Workshop No. 2: Simulation of Photosynthesis and Respiration The Photo-Blue-Bottle Experiment Irradiatiate Shake Fig. 1: Basic Photo-Blue-Bottle Experiment; (see Basic Experiments ) Fig. 2: Investigating energy conversion & storage; (see Extended Experiments ) Equipment / Materials Chemicals test tubes with screw-type cap sunlight or slide projector or LED pocket lamps glass filters (red and blue) wire leads with alligator clips, multimeter platinum electrodes or platinized razor foils snap cap vials, aluminium foil, filter paper, elastic bands, plastic syringe EDTA: ethylenediaminotetraacetic acid disodium salt ( Titriplex III ) MV 2+ : 1,1 -dimethyl-4,4 -bipyridinium dichloride (methylviologen) or EV 2+ : 1,1 -diethyl-4,4 -bipyridinium dichloride (ethylviologen) Proflavine: 3,6-diaminoacridine hemisulfate PBB-Solution Preparation of the Photo-Blue-Bottle solution for all experiments At first the following three aqueous basic solutions are prepared: Solution I: 2.8 g of EDTA in 100 ml of solution ( c = 7.5 *10 2 mol/l ) Solution II: 386 mg of MV 2+ in 10 ml of solution ( c = 1.5 * 10 1 mol/l ) Solution III: 15.5 mg of proflavine in 100 ml of solution ( c = 3 * 10 4 mol/l ) The PBB-solution (Photo-Blue-Bottle solution) is mixed from these basic solutions as following: 30 ml of solution I + 10 ml of solution II + 75 ml of solution III+ 380 ml of distilled water 495 ml of PBB-solution (The color of this solution is lemon yellow) Note: The PBB-solution for following experiments will be provided by the workshop leader Basic Experiments: Photo-Blue-Bottle in the test tube PBB-1: Fill PBB solution (see Fig. 1) in a 15-mL test tube with a screw-type cap. Approx. 1 cm of the test tube should be free of solution, that means it should be filled on the top with air. Irradiate the PBB-solution with sunlight (or a violet LED pocket lamp, or the light of a slide projector). Measure the duration needed for the complete blue coloration of the solution and note the result. Shake the test tube vigorously so that the blue solution and the air from the upper part mix. Note again the duration needed until the blue solution turns completely back to yellow. Repeat three times the described cycle blue coloration by irradiation yellow coloration by shaking and note the required durations for the blue coloration and the back coloration to yellow for each cycle. Questions, Hypothesis, Verification/Falsification Experiments: Which kind of basic chemical reactions occur in the PBB-solution when it turns blue and yellow? Which color of light is required for the reaction leading to the blue coloration of the PBB solution?
For investigating these topics, the PBB-experiment should be carried out in the following modifications: PBB-2: Fill the test tube completely with the yellow PBB-solution and irradiate it until the solution has turned completely blue. Shake it and observe the color. Open the screw cap of the test tube, remove approx. 1 ml of the solution with a pipette, close the test tube again and shake it again. Observe the color. PBB-3: Replace the air above the solution completely by nitrogen (or hydrogen, or carbon dioxide) and repeat the experiment PBB-1. (Information for the case that PBB-3 cannot be carried out: The PBB-solution turns blue by irradiation, but as in PBB-2 it does not turn yellow by shaking.) PBB-4: Repeat PBB-1 using as irradiation source a green, blue or a red LED-Pocket lamp. Alternatively you can use either sunlight or light from a halogen lamp but filtered through a green, blue or a red glass filter. Experimental findings and conclusions from PBB-1, PBB-2, PBB-3 and PBB-4: For the coloration of the PBB-solution from blue to yellow oxygen is needed. Accordingly, the coloration from blue to yellow by shaking is an oxidation reaction. Consequently, the coloration from yellow to blue by irradiation is a reduction reaction. In order to drive the reduction reaction, blue or violet light is needed. Green and red light is unable to drive this reaction. This in accordance with the well known fact that a yellow solution absorbs blue (but not green or red) light. Extended Experiments: Photo-Blue-Bottle as Energy Conversion & Storage Hypothesis: The PBB-Experiment simulates the natural cycle of photosynthesis and respiration Energy PBB-Experiment Photosynthesis and Respiration Chlorophylls and others Enzyms and others Fig. 3: Coupled reaction cycles in the PBB-Experiment and in the natural cycle of photosynthesis and respiration Before carrying out further experiments, deal with the following issues in Exercise 1: Exercise 1 (searching for analogies and differences): E1 (low and medium level) Please complete the following statements by marking a cross if there is an analogy (no cross if difference) and justify it using i) your experimental observations from the Basic PBBexperiments, ii) your knowledge on natural cycle of photosynthesis and respiration and iii) your knowledge on oxidation states and oxidation numbers respectively: Light is the driving force in the PBB-experiment as well as in photosynthesis. In the PBB-Experiment as well as in natural cycle of photosynthesis and respiration reactions occur in aqueous solutions.
