Standard solutions and titration

The language of titration

Volumetric analysis finds the amount of a substance by measuring the volume of a solution of known concentration that reacts exactly with it. In an acid–base titration:

• The titrant is the solution in the burette. The analyte is the solution in the conical flask.
• An aliquot is an accurately measured volume of solution, delivered by a pipette.
• The titre is the volume of titrant delivered from the burette.
• The equivalence point is when the acid and base have been mixed in exactly the mole ratio of the equation. The end point is when the indicator changes colour. A well-chosen indicator makes the two match closely (6I).
• Concordant titres agree within 0.10 mL (some schools use 0.05 mL). The rough titration is never included in the average.

Standard solutions and primary standards

A standard solution is one whose concentration is accurately known. You can make one directly only from a primary standard: a substance so pure and stable that its mass tells you exactly how many moles you have. A primary standard should:

• be available in very high purity (99.9% or better) with a known formula
• be stable in air: not absorb water (non-hygroscopic) or carbon dioxide, and not lose water of crystallisation
• have a high molar mass, so weighing errors are a small fraction of the mass
• dissolve readily in water and react completely and quickly in the titration.

Solutions such as NaOH and HCl are therefore made up to an approximate concentration and then standardised: titrated against a primary standard to find their exact concentration. A solution standardised this way is called a secondary standard. NaOH solutions slowly absorb CO₂, so they should be standardised on the day of use.

Glassware and how to rinse it

Rinsing is the step students most often lose marks on. The rule: rinse anything that must deliver an exact concentration with the solution it will contain; rinse anything that only holds a known number of moles with distilled water.

Pipettes are filled with a pipette filler (never by mouth) until the meniscus sits on the line, and allowed to drain under gravity, touching the tip against the inside of the flask. The small drop left in the tip is part of the calibration: don't blow it out. Burettes are read at eye level, at the bottom of the meniscus, to the nearest 0.05 mL (two decimal places, the second being 0 or 5).

Titration technique

1. Rinse and fill the burette with titrant, run some through to fill the tip with no air bubble, and record the initial reading.
2. Pipette an aliquot of the analyte into a conical flask and add 2–3 drops of indicator. Stand the flask on a white tile.
3. Do a rough titration quickly to find the approximate titre.
4. For accurate titrations, run in titrant quickly to about 1 mL before the rough end point, then add dropwise, swirling constantly and rinsing the inside of the flask with distilled water from a wash bottle.
5. Stop at the first permanent colour change (for phenolphthalein, a pale pink that lasts 30 seconds). Record the final reading.
6. Repeat until you have three concordant titres. Average them.

Titration calculations

Every titration calculation follows the same chain: known solution → moles → mole ratio → moles of unknown → concentration, using $n = cV$ with $V$ in litres.

Errors and how they affect the result

For each error, work out whether the titre is too large or too small, then follow it through the calculation. When the titrant is the known solution and you're finding the analyte's concentration, a titre that is too large gives a concentration that is too high.

These are systematic errors: they push every result the same way, so repeating doesn't fix them, although it does reduce the effect of random errors such as judging the end point colour or reading the meniscus. Concordant titres show precision (reliability), not accuracy; you can be precisely wrong if a systematic error is present.