Titration finds the exact volumes that react
Definition
Titration
A technique used to determine an unknown concentration by reacting a measured volume of one solution with a solution of known concentration.
Definition
Indicator
A substance that changes colour over a particular pH range and can show when the end point of a titration has been reached.
- A titration measures the volume of acid that exactly reacts with a fixed volume of alkali.
- It is used for strong acids and strong alkalis, such as hydrochloric, sulfuric or nitric acid with sodium or potassium hydroxide.
- A suitable single-colour-change indicator, such as phenolphthalein or methyl orange, shows when the reaction is complete.
- Universal indicator is unsuitable because its gradual range of colours makes the exact change hard to judge.
Practical
Investigation: finding the volume of acid that neutralises 25 cm3\text{cm}^3cm3 of alkali
- Use a volumetric pipette and filler to run exactly 25.0 cm3\text{cm}^3cm3 of sodium hydroxide solution into a clean conical flask.
- Add a few drops of phenolphthalein, which turns the alkali pink, and stand the flask on a white tile so the colour is easy to see.
- Fill a burette with the dilute acid, run a little through the tap to remove air bubbles, and read the initial volume at eye level from the bottom of the meniscus.
- Run the acid in quickly at first while swirling the flask, then add it drop by drop as the pink colour starts to fade.
- Stop the moment one drop turns the solution permanently colourless, and record the final burette reading.
- Work out the titre by subtracting the initial reading from the final reading, recorded to the nearest 0.05 cm3\text{cm}^3cm3.
- Do a rough titration first, then repeat carefully until two titres agree within 0.10 cm3\text{cm}^3cm3, and take the mean of those concordant results.
- Wear eye protection, because both the acid and the alkali are irritant.
Careful readings and concordant titres give reliable results
Definition
End point
The stage in a titration when the indicator shows its first permanent colour change.
Definition
Titre
The volume of solution delivered from a burette, calculated by subtracting the initial burette reading from the final reading.
- The end point is the first permanent colour change, from pink to colourless with phenolphthalein.
- Read the burette at eye level from the bottom of the meniscus to avoid a parallax error.
- Make sure there is no air bubble in the burette tip, or the titre will be too large.
- Concordant titres agree within 0.10 cm3\text{cm}^3cm3, and only these are used to calculate the mean.
- The rough titre is never included in the mean.
Common Mistake
- Do not use too much indicator, because it can slightly change the volume needed.
- Do not stop at a colour change that fades again when you swirl the flask.
- Do not include the rough titre in your mean.
Why the apparatus is chosen
- The pipette delivers one accurate fixed volume of alkali.
- The burette measures the changing volume of acid to the nearest 0.05 cm3\text{cm}^3cm3.
- A measuring cylinder is not precise enough for either volume in a titration.
- Repeating until titres are concordant reduces the random error in the result.
Self review
- What does a titration measure?
- Why is universal indicator unsuitable for a titration?
- What is the colour change of phenolphthalein at the end point?
- How do you work out a titre from two burette readings?
- Which titres are used when calculating the mean?