In the PBB-Experiment as well as in natural respiration oxygen from the air acts as oxidizing agent. Both, the endergonic reaction MV 2+ MV + in the PBB-Experiment as well as the endergonic reaction CO 2 C 6 H 12 O 6 in the natural cycle of photosynthesis require colored photocatalysts able to absorb visible sunlight. Compounds indicated in Fig. 3 as MV 2+ and MV + simulate carbon dioxide and sugar from the natural cycle of photosynthesis and respiration. In the PBB-Experiment as well as in the natural cycle of photosynthesis and respiration all reactions occur in a closed system that is a system without exchange of matter with the environment. In Basic PBB-Experiments it has been convincingly demonstrated that light energy is converted into chemical energy and stored in the product of the endergonic reaction driven by light. Experiment (Energy Conversion&Storage): PBB-5: Build up an electrochemical measuring device as shown in Fig. 2. Details of this device are shown in the adjacent scheme. It consists of two half cells connected through an electrolyte bridge which is a filter paper immersed in both half cells. Connect one of the half cells through a platinum electrode to the minus pole of digital voltmeter and prepare it for irradiation with light. Wrap the other half cell with aluminium foil to keep it dark. Irradiate now the one half and record the Irradiated PBB-solution Light source voltage indicated by the voltmeter during the blue coloration of the irradiated solution. When the solution got completely blue and the voltage does not increase any more, switch off the light and continue to record the voltage for 2-5 minutes. PBB-6: Pierce the needle of the syringe through the plastic cap of the vial with the irradiated, blue solution, unfasten the cap and gently press air from the syringe into the blue solution. Observe the color and note the voltage. Stop the introduction of air when the solution has turned completely yellow. PBB-7: Repeat the cycle PBB-5 and PBB-6 two or three times. Stirring bar Platinum electrodes Electrolyte bridge PBB-solution kept dark
Voltage [mv] Voltage [mv] Irradiation stop Irradiation start Air supply start Air supply stop Time [min] Fig. 4: Time dependence of the voltage in Extended PBB-Experiments Exercise E2: E2 (low and medium level) Please complete the following statements by marking a cross if there is an analogy and justify using your experimental observations from the Extended PBB-experiments In Extended PBB-Experiments it has been convincingly demonstrated that light energy is converted into chemical energy and stored in the reduced product until this has been oxidized by supply of oxygen. If sunlight is used to drive the device in Extended PBB-Experiments, the device would work like a solar accumulator that is a rechargeable solar storage-battery. PBB-Experiment on Video: Watch the Photo-Blue-Bottle Video on the web-side www.chemiedidaktik.uni-wuppertal.de > Teaching Photochemistry > Videos. It shows the extended PBB-experiment with an electrical motor. Note, that the motor starts, if the voltage in the PBB-device overcome a threshold of approx. 200 mv. However, the energy for the running motor is not supplied by the PBB-device. Although the voltage in the PBB-device increases up to approx. 700 mv, the current is very low. It is less than 5 μa. Exercise E3: E3 Which of the following described experimental parameters in the PBB-device consider you to be the chief cause for such a low current that it is not possible to drive a small electrical motor? The electrodes consist of inert platinum, there is no electrode of zinc as usually in galvanic cells. The electrolyte bridge is not a solution of an inorganic salt as usually in galvanic cells and batteries. The concentration of the photocatalyst Proflavine PF + (see Fig. 3) is too low. The concentration of the substrate Methylviologene MV 2+ (see Fig. 3) is too low. The voltage generating processes involve exclusively organic compounds.
