titrations

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Titrations

Titrations are normally carried out to calculate the concentration of an unknown acid or alkaline solution. The best way to master titrations is by simply carrying out a few and working through the calculations needed. Before you start this page make sure you know how to calculate the concentration of solutions using mole calculations.


The basic method for carrying out a titration is shown below.

method for carrying out a titration

Typical titration calculations

The apparatus is set up as shown above. The only improvement is that the conical flask should be sitting on a white tile. This will make it easier to identify any colour changes in the indicator. Obviously the first time you add the acid to the alkali in the conical flask you have little idea of how much acid you will need to add. So the first run through should be no more than a trial to guage roughly how much acid you are likely to have to add before the indicator changes colour marking the end-point of the neutralisation reaction. The end point is where stoichiometrically equivalent amounts of acid and alkali have been added, the indicator should change colour close to this point.
You should record your results in a suitable table:

Burette reading/cm3 trial run volume/cm3 trial 1/volume/cm3 trial 2/volume/cm3 trial 3/volume/cm3
initial reading
final reading
titre (volume acid added)

Once you have repeated the titration it is possible to calculate an average titre from you 3 titration runs that were carried out.

Example calculation

A student was trying to calculate the concentration of an unknown sodium hydroxide solution. She carried out the titration as described above and repeated it 3 times to ensure her results were concordant. She found that 20.0ml of 0.1M hydrochloric acid were required to neutralise the 25.0ml of sodium hydroxide solution in the conical flask. So to calculate the concentration of the sodium hydroxide solution we start with a balanced symbolic equation for the reaction:

NaOH + HCl → NaCl + H2O
The equation tells us that 1 mole of hydrochloric acid will neutralise 1 mole of sodium hydroxide.
We can calculate the number of moles of acid from the equation:
number of moles = concentration x volume
The concentration of the acid was 0.1M and the volume added was the average titre from the titration, which was 20.0ml (note this needs to be divided by 1000 to convert to dm3. So the number of moles of acid which reacted with the sodium hydroxide is:
number of moles = concentration x volume (in dm3)
number of moles = 0.1 x 0.02=0.002 moles
since we already know from our equation that 1 mole of acid neutralises 1 mole of alkali then 0.002 moles of acid will neutralise 0.002 moles of sodium hydroxide.
To work out the concentration of the sodium hydroxide solution we need to use the equation:
concentration = number of moles/volume(in dm3)
The volume of sodium hydroxide we know, this was the 25 ml (0.025dm2) that was pipetted into the conical flask. So all we have to do is substitute the numbers into the equation:
concentration = number of moles/volume(in dm3)
concentration = 0.002/0.025 = 0.08M

Indicators

Most of the time when you needed to use an indicator in science lessons you probably used universal indicator. Universal indicator is good at indicating if a solution is acidic or alkaline, however it is not suitable to use in titrations. Universal indicator will show a slight difference in colour over a fairly wide pH range. For titrations we need an indicator that will change colour over a very narrow and specific pH range, close to the end point of the reaction. Suitable indicators to use are phenolphthalein which is colourless in acids and pink in alkaline solutions or methyl ornage which is red in acid or yellow in alkaline solutions.

Key Points


Practice questions

Check your understanding - Questions on titrations

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