Gateway 125,126, 130
Fall 2006
Studio 11b p1
Studio 11b (11/15/06) Color Changes though Dilution?1
Group assignments: A Technician; B Leader; C Recorder
Question: Can a solution change color when you dilute it with water?
Hypothesis:
1) Reflecting on you experiences with colored solutions (the nitrate analysis, food coloring, etc.)
what happens when you dilute a colored solution with water?
2) Can you think of a way it would be chemically possible for a color change to take place
through dilution?
Experiment:
You will need ~65 mL of the 1.8 x10-4 M bromocresol green (BCG) stock solution. Carry out a
series of 9 dilutions. Add 0.50, 1.00, 1.50, 2.50, 4.50, 5.10, 6.00, 6.30 and 7.50 mL aliquots of
stock solution to separate 10 mL volumetric flasks. Dilute to the mark. (You will probably need
to transfer your solutions to 10 mL beakers or flasks as your prepare them. Be sure to rinse your
volumetric flasks with 3-4 SMALL aliquots of DI water before reusing them.
3) Record your observations.
1
Silva, C.R.; Pereira, R. B.; Sabadini, E. J. Chem. Educ. 2001, 78, 7, 939-940.
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4) Calculate the concentration of BCG in each solution. (Do you have your data organized in a
table?)
5) Propose a model of the BCG system to explain your observations.
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Studio 11b p3
One way to think about the system is to concentrate on the colors you observe. Each molecule
involved will have its own absorbance spectrum (if the molecule is red, it absorbs blue light, etc.)
(Figure 1) Sometimes molecules such as the one used as green food coloring will absorb light at
more than one wavelength to produce the color that we see.(Figure 2.)
absorbance
Figure 1: Absorbance vs. Wavelength of Red Food Coloring2
1
0.5
0
350
450
550
650
750
wavelength
Figure 2: Absorbance vs. Wavelength of Green Food Coloring3
6) Sketch a series of spectra you would expect to see if each of these two food coloring
molecules was diluted with water.
2
3
From: http://services.juniata.edu/ScienceInMotion/middle/labs/physical/chem/chalk.doc
From: http://icn2.umeche.maine.edu/genchemlabs/images/uvvis.gif
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Studio 11b p4
A solution of BCG (1.8x10-4 M) was diluted with water in the same manner that you carried out
your dilutions. The UV-vis absorbance spectrum of each solution was recorded for each sample
and a stack plot was prepared as shown in Figure 3.
Figure 3: Absorbance of BCG dilutions with water
2,5
-1
[HInd]
= 9.10M
mol L
[BCG]
= 137
0
-1
[HInd]
= 18.2M
mol L
[BCG]
= 115
0
-1
[HInd]
= 27.3M
mol L
[BCG]
= 109
0
-1
[HInd]
= 71.0 M
mol L
[BCG]
= 92.8
0
-1
[HInd]
= 81.9 M
mol L
[BCG]
= 81.9
0
-1
[HInd]
= 92.8 M
mol L
[BCG]
= 71.0
0
-1
[HInd]
= 109 M
mol L
[BCG]
= 27.3
0
-1
[HInd]
= 115 M
mol L
[BCG]
= 18.2
0
-1
[HInd]
= 137 mol L
0
[BCG]
= 9.10M
2,0
Abs
1,5
1,0
0,5
0,0
300
400
500
600
700
 / nm
7) There are two peaks in the visible region (400-800 nm): one is centered at 440 and the other
at 628 nm. How do they fit your model? (Is each peak a separate molecule or is there one
molecule that abosorbs at both wavelengths?)
8) Measure the absorbances of your solutions at 440 and 628 nm to get quantitative data.
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9) Examine the ratio of A628/A440. Would you expect the ratio to change if one molecule
absorbed at both wavelengths and was being diluted? (If you are not sure about how the ratio
will change for a single molecule, you can try some green food coloring.)
10) How does this additional data affect your model? If it disagrees, update your model.
(DO NOT discard these solutions yet!!!!!)
11) Now prepare the following solutions (aliquots of 1.00, 1.50, 2.50, 4.50, 5.10, 6.00, 6.30 mL
of the BCG stock solution diluted to 10 mL using a 0.10 mol L-1 aqueous HCl instead of water as
the solvent. What do you observe? (Set up a table including BCG concentrations and
observations and write out a once sentence summary of your observations.)
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Studio 11b p6
12) Figure 4 shows the absorbances of the BCG solutions diluted with acid. Do you have
multiple colored molecules or just one molecule in these solutions?
Figure 4: Absorbance of BCG dilutions with HCl
2,5
-1
[HInd] = 9.10 mol L
[BCG] = 0115 M -1
[HInd]0= 18.2 mol L
[BCG]
= 109 M -1
[HInd]
= 27.3 M
mol L
[BCG]
= 092.8
-1
[HInd]
= 71.0 M
mol L
[BCG]
= 081.9
-1
[HInd]
= 81.9 M
mol L
[BCG]
= 071.0
-1
[HInd]
= 92.8 M
mol L
[BCG]
= 027.3
-1
[BCG]
= 018.2
[HInd]
= 109 M
mol L
-1
[BCG]
= =9.10mM
[HInd]
115 mol L
2,0
Abs
1,5
1,0
0
0,5
0,0
300
400
500
600
700
 / nm
13) How does this new data affect your model? Write out a chemcial reaction equation that fits
the observations from both sets of diliutions and which includes both H2O and H3O+.
14) Is BCG and acid or a base?
15) Given the context that your chemical system is an equilibrium, describe how the dilution
with water caused the color change.
16) Describe how the dilutions with acid “prevented” the color change.
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Studio 11b p7
17) What is the Ka for BCG? Measure the pH of each of the water diluted solutions and find the
average Ka.
To prepare for next week, weigh ~5.00 g of OxiClean into an
aluminum weigh boat. Spread it out in the pan as much as
possible to dry more effectively. Label with your group’s name
and the weight of solid. Place in the drying oven.