Practical Activity
Titration
It is important in this core practical to use appropriate apparatus to make and record a range of volume measurements accurately. This includes the safe use and handling of liquids, and monitoring chemical changes.
This outlines one way to carry out the practical. Eye protection must be worn.
Aims
To carry out an accurate titration using dilute hydrochloric acid, dilute sodium hydroxide solution, and phenolphthalein indicator.
Method
- Use a A piece of apparatus used to measure accurate and repeatable volumes of liquid. Also called a volumetric pipette. and pipette filler to add 25 cm3 of dilute sodium hydroxide solution to a clean conical flask.
- Add a few drops of phenolphthalein indicator and put the conical flask on a white tile.
- Fill the A piece of apparatus used to add varying but measured volumes of solution during a titration. with dilute hydrochloric acid and note the starting volume.
- Slowly add the acid from the burette to the conical flask, swirling to mix.
- Stop adding the acid when the In a titration, the point at which the indicator first permanently changes colour. is reached (when the colour first permanently changes from pink to colourless). Note the final volume reading.
- Repeat steps 1 to 5 until you get Titres within 0.10 cm鲁 of each other. (see the Analysis).
Results
Record the results in a suitable table. The one here also shows some sample readings.
| Run | Rough | 1 | 2 | 3 |
| End reading (cm3) | 25.45 | 24.80 | 47.90 | 23.70 |
| Start reading (cm3) | 0.00 | 1.00 | 23.80 | 0.00 |
| Titre (cm3) | 25.45 | 23.80 鉁 | 24.10 | 23.70 鉁 |
| End reading (cm3) | |
|---|---|
| Rough | 25.45 |
| 1 | 24.80 |
| 2 | 47.90 |
| 3 | 23.70 |
| Start reading (cm3) | |
|---|---|
| Rough | 0.00 |
| 1 | 1.00 |
| 2 | 23.80 |
| 3 | 0.00 |
| Titre (cm3) | |
|---|---|
| Rough | 25.45 |
| 1 | 23.80 鉁 |
| 2 | 24.10 |
| 3 | 23.70 鉁 |
Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The Volume of one reactant needed to react completely with the other reactant in a titration. is the volume added (the difference between the end and start readings).
Analysis
Tick (鉁) at least two concordant titres. These are titres within 0.20 cm3 (or sometimes 0.10 cm3) of each other.
Worked example
Question
Calculate the mean titre.
Ignoring the rough run, and run 2 (because it is not concordant):
\(mean\ titre\ =\ \frac{23.80\ +\ 23.70}{2}\)
= 23.75 cm3
Evaluation
Worked example 1
Question
Describe four steps needed to obtain accurate results.
Make sure the burette is vertical, and take the readings from the bottom of the meniscus. Near to the end-point, rinse the inside of the flask with distilled water and add the acid drop by drop.
Worked example 2
Question
Explain why a pipette is used to measure the acid, rather than a measuring cylinder.
The pipette allows the same volume of acid to be added each time, helping to make the results repeatable.
Hazards, risks and precautions
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Possible harm | Possible precaution |
| Dilute sodium hydroxide solution | Causes skin and serious eye irritation | Wear gloves and eye protection, and use a pipette filler |
| Spilling hydrochloric acid while filling the burette | Causes eye irritation | Fill the burette slowly below eye level and use a funnel |
| Hazard | Dilute sodium hydroxide solution |
|---|---|
| Possible harm | Causes skin and serious eye irritation |
| Possible precaution | Wear gloves and eye protection, and use a pipette filler |
| Hazard | Spilling hydrochloric acid while filling the burette |
|---|---|
| Possible harm | Causes eye irritation |
| Possible precaution | Fill the burette slowly below eye level and use a funnel |