Volumetric
Analysis
(Titration)
What is it?
The concentration of solutions of acids and
bases can be determined accurately by a
technique called volumetric analysis.
This involves reacting the solution of unknown
concentration with a solution of accurately
known concentration (a standard solution).
Strong acid v weak base
This time phenolphthalein would be completely useless.
However, methyl orange starts to change from yellow towards
orange very close to the equivalence point.
You have to choose an indicator which changes colour on the
steep bit of the curve.
If the solution becomes red, you are getting further from the
equivalence point
Primary Standards
Substances that are so pure that the amount of
substance, in mole, can be calculated accurately
from their mass are called primary standards.
To be useful, a primary standard should:
• be readily obtainable in a pure form
• have a known formula
• be easy to store without deteriorating or reacting
with the atmosphere.
It is also desirable that the substance be inexpensive
and that it have a high molar mass in order to
minimise the effect of errors in weighing.
EQUIPMENT
Volumetric Analysis Equipment is measured to 4 decimal places –
any you calculate should not exceed 4 decimal places as you would
be claiming an accuracy that you do not know.
Burette – wash it with what you are filling it with
Conical flask – rinse with what you are filling it with
Volumetric Flask – rinse with water
Pipette – rinse with what you are pipetting (use a separate pipette for each solution)
Sources of Error
Titration is a very accurate procedure in measuring
volumes of liquids, however, there are many sources of
error that can account for variance in results.
1. Volumetric equipment – calibration errors so incorrect
measurements
2. Misjudging colour change of indicator
3. Incorrect reading of volumes/scales
4. Contaminated solutions (using same pipettes, not
washing with correct solutions)
Water is a very common contaminant as if it is in
equipment it will dilute solutions and cause errors in
results
Titration Calculations
If the question asks for you to take an aliquot (a
measured volume of solution, usually
delivered by a pipette), be aware that you will
need to multiply the number of mol by a
dilution factor
Eg. 20ml aliquot from a 250ml solution = 20/250
Example 1
A standard potassium carbonate solution is made by
adding 1.227 g of K2CO3 to a 250 mL volumetric fl
ask and filled to the mark with water. 20.00 mL
aliquots are taken and titrated against sulfuric
acid, H2SO4 using methyl orange indicator. The
average titre was 22.56 mL of sulfuric acid.
a Write the equation for the reaction.
b Calculate the concentration of the K2CO3 solution.
c Calculate the concentration of the sulfuric acid
solution.
Write the equation for the reaction.
K2CO3 + H2SO4
K2SO4 + H2O + CO2
22.56ml
H2SO4
20ml
aliquot
K2CO3
250ml
1.227g
20ml
K2CO3
Calculate the concentration of K2CO3
n(K2CO3) = m/M
= 1.227/138.2
= 0.008878 mol
c(K2CO3) = n/V
= 0.008878/0.25
= 0.03551 M
20ml
aliquot
K2CO3
250ml
1.227g
22.56
ml
H2SO4
20ml
K2CO3
Calculate the concentration of H2SO4
n(K2CO3) aliquot= n(K2CO3) x dilution factor
= 0.008878 x (20/250)
= 0.0007102 mol
***Stoichiometry Table***
n(H2SO4) = n(K2SO4)
= 0.0007102 mol
C (H2SO4) = n/v
= 0.0007102/0.02256
= 0.01348M
20ml
aliquot
K2CO3
250ml
1.227g
22.56
ml
H2SO4
20ml
K2CO3