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1 of 24 © Boardworks Ltd 2012 Titrations
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Titrations
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Titration
Titration is a quantitative technique that is used to accurately
determine the concentration of a substance in solution.
During a titration, a solution of known
concentration, called a standard
solution, is added to a solution of
unknown concentration.
The purpose of a titration is to determine
the volume of solution required to reach an
endpoint. An endpoint is an observable
physical change, such as a colour change.
Why is measuring volume useful?
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Finding an unknown concentration
Volume can be used to determine concentration
using the following relationship:
volume of solution (dm3)concentration =
number of moles (mol)
An unknown concentration value can be
found if the following values are known:
These values can be found using titration.
number of moles in solution
volume of solution.
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Titration equipment
The following laboratory equipment is required to perform
a titration:
stand
beaker
burette
pipette
conical
flask
safety
filler
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Indicators
The endpoint of a titration is often marked by a colour change.
This is provided by an indicator solution.
Indicators are substances which
change colour according to the
pH of a solution.
A small amount of indicator
solution is added to one of the
solutions during a titration.
When the indicator changes
colour, the endpoint of the
titration has been reached.
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Performing a titration
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Titration apparatus
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Using titration results
How are the results of a titration used to calculate the
unknown concentration of a solution?
Step 1: Write a balanced equation for the reaction.
Step 2: Calculate the number of moles of standard solution
by rearranging the concentration formula:
Step 3: Use the balanced equation to determine the number
of moles for the solution under investigation.
Step 4: Use the concentration formula to determine the
unknown concentration value:
moles = concentration × volume
concentration = moles ÷ volume
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Titration calculations – worked example
1. Write a balanced equation for the reaction:
HCl + NaOH NaCl + H2O
2. Calculate the number of moles of sodium hydroxide:
moles = concentration (mol/dm3) × volume (dm3)
= 0.1 × (20.00 ÷ 1000)
= 0.002 mol NaOH
The endpoint of a titration was reached when 20cm3 of
0.1 mol/dm3 sodium hydroxide was added to 25 cm3 of
hydrochloric acid. What is the concentration of the acid?
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volume (dm3)
Titration calculations – worked example
3. The balanced equation from step 1 shows that one mole
of HCl reacts with one mole of NaOH:
4. Use the number of moles to calculate the concentration
of the hydrochloric acid:
= 0.08 mol/dm3
concentration =moles
HCl + NaOH NaCl + H2O
0.002
(25 ÷ 1000)=
Therefore 0.002 moles of NaOH will react with:
0.002 moles of HCl.
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Titration calculations questions
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The pH scale
The pH scale shows the acidity or alkalinity of a solution.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
strong acid neutral strong alkaliweak alkaliweak acid
pH is a measure of the concentration of hydrogen ions, H+,
in solution.
The greater the concentration of H+ ions, the lower the
pH value and the stronger the acid.
The smaller the concentration of H+ ions, the larger the
pH value and the weaker the acid.
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Hydrogen ions can be neutralized by the addition of an alkali.
Alkalis release hydroxide ions, OH-, in solution:
Neutralization reactions
The OH- ions from an alkali react with H+ ions from an acid
to form water, which is pH neutral.
This is called a neutralization reaction.
Neutralization reactions often form the basis of titrations.
Hydrogen ions are released by acids in solution:
HCl H+ + Cl-
NaOH Na+ + OH-
H+ + OH- H2O
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pH change during a titration
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pH at the equivalence point
A neutralization reaction finishes when enough of one reactant
has been added to fully react with the other. This is called the
equivalence point.
How much NaOH was
required to reach the
equivalence point?
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pH ranges of indicators
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When choosing an indicator for a titration, it is important to
match its pH range (the pH values where it changes colour)
to the equivalence point of the titration.
Would litmus be a useful
indicator for a titration?
Selecting an indicator
Methyl orange has a pH range of 3–4.
Phenolphthalein has a pH range of 8–9.
Litmus has a pH range of 5–8.
Bromothymol blue has a pH range of 6–7.
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Glossary
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Anagrams
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Multiple-choice quiz
