Expt (1)
Concentration determination by colorimetry
Objective
1. To estimate the concentration of a given sample of KMnO4 solution by eye inspection
2. To determinate the concentration of the same sample by a colorimeter
Theory
(A) Human eye as a colorimeter
Our eyes are able to differentiate various wavelengths and intensities of the visible
spectrum. By providing a range of samples of known concentration of a coloured
solution, say test tubes 1, 2, 3, …to10, we can estimate the concentration of a given
unknown solution (X) by eye-matching it to the provided range of samples and
decide which one of the sample range has the same colour intensity as X. Let the
chosen one be test tube 7, then we can say the concentration of solution X is close to
that of test tube 7.
The method has lots of drawbacks. Nevertheless, this is the basic principle of
colorimetry.
For more accurate detection, instruments should be used.
(B) Graphical determination of concentration by colorimetry
The same situation as mentioned in (A) can be solved more accurately by a
colorimeter. A calibration graph is drawn by plotting intensity (absorbance)
measured with solutions 1 to 10 against concentration. By measuring the intensity of
solution X, we can deduce its concentration from the calibration graph, which is not a
straight line, as shown below:
Intensity
Intensity of X
Comc. of X
Concentration
Colorimeter ME 28 is used in the experiment. Other commercial colorimeters can
also be used, but they are more sophisticated and expensive.
Expt 1
Concentration determination by colorimetry
Page 2
Safety
Wear safety spectacles and avoid skin contact with the chemicals. Use the
minimum quantity of materials required. Dispose of chemical waste and
excess materials according to your teacher’s instruction.
EYE PROTECTION
MUST BE WORN
Materials and Apparatus
Chemicals and consumables
Qty
Equipment
Qty
4 x 10-3M KMnO4
25 cm3
Colorimeter ME 28 with connector
1
Solution X
15 cm3
Digital multimeter
1
Disposable cuvette
(12.5 x 12.5 x 45 mm)
2
Plastic dropper (large)
4
10 cm3 pipette
2
Test tube
10
250 cm3 beaker
1
Glass rod
1
Wash bottle with distilled water
1
Tweezers
1
Experimental procedures
(1) Concentration estimation by eye inspection
1. With the help of a pipette, transfer 10 cm3 4 x 10-3 M KMnO4 solution in test
tube 1 to another test tube. Add an equal volume of distilled water and stir
well. Label it as test tube 2.
2. Transfer 10 cm3 solution in test tube 2 to test tube 3, add an equal volume of
distilled water and stir well.
3. Transfer 10 cm3 solution in test tube 3 to test tube 4, add an equal volume of
distilled water and stir well.
4. Repeat procedure (3) four times until test tubes 5, 6, 7 and 8 are prepared.
Prepared solution concentrations are outlined below.
Expt 1
Concentration determination by colorimetry
Page 3
Test tube
Concentration of KMnO4/
x 10-4mol dm-3
1
40.00
2
20.00
3
10.00
4
5.00
5
2.5
6
1.25
7
0.625
8
0.3125
5. Arrange the 8 test tubes in a single row in a test tube rack or hold them in an
upright position using adhesive tape as shown below:
6. Place the test tube containing solution X close to the array of test tubes and
decide which one has nearly the same colour intensity as solution X.
Concentration of X is assumed to be equal to that of the solution in this test
tube.
(2) Concentration determination by colorimetry
(A) Set full-scale (i.e. 100 mV)
1. Connect ME 28 to a DMM and select the DC 200 mV range
2. Switch on ME 28 and select the 520 nm (green) light source
3. Fill a clean cuvette 3/4 full with distilled water
4
.
Lower the cuvette with the help of a pair of tweezers into the sample tube
cavity, ensuring the clear sides of the tube are used for the optical
experiment.
5. Turn the knob until a display of 100 is shown
6. Fix the position of the knob. The setup is ready for experiment..
Expt 1
Concentration determination by colorimetry
Page 4
(B) Concentration determination
1. Using a plastic dropper, add 40 drops of solution in test tube 1 to an empty
and clean cuvette. Place it into the sample cavity of ME 28 and record the
steady voltage reading.
2. Remove the cuvette by a pair of tweezers. Empty and clean it. Transfer 40
drops of solution in test tube 2 to this clean curvtte. Place it into the sample
cavity of ME 28 and record the steady voltage reading.
3. Repeat procedure (2) until solutions in test tube 3, 4, 5, 6, 7 and 8 are used
respectively for the experiment.
