CHM151YName: Ruo yi (Caroline) LinStudent #: 1001333112Section:
112Experiment #9 performed on February 16th, 2015: Investigating
light-matter interactions.AbstractThe effect of addition of colour
filters in the path of a light beam on the voltage generated by
that beams action on a photocell was examined by placing successive
numbers of filters within the path. Blue, green and a combination
of blue and green filters were used. The effect of the
concentration of Copper (II) Sulphate placed in the path of the
light beam on voltage was also examined, using solutions of varying
concentration. The voltage produced by the photocell had an
exponential relationship to the number of filters applied, as well
as to the concentration of the copper (II) sulphate. The absorbance
of the filters and solution were found to have linear relationships
to their number and concentration. The voltage-concentration curves
were used to find the concentration of an unknown solution of
copper sulphate. Finally, limitations of the Lambert-Beer law were
examined by observing the photofluorescence of chlorophyll
solution. These results confirm the Lambert-Beer law, while also
showing some of its limitations, proving that UV-VIS spectrometers
can be used to help identify various properties of chemical
compounds.IntroductionChemical compounds have characteristic
absorbance patterns which may be attributed to their characteristic
molecular orbitals. Spectroscopy takes advantage of these
characteristic patterns to identify compounds. One type of
spectroscopy is UV-VIS absorption spectroscopy. When a given
frequency of visible light is passed through a sample, the
intensity of the light will be reduced according to the
concentration and composition of the sample. The intensity of an
exiting beam of light may be calculated using the Lambert-Beer law,
I=Io10-nloga. In this equation, I is the intensity of the exiting
beam, Io the intensity of the original beam, n the number of units
of material in the path length of the beam, and a some constant
greater than one. The term nloga is the total absorbance of the
sample, also denoted by A. Another formulation of the Lambert-Beer
law relates intensity to the path length, b, of a light beam
passing through a solution of concentration c. This equation is
I=bc, where is a constant known as the extinction coefficient.
Using the Lambert-Beer law in conjunction with UV-VIS spectroscopy,
one may identify the concentration of a given
solution.Experimental
Figure 1. Experimental setup of a manual spectrometer. The setup
used in this experiment did not employ a diffraction grating or
lens. In part A, the holder was replaced by a series of blue or
green filters. In part B, the holder was a cuvette filled with
solutions of Copper (II) Sulphate with varying concentration. The
photodiode is referred to as a photocell in the remainder of the
report.ProcedureA power supply (two 9V batteries) and multimeter
were connected to the detector of the manual spectrometer. The
photocell was calibrated to be set in the most intense region of
the spectrum. The background voltage, Vs, was recorded. Part A. A
blue filter was placed in the front of the lamp (Figure 1, holder)
and the voltage recorded. Successive blue filters were added and
the voltage recorded until all filters were used. The measurements
were repeated using successive additions of green filters. The
measurements were repeated once more using successive additions of
one blue and one green filter each per measurement. Part B. A
cuvette filled with distilled water was placed in the holder
position and the voltage of the photocell recorded. The cuvette was
rinsed and cleaned with dry tissue. The cuvette was filled with a
0.15M solution of Copper(II) Sulphate and placed in the holder
position. The voltage was recorded. The procedure was repeated 9
times with solutions of varying concentration (see Table 1 under
Results). Part C. A spinach leaf was ground with a few drops
ethanol using a mortar and pestle. 10mL ethanol was added to create
a leaf solution. The resultant solution was collected and gravity
filtered using a conical funnel. 10mL ethanol was added to the
remaining leaf pulp, the pulp ground and the rinse gravity
filtered. The collected solution was poured into a vial, which was
placed in the holder position of the spectrometer. The vial was
observed at positions both parallel and perpendicular to the path
length of the light.Results and DiscussionPart A: Light filters
applied to spectrometerPart B: Solutions of CuSO4
Filter # (Blue)Voltage (V)Filter# (Green)Voltage (V)Filter#
(Blue+Green)Voltage (V)Stock #CuSO4 (M)Voltage (V)
11.9413.6911.61003.78
2123.0520.7810.151.63
30.6532.5430.5420.12.23
40.542.1440.4430.072.37
50.4351.8450.440.052.78
60.461.5860.3850.023.17
70.3871.3170.3660.013.45
80.3781.1680.3670.0073.48
90.3691.0280.0053.52
100.37100.990.0023.64
100.0013.66
Table 1. The results of the experiment for parts A and
B.Description of qualitative results. When the chlorophyll solution
from part C was observed from a slide parallel to the light beam,
it appeared to transmit green light. However, when the same
solution was observed from a side perpendicular to the light beam,
it appeared to transmit red light. The red light may be explained
by photofluorescence. Typically, the energy absorbed by chlorophyll
in a plant cell is used for photosynthesis. However, because the
chloroplasts are destroyed when making the chlorophyll solution,
the energy absorbed by the chlorophyll is instead re-emitted as red
light. When a blue filter is placed in front of the light source,
only blue light is transmitted through the filter. When the same
chlorophyll solution is placed in front of the filtered light, the
solution no longer appears red when viewed perpendicular to the
light beam. This is perhaps because the blue filter does not
transmit the light wavelengths which the chlorophyll absorbs.Sample
Calculations. Graphs for absorbance and voltage as a function of
number of filters or concentration of copper (II) sulphate were
created using Excel. The value of Vo was found to be 3.78V from the
experiment. The value of Vs (voltage from background light only)
was found to be 0.36V. This was the plateau voltage for the blue
and blue+green filter experiments.The transmittance of a given
experimental exposure is given by T=(V- Vo)/( Vs- Vo). For the blue
filter experiment with number of filters = 1,
T=(1.94-0.36)/(3.78/0.36)=0.462. The absorbance A is given by
-logT. The absorbance for 1 blue filter is log0.462=0.335.
