Core Practical: accurate acid-alkali titration

One titration, start to finish

Step through the run. At each stage ask: what does this make more accurate?

1. 1Trial run firstStart with a rough titration. It's a practice run that estimates how much titrant it takes to neutralise the analyte, and that makes the careful titration more accurate.+
2. 2Analyte into the flaskMeasure 25 cm³ of the analyte (the solution of unknown concentration) into the conical flask.+
3. 3Record the starting volumeRead the burette and write down the initial volume of titrant before anything is added.+
4. 4Add titrant, swirlingLet titrant run in from the burette while you swirl the conical flask. Swirling mixes the two solutions.+
5. 5Drop by drop near the end pointAs you get close to the end point, slow right down and add the titrant drop by drop.+
6. 6Record the final volumeWrite down the final burette volume. The titre is the volume of titrant used to reach the end point: final volume minus initial volume.+
7. 7Repeat until concordantRun it again, and again if you need to, until you have concordant titres: titres within a small range of each other, normally within ±0.10 cm³.+

The set-up

Tap a part to see its job. Switch the view to compare an indicator with a pH meter.

The usual set-up: an indicator shows the end point, and the white tile helps you see it.

Tap a part to see its job. Switch the view to compare an indicator with a pH meter.

Stop or carry on?

You're adding titrant drop by drop. Make your call before you look.

A small colour change flashes up in the flask, but it's only brief. Has the end point been reached?

What you need to know

• Titrations find the exact volume of one solution that reacts completely with another, which helps identify unknown concentrations.
• The titrant (known concentration) goes in the burette; the analyte (unknown concentration) goes in the conical flask.
• Have a goTwo bottles on the bench: one labelled with its concentration, one marked 'unknown'. Which one goes in the conical flask, and what's it called?

  The unknown one. It's the analyte. The known one is the titrant, in the burette.

  The titrant is the solution of known concentration, added from the burette. The analyte is the unknown one that sits in the flask.

• Start with a trial (rough) titration to estimate how much titrant you need, which makes the proper titration more accurate.
• Measure 25 cm³ of analyte into the flask, record the initial burette volume, then add titrant while swirling to mix.
• Near the end point, add the titrant drop by drop, then record the final burette volume.
• Methyl orange (an indicator) is red in acidic solutions and yellow in alkaline ones; a pH meter detects neutralisation more accurately.
• Have a goYou've added methyl orange and the solution in the flask looks yellow. Is it acidic or alkaline?

  Alkaline.

  Methyl orange is yellow in alkaline solutions. It would be red in acidic ones.

• A white tile under the flask helps you see the colour change, and the end point needs a persistent colour change.
• Read the burette at eye level from the bottom of the meniscus, to two decimal places (the second one is 0 or 5).
• Have a goPriya's meniscus sits between the 12.4 and 12.5 marks. She writes '12.4, close enough'. What should she write, and why?

  12.45 cm³ — two decimal places, ending in 5.

  Readings go to two decimal places. The second is 0 if the meniscus is exactly on a mark, and 5 if it is between two marks.

• Work out each titre as final volume − initial volume, and repeat until titres are concordant: normally within ±0.10 cm³.
• Excluding titres outside that range, take the mean of the concordant titres only.

The big picture

A titration finds the exact volume of one solution that reacts completely with another. Accuracy comes from a trial run, drop-by-drop addition near the end point, a persistent colour change, burette readings to two decimal places, and a mean of only the concordant titres.

⚠ Watch out

Stopping at the first brief colour change, then averaging every titre you collected. The end point needs a persistent colour change, and only concordant titres (normally within ±0.10 cm³) go into the mean.

Memory hook

Stays, then agrees: the colour must STAY before you stop, and the titres must AGREE before you average.

Check yourself

Without looking back: put one titration run in order, from the trial titration to concordant titres. Then say when you stop adding titrant, and which titres go into the mean.