Acids and alkalis react together to form salts and water. If the salt is soluble, it is very difficult to separate it from the soluble acid and alkali. The method of titration is used here to make a pure sample of a soluble salt.
The titration apparatus
- Open Yenka file Model 1.
- Give two safety precautions you would take while performing this experiment in the laboratory.
AnswerWear safety goggles and gloves.
- What is the name of the type of reaction that occurs when an acid and an alkali react together?
- Which ions must be present for a substance to be an acid, and which must be present for a substance to be an alkali?
Ions present in acid Ions present in alkaliAnswerH+AnswerOH-
- Pour the sodium hydroxide (NaOH) into the beaker for the alkali. Take the pipette and dip it into the solution in the beaker. It should fill with exactly 20 cm3 of alkali. Now drag the full pipette over to the conical flask and hold it there until the pipette has emptied into it. What would you expect the pH of the alkali to be?
- Transfer some of the hydrochloric acid into the beaker. Now pour it from the beaker into the burette. Use the 'Make to volume' button beside the burette to make it up to the 0 cm3 line. Drag the full burette over the conical flask and use the slider to add acid slowly to the alkali in the flask. The graph shows how the pH of the mixture changes. Does it alter quickly or slowly as the acid is added?
- Continue to add the acid until the pH changes very sharply and then pause the simulation. Use the graph to determine precisely how much of the acid was needed to exactly neutralise the 20 cm3 of alkali in the flask. Record this volume below.
- Click on the reaction details of the flask. What are the two substances present in solution?
AnswerSodium chloride and hydrochloric acid.
- Now you need to reload the simulation (F5) and repeat the experiment. However, this time you need to add the acid very slowly near the point at which the pH starts to change rapidly, and stop adding acid just as the pH drops through 7. At this point you will have exactly neutralised the alkali. You may need several attempts to get it just right! Use the reaction details to find out what is in solution in the flask at this point.
AnswerOnly sodium chloride.
- Select the conical flask and copy it. Click on Scene 2 and paste the flask into the simulation. Use the apparatus there to prepare a pure dry sample of sodium chloride (you may wish to use the slider in the main toolbar to speed up the rate of the simulation to ×10). What mass of solid do you obtain?
Using specialist equipment it is possible to determine exactly the volume of an acid needed to neutralise a known volume of an alkali. This technique is known as a titration. A salt and water are the only products, and the salt can be obtained by evaporating off the water.
- This exercise can be used to introduce pupils to the method of titration.
- It relies on the use of a pH graph to determine the point at which neutralisation occurs.
- A related exercise on the formation of soluble salts from insoluble metal oxides and carbonates can be found in the activity Salt Preparation Methods 1.