Results
Test tube
Concentration of KMnO4/
x 10-4mol dm-3
DMM reading (pre-set blank
sample at 100 mV)
1
40.00
4.7
2
20.00
9.5
3
10.00
17.4
4
5.00
39.5
5
2.5
68.1
6
1.25
85.3
7
0.625
93.1
8
0.3125
96.4
DMM reading of solution X = __________________
Concentration of X deduced from the calibration graph = ________________
Treatment of data
Activate the Microsoft “Excel” spreadsheet software. Input concentration and DMM
reading data. Plot the scatter diagram.
Result (calibration graph)
Intensity vs concentration
120
100
Intensity
80
60
40
20
0
0
0.001
0.002
0.003
0.004
0.005
Concentration / mol dm-3
Expt 1
Concentration determination by colorimetry
Page 5
Why the “DMM Display” technique?
Two reasons:
(1)
Low-cost and supplier-free availability are advantages of instruments using this
technique over commercial models. In addition, skills required to assemble the
set, i.e. homemade, are not very demanding. Electronic hobbyists are capable of
finishing the assembly with local resources.
(2)
The technique is especially useful at school level. Teachers found school
chemistry practical have the following problems
(i)
Some commercial instruments for tertiary education are designed for
specific teaching topics. As such, their terminal display usually
incorporate units specific to the topic, like absorbance for optical
spectroscopy (colorimeter being the most typical example). Teachers
face the dilemma whether or not they have to teach variable dimensions
on top of experimental details.
(ii)
Things should be made easy for school students, as opposed to their
undergraduate counterparts. Sophisticated appearance of commercial
models makes students less confident of their procedure dexterity.
Personal error may be due to such situations. Equipment based on the
technique treat this as one of the design rationale. Instruments do not
just look simple, their operations are also straight forward. In addition,
they carry no display units and can be used by any levels of student.
Questions for discussion
1. Briefly explain whether or not it is a scientific method to use eye inspection to
estimate concentration of solutions when a colorimeter is not available.
____________________________________________________________________
____________________________________________________________________
In the absence of scientific instruments, eye inspection is a practical trial-and-error
scientific method.
2. Name one experimental method to determine concentration of solutions in addition to
colorimetry. Is the suggested method better than colorimetry?
____________________________________________________________________
Method of titration. Titration technique can only detect concentrations of metallic
and non-metallic ions up to 10-4M because after all end point detection has to depend
on human eye. Colorimetry or Atomic Absorption Spectroscopy techniques are able
to detect 10-5 M or less.
Expt 1
Concentration determination by colorimetry
Page 6
3. The following is about Beer’s law.
(i)
Can we calculate a value of concentration other than the value obtained by
reading the calibration graph?
_____________________________________________________________
Yes
(ii)
What is required for such calculation?
_____________________________________________________________
A mathematical relationship (equation) between concentration and intensity
of transmitted light is required.
(iii)
What kind of mathematical function does the graph likely to represent,
bearing in mind that all natural phenomena follow such a functional change?
_____________________________________________________________
Exponential variation
(iv)
Try to plot another graph, using the proposed function of intensity instead of
numerical values of intensity. It is advisable to consider only positive values
(v)
Does it look like a straight line graph passing through the origin? Do we need
to discard some points and why?
______________________________________________________________
______________________________________________________________
___________________________________________________________
Having discarded readings of high (relatively) concentrations, the graph
appears to be a straight line through the origin.
(vi)
Propose one experimental condition that controls the linearity of the graph.
_____________________________________________________________
Concentration considered should be very dilute.
(vii)
For the same experimental run, calibrated and un-calibrated colorimeters
have different values of intensity. How would you, using simple mathematics,
ensure that the un-calibrated data can still be used, in case you forget to
calibrate the meter?
______________________________________________________________
____________________________________________________________
Consider (I/Io) instead of I alone, where Io is the blank sample intensity. For
calibrated colorimeters, Io is set equal to 100. For un-calibrated colorimeters,
Io is the reading of the blank sample.
Expt 1
Concentration determination by colorimetry
(viii)
Page 7
Finally propose a mathematical function that is directly proportional to
solution concentration.
_____________________________________________________________
-ln(I/Io). A negative sign is adopted because the ratio I/Io is less than 1.
The quantitative variable that relates transmission intensity and solution concentration
I
I
is –ln( ), or conc. = A [-ln( )] where A is a proportional constant and Io is the
Io
Io
I
intensity of the blank sample. The variable –ln( ) is called absorbance. Thus for a
Io
dilute solution and a constant transmitted light path (i.e. using the same cuvette
dimensions), absorbance is proportional to concentration. This is called Beer’s law.
Commercial colorimeters use absorbance as measurement unit.
Conclusion
_______________________________________________________________________
_______________________________________________________________________