C3: Titrations

Titration is an accuarate technique that you can use to find out how much of an acid is needed to neutralise an alkali.

When neutralisation takes place, the hydrogen ions (H+) from the acid join with the hydroxide ions (OH-) from the alkali to form water (neutrality).

H+  +  OH  ---->  H O

1. Wash and rinse a pipette with the alkali that you will use.

2. Use the pipette to measure out a known and accurate volume of alkali.

3. Place the alkali in a clean, dry conical flask. Add a suitable indicator (e.g. phenolpthalein)

4. Place the acid in a burette that has been carefully washed and rinsed with an acid. Take a reading of the volume of acid in the burette (initial reading).

5. Carefully add the acid to the alkali until the indicator changes colour to show neutrality (pheno turns pink). This is called the end point. Take a reading of the volume of acid in the burette (final reading).

6. Calculate the volume of acid added (i.e. initial reading - final reading)

This method can then be repeated to check results and can then be performed without the indicator to obtain a the salt.

Indicators: Different strength acids and alkalis can react together to form a neutral solution. You must use suitable indicators for titrations. For example, if you have a strong acid and strong alkali you should use any suitable acid-base indicator (e.g. litmus).

Calculations: Titration can be used to find the concentration of an acid/lkali providing you know...

• the relative volumes of acid and alkali used or
• the concentration of the other acid or alkali.

1. Write down a balanced equation for the reaction in order to determine the ratio of moles of acid to alkali involved.

2. Calculate the number of moles in the solution of known volume and concentration/ (You will know number of moles in the other solution from your previous concentration.)

3. Calculate the cocentration of the other solution using this formula: