Molar absorptivity

CHM364

Molar absorptivity

Part I: (Spectra and Molecular Structure, UV-VIS Analysis)

Introduction

In this experiment, students will learn how to use the UV-visible spectrophotometer to determine the molar extinction coefficient of organic dyes. In the same time they will learn how to use UV-vis spectroscopy to construct a calibration plot for light absorbing compound. Then, they will be able to identify unknown concentrations of this compound, in this case the organic dye, based on their calibration plot.

In the second part of this experiment (Part II), you will conduct a photocatalytic degradation exercise using the xenon lamp in the light cabinet as the energy source to effect the degradation. Careful data acquisition and analysis will enable you to follow and determine the rate of the reaction.

Theory

When monochromatic light passes through a sample, the ratio of power transmitted to the original power is called the transmittance T. The Absorbance is equal to the logarithm of the reciprocal of the transmittance:

A = -log T = log (1/T)

In this case, the light absorbance of the sample depends on the molar concentration (c), light path length in centimeters (L), and molar extinction coefficient (ε) for the dissolved substance. The molar extinction coefficient (ε) is a term that determines how strongly a substance absorbs light at a given wavelength per molar concentration.

Therefore, the concentration of the substance dissolved in the solvent can be determined experimentally using a spectrophotometer and based on Beer’s law: A= εLC

Plotting the absorbance against the concentration for a series of dilutions will result in a linear line with the slope = ε, where the light path length (through the sample solution) is 1 cm.

Procedure

1- Carefully perform every step in this procedure, as the grade for this lab will be based on precision and accuracy.

2- In a set of 20 mL glass vials, prepare a series of aqueous solutions of the methylene blue solution, 10, 30, 50, 70, and 100 M. You may prepare a sample of the highest concentration (100 M) and then perform serial dilutions.

3- In a set of 20 mL glass vials, prepare a series of aqueous solutions of the rhodamine B solution, 10, 30, 50, 70, and 100 M. You may prepare a sample of highest concentration (100 M) and then perform serial dilutions.

4- Set the UV-vis spectrophotometer for scan absorbance over the range of 300 − 700 nm.

5- Use DI water to obtain baseline readings of the spectrophotometer.

6- Measure the UV-vis spectra for your series of methylene blue solutions using disposable cuvettes over the range on 300 − 700 nm.

7- Measure the UV-vis spectra for your series of rhodamine B solutions using disposable cuvettes over the range on 300 − 700 nm.

8- Perform the analysis of each concentration in triplicate.

9- Measure the absorbance of the unknown concentration of each dye.

Data Manipulation and Calculations

1- Export data to Excel and calculate average absorbance of each sample.

2- Plot average absorbance vs wavelength and identify the peak maximum.

3- Plot the absorbance at peak maximum against concentration of the dye.

4- Find the slope and regression of the linear fit line. Slope equals extinction coefficient (ε).

5- Calculate the concentration of the unknown sample using extinction coefficient and absorbance.

Equipment

20 mL Glass Scintillation vials

UV/VIS Spectrophotometer

Disposable cells (cuvettes) for UV-VIS

Part II: (Photocatalytic dissociation of organic dyes by metal oxide semiconductor nanomaterials)

Introduction

Catalysis is very important process that facilitates (enables) reactions at lower energy input. Catalyzed reactions have lower activation energy than the corresponding uncatalyzed reaction. The result is a higher reaction rate (at the same temperature) for the same reaction concentrations. It is estimated that 70 % of chemical products depend on catalysis. Even, in the human body, many vital biochemical processes involve enzymatic catalysis.

Studying the photocatalytic degradation of organic dyes using metal oxide nanomaterials allows one to have practical experience with several physical chemistry concepts, including band gap energy of semiconductors, photoexcitation, light absorption of organic dyes, different energy bands of light, kinetics of catalytic reaction, the role of catalysts and photon to electron conversion.

Theory

Scheme 1

Nano and micro-scale metal oxide semiconductors have photo-induced electrical properties. In metal oxide semiconductors, the valance band and the conduction band are separated by energy band gap. When a photon with energy higher than that of the band gap hits the metal oxide semiconductor, electrons migrate from the valence band to conduction band leaving holes in the valence band. The photogenerated electron and hole couples leads to reduction/oxidation reaction, in which water molecules are reduced by electrons to generate hydrogen and oxidized by holes to generate oxygen (Scheme 1). These charge carriers can drive the degradation reaction of organic compounds. In this approach, the electron from the conduction band transfer to the dissolved oxygen to produce super oxide radical that is very potent in its ability to oxidize many organic compounds, including the organic dyes prepared in Part I of this experiment. The holes in the valence band produce hydroxyl radicals, which then go on to degrade organic compounds. All reactive species generated as a result of the photoexcitation contribute to the photocatalytic degradation of the organic dye.

Studying the kinetics of the photocatalytic degradation reaction gives insight to the activity of the catalyst. It is generally accepted that the rate of this reaction follows first order kinetics for the dye and pseudo-first order for the overall reaction.

Where is the observed pseudo–first-order rate constant (h1) and is the aqueous phase concentration of the organic dye at time t and is the initial concentration of the organic dye.

Procedure

The instructor will provide bismuth vanadate as the catalyst

1- Carefully perform every step in this procedure, as the grade for this lab will be based on precision and accuracy.

2- Weigh out 10 mg of the catalyst (bismuth vanadate).

3- Prepare 20 mL of a 20 mg/L aqueous solution of each dye (Rhodamine B and Methylene Blue) in the 20 mL glass scintillation vials.

4- Add the 10 mg of the catalyst into on of the dye solutions.

5- Sonicate the solution for 5 min to attain dispersion.

6- Add a Teflon-coated magnetic stir bar into the vial.

7- Place the vial on the stir plate against the solar simulator.

8- Take 1 mL aliquot at 0 min into an Eppendorf tube (Note: 0 min = before the start of irradiation).

9- Turn on the solar simulator to start the light irradiation (make sure to put on proper safety glasses).

10- Take 1 mL aliquots at 2, 4, 6, 8, 12, 16, and 20 minutes of irradiation.

11- Repeat steps 3 − 9 for the second dye solution.

12- Place the samples in the centrifuge and turn on to begin spinning.

13- Carefully transfer the supernatant into cuvettes (for UV-VIS analysis).

14- Measure the absorbance of the samples over the wavelength range of 300 − 700 nm.

Data Manipulation and Calculations

1- Save the UV-VIS measurements as both spectrum and data print table.

2- Export data to Excel and calculate average absorbance of each sample.

3- Transfer your data to a USB drive for later manipulation.

5- Find the wavelength at the absorption maxima at 0 min.

6- Record the absorbance at the same wavelength for all samples of the

same dye.

7- Convert the absorbance to concentration using the experimental data from Part I of this 2-part experiment.

8- Plot a graph of the time against natural logarithm of concentration at time t, over concentration at t = 0 min.

9- Find the linear best-fit line. Hint: use the linear portion of your data.

10- The slope of the linear line is the rate constant of the reaction.

Equipment

20 mL Glass Scintillation vials

UV/VIS Spectrophotometer

Disposable cells (cuvettes) for UV-VIS

Eppendorf tubes

Xenon Arc Lamp (Solar simulator)