Practical activity - carrying out a titration
Titration
There are a number of ways that you could carry out a A quantitative procedure in which two solutions react in a known ratio, so if the concentration of one solution is known and the volumes of both are measured, the concentration of the other solution can be determined. in Chemistry. This is an outline of the required steps to undertake one of these methods.
It is important in this practical to use appropriate apparatus to make and record a range of volume measurements accurately.
Aims
To carry out an accurate titration using dilute hydrochloric acid, dilute sodium hydroxide solution, and phenolphthalein A substance that has different colours, depending upon the pH of the solution it is in..
Titration is a practical technique used to determine the amount or concentration of a substance in a sample. It is an example of Results that can be expressed using numerical values. analysis. An Corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.-A base which is soluble in water. titration can be used to find out what volume of acid (or alkali) of known concentration exactly When a substance is neither acidic nor alkaline, and has a pH of 7. a known volume of alkali (or acid) of unknown concentration. This concentration can then be calculated.
To obtain A measurement, test or observation that can be repeated or reproduced to get similar data to support a conclusion. results, it is important that measurements are A measurement of the spread of numerical results. Data is precise if values are clustered closely together. and When the value of a measurement is close to the true value.. This can be achieved by using a standard procedure for carrying out a titration.
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 alkali solution to a clean conical flask.
- Add a few drops of a suitable A substance that has different colours, depending upon the pH of the solution it is in. 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 acid. Flush the tap through to remove any air bubbles. Ensure the burette is vertical.
- Slowly add the acid from the burette to the conical flask, swirling to mix. (The mixture may at first change colour, and then back again when swirled.)
- 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). Note the final volume reading.
- Repeat steps 1 to 5 until three results are When a measurement gives a very similar result if it is taken many times over in the same experiment. (in close agreement). Ideally these should lie within 0.10 cm3 of each other.
Results
Record the results in a suitable table. The one here also shows some sample readings.
| Run | Rough | 1 | 2 | 3 | 4 |
| End reading (cm3) | 25.45 | 24.80 | 47.90 | 23.70 | 23.90 |
| Start reading (cm3) | 0.00 | 1.00 | 23.80 | 0.00 | 0.00 |
| Titre (cm3) | 25.45 | 23.80 | 24.10 | 23.70 | 23.90 |
| Run |
|---|
| Rough |
| 1 |
| 2 |
| 3 |
| 4 |
| End reading (cm3) |
|---|
| 25.45 |
| 24.80 |
| 47.90 |
| 23.70 |
| 23.90 |
| Start reading (cm3) |
|---|
| 0.00 |
| 1.00 |
| 23.80 |
| 0.00 |
| 0.00 |
| Titre (cm3) |
|---|
| 25.45 |
| 23.80 |
| 24.10 |
| 23.70 |
| 23.90 |
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). The burette is marked in 0.10 cm3 graduations. It is possible to record results to the nearest 0.05 cm3 by noting if the The curve in the upper surface of a liquid contained within a glass container. Where the container is narrow, like a burette, the meniscus is most noticeable. lies between markings.
Analysis
Example
Calculate the mean titre. In the calculation, ignore the rough run and any results that are not in close agreement (24.10 cm3 in the table above).
Ignoring the rough run, and run 2 (because it is not concordant):
\(mean\ titre = \frac{23.80 + 23.70 + 23.90}{3}\)
= 23.80 cm3
Evaluation
Question
Explain why a pipette is used to measure the acid, rather than a measuring cylinder.
A pipette is more precise than a measuring cylinder. Adding slightly different volumes of alkali to the flask will result in a An error in measurement which differs from the true value by the same amount each time.This could be due to the equipment, how the experiment is 'carried out', or the environment.. The pipette allows the same volume of acid to be added each time, helping to make the results repeatable.
Question
Describe two steps needed to obtain accurate results.
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.
Question
Explain the importance of a suitable indicator in obtaining accurate results.
The indicator must change colour sharply when the solution in the flask is neutralised. This means the volume of acid measured is very close to the The actual value that a measurement should be.. The white tile makes it easier to see the colour change. Misjudging the colour change could result in a An error in measurement where results vary in an unpredictable way. This is sometimes due to human judgement..
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.
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, using 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, using a funnel |