Acid-alkali titrations
The concentration of an Corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions. or A base which is soluble in water. can be calculated by carrying out an experiment called 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..
Materials
The apparatus needed includes:
- a pipette to accurately measure a certain The volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml. of acid or alkali
- a pipette filler to use the pipette safely
- a conical flask to contain the liquid from the pipette
- a burette to add small, measured volumes of one reactant to the other reactant in the conical flask
Method
This is an outline method for carrying out a titration in which an acid is added to alkali.
- Use the pipette and pipette filler to add 25 cm3 of alkali to a clean conical flask.
- Add a few drops of 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 burette with acid and note the starting volume.
- Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
- Stop adding the acid when the end-point is reached (the appropriate colour change in the indicator happens). Note the final volume reading.
The same method works for adding an alkali to an acid 鈥 just swap around the liquids that go into the conical flask and burette.
The titre [Higher tier 鈥 GCSE Chemistry only]
The difference between the reading at the start and the final reading gives the volume of acid (or alkali) added. This volume is called the titre.
For example, if the reading at the start is 1.00 cm3 and the final reading is 26.50 cm3, then the titre is 25.50 cm3 (26.50 鈥 1.00). The titre will depend upon the volume of liquid in the conical flask, and the concentrations of the acid and alkali used. Repeat the titration until you have at least two readings that are within 0.20 cm3 of each other.
Titration results are recorded in a table like this:
| Titration | Rough | 1 | 2 | 3 |
| End vol (cm3) | 26.80 | 24.60 | 24.50 | 25.00 |
| Start vol (cm3) | 1.00 | 0.00 | 0.40 | 0.20 |
| Titre (cm3) | 25.80 | 24.60 | 24.10 | 24.80 |
| Titration | End vol (cm3) |
|---|---|
| Rough | 26.80 |
| 1 | 24.60 |
| 2 | 24.50 |
| 3 | 25.00 |
| Titration | Start vol (cm3) |
|---|---|
| Rough | 1.00 |
| 1 | 0.00 |
| 2 | 0.40 |
| 3 | 0.20 |
| Titration | Titre (cm3) |
|---|---|
| Rough | 25.80 |
| 1 | 24.60 |
| 2 | 24.10 |
| 3 | 24.80 |
You can ignore the first (rough) titration result. Titrations one and three have results that are within 0.20 cm3 of each other but the result for titration two is too low. You would calculate the mean titre for titrations one and three, and ignore titration two:
Mean titre = (24.60 + 24.80) 梅 2 = 49.40 梅 2 = 24.70 cm3