Abstract

An experiment simulating the natural cycle of photosynthesis and respiration – and dealing with both matter and energy conversion – is presented and theoretically elucidated. Teaching recommendations concerning the integration of the Photo-Blue-Bottle experiment into lower chemistry and biology education according to the 5E Instructional Model are provided as well as supplementary materials.

1. Introduction

In the ongoing scientific research, outstanding efforts are dedicated to solve a big problem: How would it become possible to use solar light as the main energy source in chemical synthesis as well as in the industrial production of “green fuels”? In order to solve this problem, the firm conviction in the scientific community is: Learn from nature! ^{1, 2, 3}. We share this opinion, and we complete it concerning research in science education with the slogan:

2 teach 4 future is

2 learn from nature.

Indeed, in science education there is the need for a permanent adjustment of teaching contents and methods to the state of the art in science and technology is. Therefore, our research centres on the inclusion and usage of solar light energy in the science education of the young generation. Light is the driving force for life on our planet, and we are deeply convinced that basic insights into the energy and matter conversions occurring in the natural cycle of photosynthesis and respiration must be given to students right from the beginning, that is, already in the lower science education phase. In order to achieve this aim, convincing experiments, scientifically consistent concepts, and adequate teaching materials are needed.

2. Background

The Photo-Blue-Bottle (PBB) experiment formerly published in J. Chem. Educ. ⁴ has been completely renewed: i) The toxic substrate methylviologene (MV²⁺) has been substituted by nontoxic ethylviologene (EV²⁺), ii) harmless LED torches (or sunlight) serve now as irradiation sources instead of hazardous high-pressure mercury lamps, and iii) microscale 5-10 mL vials replace the 450 mL reaction vessel from the original experimental setup ^{5, 6}. Additionally, titanium dioxide (TiO₂) has been alternatively used as photocatalyst instead of proflavine (PF⁺) ⁷. The renewed versions of the PBB experiment work with an aqueous solution containing the substrate ethylviologene (cf. Figure 1), the photocatalyst proflavine (cf. Figure 1), and EDTA as sacrificial donor (see details in section 3).

The reduction-oxidation cycles of the substrate are visualized as color changes (yellow–blue–yellow) and as changes in the electrochemical potential (cf. Figure 4). During the irradiation, the color of the PBB solution changes from yellow (caused by PF⁺) to deep blue (caused by EV⁺). If oxygen is introduced into the blue solution (for instance by shaking the vial), EV⁺ is being reoxidized into the colorless EV²⁺. The endergonic reduction of an ethylviologene dication EV²⁺ to a monocation EV⁺ is photocatalized by proflavine (see Figure 2).

Hereby, the excited state of the proflavine monocation PF^+* initially generated by light absorption (λ_max = 445 nm) acts as reducing agent for EV²⁺, while oxidizing itself to PF²⁺. The photocatalyst monocation PF⁺ is regenerated by electron capture from the sacrificial donor EDTA to PF²⁺. Consequently, EDTA is irreversibly consumed during irradiation, whereas proflavine goes through many turnovers, i.e. photocatalytic oxidation-reduction cycles, without being consumed. (Note that the concentration of proflavine in the PBB solution is only approximately 1% of the concentration of ethylviologene - see Experiments).

Due to the fact that the concentration ratio c(EV⁺)/c(EV²⁺) increases by irradiation, the electrochemical potential of the PBB solution decreases according to the Nernst equation ⁴. In order to visualize this, a microscale galvanic concentration cell as shown in Figure 3 can be assembled and operated even by students. Using this type of cells, voltages up to 240 mV can be reached.

In order to understand why the PBB experiment works as described above, the redox potentials of the involved redox pairs have to be considered. Since E⁰ (EV²⁺/EV⁺) = -0,48 V, a redox pair with E⁰ < -0,48 V is needed for the reduction process EV²⁺ → EV⁺. This becomes available after the electronic excitation PF⁺ → PF^+*. However, the redox potential of the photocatalyst changes dramatically by electronic excitation from E⁰ (PF⁺)/PF²⁺) = + 1,1 V to E⁰ (PF⁺)/PF²⁺)* = - 0,6 V (see the yellow arrow in Figure 4). Indeed, the electronic excitation by absorption of a photon is always the very beginning elementary process of every light driven reaction or multistep cascade of reactions. The numbered redox reactions in Figure 4 correspond to the reactions taking place in the coupled cycles shown in Figure 2. It should be emphasized that except for the initial electronic excitation 1, all other elementary processes consist of a reduction/oxidation pair. These are the pairs 2/3, 5/4, and 7/6 highlighted in Figure 4.

