Uddingston Grammar High School
CfE Higher Chemistry
Unit 3: Chemistry in Society
Sub-Unit (e):
Chemical Analysis
1
Redox Titrations
As with volumetric titrations there has to be a way of determining the end-point of the
reaction. For many reactions an indicator must be used however, for some redox
reactions the end-point can be recognised from a colour change in one of the reactants,
for example, when potassium permanganate solution (purple) reacts with iron(II)
sulphate solution, the permanganate ions are reduced to colourless manganese ions.
When a colour change involving one of the reactants is used to determine the end-point
the reaction is said to be self-indicating.
Worked example:
20cm3 of iron(II)sulphate were titrated with 0.01mol-1 potassium permanganate solution
until a permanent pink colour was observed. If the volume of potassium permanganate
used was 25.6cm3, what is the concentration of the iron(II)sulphate solution?
Step 1:
write the ion-electron equations and combine to give the redox
equation.
MnO4
Fe2+
+ 8H+ + 5e-
-
Fe3+ + eMn2+ + 4H20
5Fe2+ + MnO4- + 8H+
mole
ratio
Step 2:
5
:
(x5)
5Fe3+ + Mn2+ + 4H2O
1
calculate the number of moles of the ‘known’ substance
Moles
MnO4-
=
c x v
0.01 x 0.0256
0.000256 moles
using the mole ratio:
5
:
1
0.00128 : 0.000256
Step 3:
calculate the concentration of the iron(II)sulphate solution
C =
moles / v
0.00128
0.02
= 0.064moll-1
2
Chromatography


Chromatography is an important analytical technique because it allows
chemists to separate substances in complex mixtures.
Chromatography is a method of separating and analysing a mixture of
soluble chemical substances.

There are a variety of types of chromatography, which can be used in
different contexts.

In chromatography, substances are separated as they travel in a mobile
phase which passes through a stationary phase.

Different substances travel at different speeds, so some move further
than others in a given time.
Paper chromatography

In paper chromatography, the stationary phase is a sheet of
chromatography paper.

The mobile phase may either be an aqueous (water-based) liquid or a nonaqueous (carbon-based) organic solvent.

An example of an organic solvent is propanone - which is the main
chemical in nail varnish remover.

During paper chromatography a small sample of the mixture being tested
is spotted onto the base line (a straight line usually drawn in pencil above
the level of the solvent) on the filter paper. The filter paper is then
placed in a solvent.

By capillary action the solvent moves up the paper. This is when different
components of the mixture are separated.
3

Components can move quickly or slowly up the paper depending on the
solvent used.

For each chemical in the sample, there is a dynamic equilibrium between
the stationary phase and the mobile phase.

The overall separation depends upon how strongly attracted the
chemicals are to the mobile and the stationary phases.

A chromatogram can also be produced where different samples can be
compared to a reference material.
Thin layer chromatography

Thin layer chromatography (TLC) is similar to paper chromatography but
instead of paper, the stationary phase is a thin layer of an inert
substance (eg silica) supported on a flat, unreactive surface (eg a glass
plate).

TLC has some advantages over paper chromatography. For example:
1.
the mobile phase moves more quickly through the stationary phase
the mobile phase moves more evenly through the stationary phase
3. there is a range of absorbencies for the stationary phase
2.
4

TLC tends to produce more useful chromatograms than paper
chromatography, which show greater separation of the components in the
mixture - and are therefore easier to analyse.
What factors Affect how far a sample travels in Paper or Thin Layer
Chromatography?

The distance a sample travels can depend on the size or the polarity of
the molecules involved.

Larger molecules take longer to move up the chromatography paper or
TLC plate, whereas smaller molecules are more mobile.

Therefore, smaller molecules move further up the filter paper
compared to larger molecules.

Likewise, the polarity of the molecules can affect how far the spots
travel, depending on the type of solvent used.

Polar molecules will be more strongly attracted to polar solvents, and
so would move further if a polar solvent was used as opposed to a nonpolar solvent.

