Light from the Chemist’s Retort
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The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (5/2014):
Chemiluminescence is the phenomenon of light emission produced as a result of specific chemical reactions. Many substances exhibit chemiluminescence during the course of reactions, usually oxidation: luciferin in the presence of the enzyme luciferase, luminol, the pyrogallol-formaldehyde system, white phosphorus, and even metallic sodium [1]. This phenomenon can also be observed in singlet oxygen.
Oxygen is a gas that naturally exists as diatomic molecules. For molecular oxygen (O2), the ground state is the triplet form (3O2), characterized by two unpaired electrons, making it a radical (Fig. 1A). In contrast, an excited oxygen molecule with all electrons paired exists in the singlet state (1O2). There are two forms of singlet oxygen, differing in the distribution of electrons in the molecular π*2p orbitals:
- Two paired electrons on the same π*2p orbital (Fig. 1B),
- Two electrons with opposite spins, one on each π*2p orbital (Fig. 1C).
Oxygen exhibits an unusual property — triplet oxygen (3O2), which is a radical, is the ground state, while singlet oxygen, with fully paired electrons, corresponds to an excited state with higher energy.
As one might expect, the singlet form of oxygen is unstable and can spontaneously transition to the triplet state. According to the law of energy conservation, the excess energy can be released, for example, during an oxidation reaction or in the form of electromagnetic radiation within the visible (or adjacent) spectrum.
For an isolated oxygen molecule, however, the transition from the triplet to the singlet state is strictly forbidden by selection rules governing interactions with electromagnetic radiation. This makes such a transition highly improbable. Sensitizers (mainly organic dyes such as methylene blue or porphyrins) can facilitate this process [2].
Singlet oxygen can also be generated chemically, for example, by passing a stream of chlorine gas through an alkaline, cooled 30% hydrogen peroxide solution (Photo 1). Although this method is commonly suggested, it presents a significant drawback from a teaching perspective: the necessity of handling chlorine gas, which is highly toxic and is difficult to store safely [3].
Therefore, I propose an alternative, simpler, and safer method for producing singlet oxygen and observing its chemiluminescence. This requires only two relatively accessible substances:
- Hydrogen peroxide H2O2 30% (perhydrol),
- Sodium dichloroisocyanurate C3N3O3Cl2Na.
Sodium dichloroisocyanurate is commonly used as a disinfectant for sanitary applications. The commercial product, usually available in tablet form (Photo 2), typically contains at least 80% sodium dichloroisocyanurate and is well-suited for this experiment.
Perhydrol is highly corrosive and causes necrotic damage upon contact with skin. Avoid contamination of eyes or skin at all costs! The use of gloves and protective eyewear is essential! Sodium dichloroisocyanurate is harmful, and when it comes into contact with water or acids, it releases toxic gases. During the reaction, chlorine gas is released, which is highly toxic — work must be conducted under a fume hood or outdoors!
The experiment itself is simple. Pour approximately 30 cm3 (1.01 fl oz) of perhydrol into a 100-200 cm3 (3.38-6.76 fl oz) beaker. Then, darken the room and add sodium dichloroisocyanurate in powder or tablet form. A vigorous reaction occurs, releasing a large amount of gas and producing foam. Simultaneously, chemiluminescence can be observed — a beautiful, bright red glow (Photo 3).
Explanation
Initially, water reacts with the dichloroisocyanurate anion:
This reaction produces hydrogen cations and anions: cyanurate and hypochlorite [4]. The hypochlorite ion further reacts with hydrogen peroxide:
The resulting peroxychlorate anion is unstable and decomposes into oxygen and chloride anions:
The oxygen initially exists in the singlet state. During its transition to the more stable triplet form, the energy difference must be released into the surroundings according to the law of energy conservation, often in the form of emitted light.
The energy difference between the ground state and the singlet state with two paired electrons in the same π*2p orbital is 94.3 kJ/mol, corresponding to a wavelength of approximately 1270 nm, which lies in the near-infrared range. This chemiluminescence would, of course, be invisible to the naked eye, as the emitted light is outside the visible spectrum.
However, at high concentrations of singlet oxygen, another emission occurs at 634 nm (red light). This happens when two 1O2 molecules interact. This phenomenon is what we observe in the experiment.
The reaction can be schematically represented using the general mechanism of chemiluminescence:
In this process, the reactant (or reactants) X leads to the formation of an excited intermediate [Y]*. The excited state is high-energy and therefore unstable. The intermediate spontaneously converts into the final product Y, which has lower energy. The excess energy is released as radiant energy hν. In this case, the substrate X, the excited intermediate [Y]*, and the final product Y correspond to peroxychlorate ClOO-, singlet oxygen 1O2, and triplet oxygen 3O2, respectively.
References:
- [1] Ples M., Chemiluminescencja metalicznego sodu (eng. Chemiluminescence of metallic sodium), Chemia w Szkole (Chemistry in School), 2014, 1, pp. 5-7 back
- [2] Laingl M., The Three Forms of Molecular Oxygen, Journal of Chemical Education, 1989, 66 (6), pp. 453-455 back
- [3] Roesky H.W., Möckel K., Niezwykły świat chemii, Wydawnictwo Adamantan, 2001, pp. 161-163 back
- [4] Pinto G., Rohrig B., Use of chloroisocyanurates for disinfection of water, Journal of Chemical Education, 2003, 80(1): pp. 41–44 back
All photographs and illustrations were created by the author.
Addendum
The effect of this experiment can be seen in the following video:
Marek Ples