Titration equipment
A titration is a very commonly used type of quantitative analysis. It is based on accurately measured volumes of chemicals. A pipette is used to accurately measure a fixed volume of liquid and is filled using a pipette filler to a line on the upper thin part of the tube. A burette is used to accurately measure the volume of liquid that has been allowed to pour out of it.
Watch this demonstration of the correct titration procedure
Titrations
In a titration the liquid in the burette is allowed to slowly run into the conical flask until an end point is reached. The end point can be detected by the colour change of an indicator in the flask or by measuring pH or conductivity.
Since the concentration and volume of the solution added from the burette are known, it is possible to work out the number of moles of the reactant that were added to the conical flask to reach the end point.
A pipette is used to put an accurate volume of reactant in the conical flask. It is therefore possible to use all this information with a balanced equation to work out the concentration of the reactant in the conical flask.
Titrations are carried out quickly the first time to get a rough idea of the approximate volume that is needed to reach the end point. This value is too big since it is unlikely to have been stopped exactly at the endpoint. This reading is the "rough titre" and is not used to calculate the average. The titration would then be carried out very carefully at least two more times and an average taken.
Working out the titre
The following table shows the results from a titration in which sulfuric acid of concentration \(0.1 moll^{-1}\) was added from the burette into a conical flask containing 20 cm3 of sodium hydroxide solution.
| Start volume | Final volume | Volume of acid added |
| 0.0 | 12.0 | 12.0 (approx) |
| 12.0 | 23.2 | 11.2 |
| 23.2 | 34.3 | 11.1 |
| 5.5 | 16.7 | 11.2 |
| Start volume | 0.0 |
|---|---|
| Final volume | 12.0 |
| Volume of acid added | 12.0 (approx) |
| Start volume | 12.0 |
|---|---|
| Final volume | 23.2 |
| Volume of acid added | 11.2 |
| Start volume | 23.2 |
|---|---|
| Final volume | 34.3 |
| Volume of acid added | 11.1 |
| Start volume | 5.5 |
|---|---|
| Final volume | 16.7 |
| Volume of acid added | 11.2 |
(All volumes are in cm3)
The start volume is read from the burette before the titration is started and at the end point the final volume is read from the scale on the burette.
The volume of acid added is the final volume minus the start volume.
To find the average titre (titration volume) the values are added together and divided by the number of readings that were taken. Notice the rough volume is not used to calculate the average.
\(Average\: titre = \frac {(11.2 + 11.2 + 11.2)}{3} = 11.17cm^{3}\)