For example, if a mixture contains very polar molecules and non-polar
molecules and a polar solvent, such as ethanol, is used the polar
molecules move much quicker up the filter paper.

This is because the less polar molecules will be more attracted to the
stationary phase. i.e. the paper

The distance that spots move can be compared to the overall distance the
solvent has moved and comparisons and measurements made.
5
The Retention Factor (Rf) Values

The Rf factor can be used to compare the different components found in a
sample. The Rf values of a mixture being tested can be compared with known
samples.

Note: if two substances on chromatography paper have the same Rf value it is
likely that they are the same compound. If they do not have the same Rf value
then they are definitely different compounds.
The Rf value of the
red, green and blue
particles can be
measured and
calculated to prove
that certain
molecules are
present in the
mixture.
Gas chromatography

In gas chromatography (GC), the mobile phase is an inert gas (eg
helium).

The stationary phase is a very thin layer of an inert liquid on an inert
solid support - such as beads of silica packed into a long thin tube (this
flexible tube is coiled many times inside a thermostatically-controlled
oven to keep it at a constant temperature).

GC is used to separate complex mixtures.

It is much better at this than thin-layer or paper chromatography.

This is because it is more sensitive - allowing the determination not only
of what chemicals are in the mixture, but also how much of each chemical
there is.
6

The mixture to be analysed is injected into the stream of carrier gas.

As it passes along the column (long thin tube) it separates into the
different substances.

Substances with a greater affinity (attraction) for the mobile phase
reach the detector at the end of the column more quickly.

Substances with a greater affinity for the stationary phase move more
slowly through the column.

Gas chromatography can be used to detect banned substances in urine
samples from athletes, or by forensic investigators to detect the
presence of fuels that may have been used to deliberately start fires.

A gas chromatogram might show the time along the x-axis and the
strength of response along the y-axis.

The amount of time that a substance takes to pass through the column is
called it retention time.

The retention time of an unknown substance can be compared with
standard reference data to help to identify it.

Three main pieces of information can be gathered from a gas
chromatogram:
1.
the number of compounds in the mixture - represented by the number of
peaks
how much of each compound is present - represented by the height of
the peak (higher = more)
3. the retention time - indicated by the position of the peak
2.
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This gas chromatogram shows that:






substance A was present in the smallest quantity (it has the smallest
peak)
substance A had the shortest retention time
substances B and C were present in equal amounts
substance F had the longest retention time
substance F was present in the greatest quantity (it has the largest peak)
substance F had the greatest affinity for the stationary phase
8
Standard Solutions

Concentration is measured in moles per litre ( moll-1)

This means the number of moles of solute in 1 litre of water.

A solution labelled 1 moll-1
contains 1 mole of solute in 1
litre of water.

E.g. If it was sodium chloride,
you would dissolve 58.5 g
1 litre of water
in 1 litre of water.
1 mole of solute

A standard solution is a solution whose concentration is known
accurately.

Stage 1 : When making up a standard solution it is important that the
correct mass of substance is accurately measured.

Stage 2 : Then dissolve the measured solid in a small quantity of water in
a beaker.

Stage 3 : Once the solid has dissolved, the solution is carefully poured
into a standard flask. To remove every last trace of solution left in the
beaker, the beaker needs to be rinsed with water and the rinsings added
to the standard flask. This process should be repeated 2 or 3 times.

Stage 4 : Water is then added to the standard flask until it is filled
exactly to the mark on the flask.

Stage 5 : The flask is stoppered and inverted to ensure the solution is
thoroughly mixed.
9
Preparing a Standard Solution
The correct mass
of substance is
accurately
measured.
Water is then added to the
standard flask until it is
filled exactly to the mark on
the flask.
Dissolve the measured
solid in a small quantity of
water in a beaker
Once the solid has
dissolved, the solution
is carefully poured
into a standard flask.
To remove every last
trace of solution left
in the beaker, the
beaker needs to be
rinsed with water and
the rinsings added to
the standard flask.
This process should
be repeated 2 or 3
times.
The flask is stoppered and
inverted to ensure the
solution is thoroughly
mixed.

