We can use titration to make soluble salt from base and an acid.

An acid-alkali titration is used to find out how much acid is needed to react exactly with a solution of an alkali.

We use indicator to find when the acid just reacted with all the alkali.

We call this the end point of the titration.At the end point indicator changes colour.

What indicator we will choose depend on whether we use a strong or weak acid or alkali:

For a strong acid and alkali we can use any indicator.

If we are making a salt from a weak alkali (e.g. ammonia), we titrate with a strong acid. We use methyl orange indicator, which goes from orange to red when there is excess acid.

If we are making a salt from a weak acid (e.g. ethanoic acid), we use a strong base. We use phenolphthalein indicator, which goes from pink to colourless when there is excess acid.

You use the following sequence to carry out a titration:

1. Measure a known volume of alkali into the titration flask using a volumetric pipette (first wash the pipette with a little of the alkali you are using).

2. Add a few drops of indicator solution to the alkali in the flask.

3. Fill a clean burette with acid (first wash the burette with a little of the acid you are using).

4. Record the burette reading.5. Open the burette tap and let the acid flow into

the flask. Keep swirling the flask gently to make sure that the acid and alkali mix and react.

6. Keep adding the acid slowly until the indicator changes colour. This is the end point.

7. Record the reading on the burette. The final reading minus the initial reading is called titre.

8. Repeat this process at least three times. You can add acid rapidly until you are a few cm3 from the end point. Then add acid drop by drop so that you get accurate titre.

9. If you are doing calculations to find the concentration of the alkali in the flask, you take average of the accurate titres. You can ignore any titres that appear

We can make a soluble salt by titrating an alkali with an acid.

Example: if we want to make potassium sulphate, we carry out reaction:

2KOH(aq) + H2SO4(aq) → K2SO4(aq) + H2O(l)

potassium hydroxide + sulphuric acid → potassium sulphate + water

We first carry out a titration using an indicator to find the correct volumes of solution to mix.

Then we carry it out without an indicator to prepare a sample of the salt uncontaminated by indicator.