Elementary Processes in the PBB-Experiment: For a deeper understanding of the endergonic reduction of MV 2+, which is driven by light using PF + as photocatalyst, the energy level model is useful. E [V] Redox Potentials hν 1 2 3 Fig. 5: Elementary processes 1-2-3 for a photocatalized endergonic reduction 1: By absorption of a photon hν of adequate energy in the photocatalyst particle PF + an electron jumps from the highest occupied energy level into the lowest unoccupied energy level, generating the excited state (PF + )*. 2-3: The photoelectron transfer from (PF + )* to the substrate particle MV 2+ leads to the reduced substrate and the oxidized photocatalyst PF 2+. The energy of the absorbed photon has been partially used for the endergonic reduction MV 2+ MV +. Exercises E4 E10: E4 (low level) Choose the accurate denomination for each elementary process (1, 2 and 3) by marking a cross at the adequate position: 1: reduction electronic excitation oxidation 2: reduction electronic excitation oxidation 3: reduction electronic excitation oxidation Fig. 6 Redox potentials of redox pairs involved in PBB-Experiment compared to NHE E o (H 2/2H + ) E5 (medium level) The arrows and encircled numbers in Fig. 6 indicate the succession of elementary reaction steps in the complete reaction cycle yellow blue yellow from the PBB-experiment. Complete the following sentences relating to the encircled numbers by including fitting terms: 4: This indicates the. of the sacrificial donor particle EDTA 5: This indicates the reduction of the oxidized particle PV 2+, which is generated in step.. 6: This indicates the. of the reduced particle MV + generated in step.. 7: This indicates the reduction of. from the air introduced into the blue PBB-solution. E6 (medium level) After having solved E4 and E5, please transcribe all seven encircled numbers from Fig. 6 into Fig. 3, placing each of them beside the appropriate bended arrow. E7 (medium and advanced level) Whereas the concentration of the photocatalyst in the PBB-solution is only approx. 1% of the substrate concentration, the concentration of the sacrificial donor is much more higher than the substrate concentration. Give reasons for these optimized experimental conditions. The photocatalyst PF + :.. The sacrificial donor EDTA:..
E8 (advanced level) In the PBB-experiment as well as in any other photocatalytic endergonic reduction, energy of the absorbed light is only partially converted into chemical energy. Discuss this fact with your teacher using the terms and the representation from Fig. 5. For a more detailed argumentation you can use additional terms and acronyms such as electronic excitation, hν, vibrational relaxation, free reaction enthalpy ΔG R and others. E9 (advanced level) The reason for the generation of a voltage in Extended PBB-Experiments consists in the changes of the ratio between the reduced and the oxidized substrate methylviologene. Comment and exemplify this statement dealing with the NERNST S equation indicated below: E10 (advanced level) Assume, that in the experiment PBB-5 a voltage of U = 600 mv has been measured. Calculate the ratio c(mv + )/c(mv 2+ ) applying NERNST S equation. Colors and Water Solubility of PBB-Components Fig. 7: Formula of components in the PBB-experiment: Substrate MV 2+ Photocatalyst PF + and sacrificial donor EDTA; Exercises E11 E12: E11 (low and medium level) All components involved in the PBB-experiment (excepting oxygen) are soluble in water. Justify this using formula from Fig. 7. Proflavine PF + : Methylviologene MV 2+ :.. EDTA:.. E12 (medium and advanced level) Otherwise than proflavine PF +, methylviologene MV 2+ does not absorb light in the visible region. Find and formulate a reason for this fact using formula from Fig. 7. (Hint: Molecular 3D-Models on the computer could be helpful for this task.)