7.13.3
Colorimetry
The use of colorimetry to determine the percentage of iron in aluminium foil and, by Job's method, the formula of a complex such as that between copper(II) ions and ammonia.
(The theory of colorimetry is not required.)
Colorimetry
This
is an analytical technique which depends on the measurement of colour intensity
of solutions.
In
a colorimeter a beam of light passes through the test solution towards a
sensitive photocell connected to a meter. A filter is used to select the most
appropriate colour (i.e. wavelength) of
light to be used. The reading generated on the meter depends on the amount of
light transmitted by the solution. This in turn depends on the colour intensity
of the solution.
The
greater the concentration of the coloured complex in the solution, the more
intense the colour, and the greater the absorbance.
Determination of Iron in Aluminium Foil
Commercial
aluminium foil contains small amounts of iron. The percentage of iron in the
foil can be determined by colorimetry.
The
metal is dissolved in hydrochloric acid and hydrogen peroxide. After
evaporation to crystallisation the iron(III) thiocyanate colour is developed in
nitric acid solution and the intensity of the colour measured colormetrically
at 480 nm.
Practical
Dissolve
about 0.1 g of aluminium foil (a 5 cm x 5 cm square) in a 250 cm3
beaker with 5 cm3 of concentrated hydrochloric acid and add 1 cm3
of 10 vol. hydrogen peroxide solution. Boil down until crystallisation
commences. Add 8 cm3 of 4M nitric acid, boil for 4 minutes, and
cool. Transfer quantitatively to a 100 cm3 volumetric flask and
dilute to the mark with distilled water. Mix well.
Calibration graph
Place
the following solutions in 100cm3 volumetric flasks and dilute to
the mark with distilled water. Mix well. The 0.1 mg cm-3 solution is
dispensed from a 50 cm3 burette. The 20% KSCN solution is added
using a 10 cm3 measuring cylinder as is the 4M nitric acid.
Solution |
Volume
0.1 mg cm-3 Fe(III) (cm3) |
Volume 20% KSCN solution (cm3) |
Volume
4M HNO3 (cm3) |
A |
0 |
10 |
6 |
|
B |
2 |
10 |
6 |
|
C |
4 |
10 |
6 |
|
D |
6 |
10 |
6 |
|
E |
8 |
10 |
6 |
|
F |
10 |
10 |
6 |
|
G |
25
(sample solution) |
10 |
6 |
Measure
the absorbance of each solution in 1 cm glass cells, using distilled water to
set up 0% absorbance (100% transmission).
Plot
a calibration graph of absorbance against mg Fe per 100 cm3. From
the calibration graph deduce the amount of iron(III) present in the dissolved
sample and hence the percentage of iron in the aluminium foil.
Formula
of the Complex Between Cu (II) ions and Ammonia.
Cu2+ (aq)
+ xNH3 (aq) [Cu(NH3)x]2+
(aq)
The value of x can be determined by the method of continuous variations (Job’s method).
Mixtures
of Cu2+ (aq) and NH3
(aq) are made up containing ratios of Cu2+:NH3 which vary
continuously from 10:0 to 0:10. The intensity of the colour is measured
colormetrically and hence the mole ratio giving the maximum colour intensity
determined.
Practical
Mix
the following solutions together in separate test tubes.
|
Tube |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
|
Volume
of 1M CuSO4 Soln. (cm3) |
10 |
9 |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
Volume
of 1M NH3 (aq) soln. (cm3) |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
Measure
the absorbance of each solution in 1 cm3 glass cells, using
distilled water to set up 0% absorbance and a yellow filter (complementary to
the blue colour of the solution).
Plot the absorbance against
the volume of 1M CuSO4 solution.
Absorbance %
0 5 10 vol. of CuSO4
10 5 0 vol. of NH3 (aq)
the ratio giving the darkest coloured solution (i.e. maximum absorbance) corresponds to the molar proportions which just react completely lving neither in excess. Once this is determined the formula for the complex can be written.
In
this case a mole ratio Cu2+:NH3 of 2:8 (i.e. 1:4) gives
the deepest colour.
Therefore
Cu2+ (aq)
+ 4NH3 (aq) [Cu(NH3)4]2+
(aq)