acidity is exactly what a brewer or a winemaker wants, regardless of which acids contribute.

Handling. Sodium hydroxide solution is corrosive — it attacks skin and especially eyes, and it does so without much warning because it feels soapy rather than painful. Goggles throughout, cover skin, and rinse any splash with plenty of running water. Corrosive is not the same as toxic: this blueprint is not age-gated, and lye is a routine material in soapmaking, food preparation and metal conservation. Treat it with the same respect as a hot pan, and clean up spills immediately — dried NaOH is invisible.