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.