3. Experimental Procedure and Results

The PBB-solution is prepared by dissolving

- 1 g of EDTA (Ethylene-diaminetetraacetic acid disodium salt, Merck/Sigma-Aldrich, CAS No. 6381-92-6),

- 561 mg of ethylviologene (1,1′-Diethyl-4,4′-bipyridinium dibromide, Merck/Sigma-Aldrich, CAS No. 53721-12-3), and

- 15 mg of proflavine (3,6-Diaminoacridine-hemisulfate, Merck/Sigma-Aldrich, CAS No. 1811-28-5)

in 500 mL of distilled water which is stirred continuously. The solution should be stored in a closed brown glass flask. In this case, it remains stable and usable for several months and suffices for at least 15 groups of students if they use microscale equipment as shown in Figure 5 and Figure 6.

The first step of the basic version of the Photo-Blue-Bottle experiment (Figure 5) consists of the irradiation of approx. 3 mL of PBB-solution in a closed screw cap vial with light of different colors from LED torches. Doing so, a blue compound is generated when blue or white light is used, but not with red or green light. Certainly, the blue compound is also generated by irradiation with sunlight. Furthermore, UV light can be used, but for both safety and yield reasons we recommend using blue light.

In order to introduce air into the blue solution, the closed vial simply has to be shaken. Doing so, the blue compound disappears while the PBB-solution regains its initial yellow color. The described cycles yellow-blue-yellow can be repeated several times, depending on the volume of air above the solution. Each cycle consists of a photocatalytic reduction of the substrate EV²⁺ and an oxidation of the reduced substrate EV⁺ with oxygen from air.

In order to assemble photogalvanic concentration cells, two half cells containing PBB-solution have to be combined according to the detailed indications in Figure 3 (see section 2). Actually, these versions of micro solar accumulators have been realized as shown in Figure 6 using round snap cap vials or squared micro containers respectively. During irradiation, the voltage increases up to a constant value between 230 mV and 240 mV.

Even after the LED torch is turned off, neither the blue color nor the voltage changes. If oxygen from the air above the solution is added by shaking, the color of the formerly irradiated solution changes from blue to yellow, and the voltage breaks down simultaneously. This is the proof that during irradiation light energy is converted into chemical energy by the endergonic photocatalytic reduction of the substrate EV²⁺ and stored in the reduced substrate EV⁺ until its re-oxidation by reaction with oxygen from air.

4. Teaching recommendations

According to Jean Piaget’s philosophy of education (cf. e.g. ⁸), we adapted the 5E Instructional Model ⁹ as framework for the integration of the Photo-Blue-Bottle experiment into the curriculum for the lower science education phase. Consequently, neither chemical formula nor sophisticated terminology should be used. The pictogram of the framework, figuratively shaped as a spiral loop, is shown in Figure 7. Comments, teaching specifications and recommendations for the 5E phases are summarized in Table 1.

Note that the main goal of this unit is to make learners well aware of the cyclic transformations of matter and energy conversions in the natural cycle of photosynthesis and respiration.

Therefore, the phases 2 and 3, that is Exploration and Explanation, represent the core of this teaching unit. Learners are requested to find the solution of a problem, but without giving them concrete prescriptions for the experiments (see E1 in Figure 8). An experimental box containing the suitable equipment – and some additional, thought-provoking material allowing for some further exploration -- is provided instead. So the learners are repeatedly forced to develop a hypothesis, plan and carry out an experiment in order to confirm or reject the hypothesis – if necessary, to develop a new hypothesis, test it again, and so on. This way of investigation gives them an insight into the basic method of scientific research. Assignment E2 substantiates and broadens this approach by covering further relevant aspects. Here, the idea that oxygen might play a crucial role is touched upon as this does not directly arise from the main ideas put forward in E1.

5. Conclusion: Crossover with Light

Depending on the concrete educational programs in chemistry, biology and physics at secondary and tertiary level, the following interdisciplinary topics can be addressed in connection with the Photo-Blue-Bottle experiment:

- the carbon cycle in animate nature - deeper and more detailed evaluation of the PBB experiment from the perspective of organic chemistry and biology;

- energy conversion and energy storage - deeper and more detailed evaluation from the perspective of electrochemistry and physics (see also ¹²);

- color by light absorption and light emission - PBB and other experiments for elucidating the interaction light-matter and the generation of color from the perspective of chemistry and physics (see also ¹³);

- thermodynamic equilibrium and photosteady state – PBB and other experiments for elucidating the fundamental difference between these “unequal equilibria” and their importance in the atmosphere, the biosphere and the technology of smart materials;

- photocatalysis with solar light - deeper and more detailed evaluation of the PBB experiment from the perspective of chemistry, physics, technology and economy pursuing the goal “from CO₂ biology to CO₂ economy”.

Supporting materials with additional information, videos, TV-tutorials (cf. Figure 11), and worksheets is provided open access on our website ¹⁴ and in references ¹⁵ and ¹⁶.

Acknowledgements

We acknowledge the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for supporting the projects “Photoprocesses in Science Education” (Photo-LeNa, TA 228/4-1 and Photo-MINT, TA 228-2), and the Beilstein Institute for the production of TV-tutorials “The Fascinating World of Photochemistry”. Additionally, we acknowledge the Chemical Industry Fund (FCI) for supporting the project “Photoprocesses in Bilingual Chemistry Education”.

References