Titration is an important method to determine the concentration of either an acid or alkali. It works by knowing the volume and concentration of one solution needed to *neutralize* the other. From this, you can calculate the unknown concentration.

When an acid is neutralized by a base, the pH of the solution changes. We can follow this pH change using an indicator to find out how much base just reacts with the acid to produce a neutral salt solution. * This method of adding one solution from a burette to another solution in order to find out how much of the two solutions will just react with each other is called titration.* When the two solutions just react and neither is in excess, we have found the endpoint of the titration.

## How to carry out a titration?

In this **titration experiment**, we will use **phenolphthalein** to determine how much **hydrochloric acid** just reacts with 25 cm^{3} of **sodium hydroxide** solution of concentration 1 mol/dm^{3}.

1) Measure 25.0 cm^{3} of sodium hydroxide solution containing 1.0 mol/dm^{3} (1.0 M NaOH(aq)) into a conical flask using a pipette (Figure 1).

2) Add 5 to 10 drops of phenolphthalein and note the colour.

3) Add 5.00 cm^{3} of 1.0 mol/dm^{3} hydrochloric acid from a burette (Figure 2), mix well and record the colour again.

4) Record the colour also when 10.00, 15.00 and 20.00 cm^{3 }of hydrochloric acid have been added.

5) Now add 1.00 cm^{3} of hydrochloric acid and record the colour again.

Repeat the addition of 1.00 cm^{3} nine more times and note the colour each time.

**Table 1 shows the results that we should get. **

Notice that the indicator is colourless when 25.00 cm^{3} of hydrochloric acid has been added.

So, 25.00 cm^{3} of 1.0 mol/dm^{3} hydrochloric acid just neutralise 25.0 cm^{3} of 1.0 mol/dm^{3} sodium hydroxide.

So, 0.025 mol of HCl react with 0.025 mol of NaOH

⇒ 1 mol of HCl reacts with 1 mol of NaOH

**Thus, the Titration experiment is carried out.**