Tap water should not be used when making a standard solution.

Tap water contains dissolved salts which could react with the
compound in the standard solution.

This would affect the concentration of the standard solution and
reduce the accuracy.

Instead, deionised water should be used to make a standard solution
since the salts / ions have been removed.
10
How to Make a Standard Solution
Question: Make 250 cm3 of 0.25 moll-1 ammonium sulphate
solution
Step 1: Calculate the mass of ammonium sulphate required

Number of moles ammonium sulphate = c
x
v
= 0.25 x 0.25
= 0.0625 moles

Formula of ammonium sulphate = (NH4+)2SO42-

gfm = ( 14 x 2 ) + ( 1 x 8 ) + (32.1 x 1 ) + ( 16 x 4 ) = 132.1 g

Mass of ammonium sulphate required =
n
x
gfm
= 0.0625 x 132.1 = 8.256 g
Step 2: Make the Standard Solution

Stage 1 : Measure 8.256 g of ammonium sulphate accurately using a balance.

Stage 2 : Then dissolve the measured solid in a small quantity of deionised
water in a beaker.

Stage 3 : Once the solid has dissolved, the solution should be carefully poured
into a standard flask. To remove every last trace of solution left in the beaker,
the beaker needs to be rinsed with water and the rinsings added to the
standard flask. This process should be repeated 2 or 3 times.

Stage 4 : Water should then be added to the standard flask until it is filled
exactly to the mark on the flask. The bottom of the meniscus should be touching
the mark.

Stage 5 : The flask should then be stoppered and inverted to ensure the
solution is thoroughly mixed.
11
Volumetric Analysis

A known volume of the solution of unknown concentration is pipetted into a clean
conical flask, to which a few drops of suitable indicator are added.

A suitable indicator is one which changes colour when the reaction is just
finished – this is called the end point of the reaction.

The burette is then filled with the solution of known concentration (standard
solution) and the meniscus is set at zero, ensuring that the jet below the tap
does not contain air.

A rough titration is carried out first. The titre is added 1cm3 at a time whilst
the solution in the flask is gently swirled. This will give an end-point in the range
of 1cm3 eg 25-26cm3.

The whole procedure is then repeated but this time approx 25cm3 can be added
carefully, then added dropwise near the endpoint until the indicator changing
colour marks the end-point.

This accurate titration is then repeated until concordant results are obtained
and an average is calculated. Concordant results are within 0.2 cm3 of each
other.
Burette
Conical Flask

First the burette must
be rinsed with the
standard solution to
be put into it.

Use a filter funnel to
fill the burette with
the standard solution
above the zero mark.

Remove the filter
funnel.

Pipette
Drain the tap to
ensure there are no
air bubbles.

Drain until the
meniscus of the
solution is sitting on
the 0.0 mark on the
scale.
12
Looking at Titration Results
The rough titre volume of 26.3 cm3
would be ignored.
An average titre volume would be
calculated using the 1st and 2nd titre.
Average titre = 25.0 cm3
13
When are Titrations Carried Out ?
1. Acid/Base Titrations

Acid/base titrations are neutralisation reactions and an indicator is
always required.

The choice of indicator is very important as it must cover the pH range
over which the change takes place.
2. Redox Titrations

Redox titrations are based on redox reactions.

The two most common systems in use are those which use potassium
permanganate and iodine as oxidising agents.
Using Potassium Permanganate
Potassium manganate (VII) – potassium permanganate is widely used in
redox titrations as it can act as its own indicator. It becomes
decolourised in a redox reaction and therefore is able to indicate the
end-point. The colour change associated with KMnO4 in a redox reaction
is;
Purple
Clear (pale pink)
(MnO4- titrations are sometimes difficult to read due to the dark colourremedied by reading top of meniscus rather than the bottom).
Using Iodine
Iodine will produce a blue/black colour in the presence of starch, it can
therefore be used as an indicator.
I2 (s) + 2e-
2I- (aq)
14