Additional values were calculated with Excel. Graphs are attached
in an appendix. Additional Discussion. The trendlines in excel use
base e rather than base 10. In this case, the absorbance values
indicated by the trendlines are equal to nlna, not nloga. Notice
also that the y intercepts of the absorbance graphs were fixed to
0, as the absorbance of the filters when there are n=0 should be
zero.The coefficients for the voltage graphs give Vo, while the
coefficient in front of x (x is number of filters) gives the
absorbance indirectly (note that the value given is for nlna and
must be converted to nloga). The slopes of the absorbance graphs
indirectly give the value of a for the filter experiments (again, a
ln to log conversion must be done), and the extinction coefficients
for the concentration experiments. The y intercepts for the
absorbance experiments were fixed to 0, as discussed above.The
voltage of the unknown sample of Copper (II) Sulphate was 3.60.
Subtracting 0.36 to correct for background voltage gives 3.24. The
exponential trendline which relates voltage as a function of
concentration is Voltage=3.2934e-6.273x, where x is concentration
in M. Plugging 3.24 in for voltage, we get 3.24=3.2934e-6.273x
yields x=0.00261M. Thus, the concentration of the unknown is
2.61x10-3M.ConclusionsThe Lambert-Beer law in conjunction with a
simple spectrometer can be used to find the concentration of an
unknown sample of a given chemical compound. For this experiment,
and unknown concentration of Copper Sulphate which gave a voltage
of 3.60V was calculated to have a concentration of 2.61x10-3M.
Using coloured filters, the validity of the Lambert Beer law in an
experimental setting was confirmed. Limitations of the Lambert-Beer
law were observed using the phenomenon of chlorophyll
photofluorescence. In conclusion, the Lambert-Beer law is shown to
be a useful tool for the identifications of the concentrations of
compounds, although not without limitations.Post-lab Questions1. As
the number of filters approaches infinity, the term nloga will
approach negative infinity. This means that the intensity of the
exiting beam, and therefore the voltage, should approach 0 (given
by taking the limit of I=Io10-nloga as n approaches infinity). In
fact, this is exactly what occurs in the blue and blue-green filter
experiments, evidenced by the plateaus in both graphs. However, the
plateau is at 0.36 rather than at 0. It must thus be concluded that
contaminating light from the background is causing the plateau to
be at a higher value than expected.2. The number of filters is n,
and A=nloga, where a is a constant. This equation has the same form
as the equation of a line, y-mx+b, where b=0. Thus, absorbance is
expected to have a linear relationship with number of filters.
Increasing concentration has a similar function to increasing the
number of filters, so it is predicted that the relationship between
concentration and absorbance also be linear.3. Absorbance has a
linear relationship with the number of filters added, so it is
expected that the blue+green absorbance line has a slope equal to
the addition of the blue absorbance slope and the green absorbance
slope. This is actually the case. The slopes for the blue and green
lines are 0.2978 and 0.0775 respectively, which add to 0.3753. The
actual slope for the blue+green line is 0.3602.4. Although the
chlorophyll emits red light in all directions, the fluorescence of
the chlorophyll can only be seen viewed perpendicular to the beam.
This is because the intensity of the green light which the
chlorophyll transmits is much higher than the intensity of the
emitted light, and when viewing the chlorophyll from directions
where there is much transmitted light passing through, the emitted
light is drowned out by the transmitted light.References1. Hollas,
Michael J. Modern Spectroscopy. John Wiley & Sons, 1996.2.
Sawyer, Heineman, and Beebe. Chemistry Experiments for Instrumental
Methods, John Wiley & Sons, 1984.3. Skoog, Holler and Nieman.
Principles of Instumental Analysis, 5th ed., Saunders College
Publishing, 1998.
Appendix: Graphs of voltage and absorbance as functions of
number of filters/concentration.
Graph 1: Voltage as a function of number of blue filters. x is
the number of filters.
Graph 2: Voltage as a function of number of green filters. x is
the number of filters.
Graph 3: Voltage as a function of number of blue and green
filter pairs. x is the number of filter pairs.
Graph 4: Absorbance as a function of the number of blue filters.
x is the number of filters.
Graph 5: Absorbance as a function of the number of green
filters. x is the number of filters.
Graph 6: Absorbance as a function of the number of blue-green
filter pairs. X is the number of filters.
Graph 7: Voltage as a function of the concentration of copper
sulphate. X is concentration.
Graph 8: Absorbance as a function of the concentration of copper
sulphate. X is concentration.