Distance learning: an interdisciplinary experiment on Rayleigh scattering

Introduction

Distance learning became crucial during the SARS-Covid-19 pandemic. We explored a non-traditional distance learning mode that involved the sending of microscale chemistry kits (Ibanez, 2011) to the students’ homes for remote experimentation. This allowed the performance of ∼25 experiments per person in each of the Fall-2020 and Spring-2021 semesters in an elective undergraduate chemistry course. As part of this course, students were requested to design experiments on their own and have the entire group reproduce them in their homes. Published outcoming examples include the fabrication of a fuel cell from solar electrolysis (Cesin-AbouAtme et al., 2021), bismuth plating by galvanic displacement (Aguilar-Charfen et al., 2021), tin electroplating (Herrera-Loya et al., 2022), homemade colloidal silver (Covarrubias-Montero and Ibanez, 2022), and homemade CO₂ capture and conversion (Acuña-Girault et al., 2022). The present experiment, Rayleigh scattering, accomplishes the intersection of physics, environmental science, and electrochemistry.

Rayleigh scattering is the dispersion of light caused by the presence of isotropic particles acting as electric dipoles in a fluid through which it passes (Bender, 1952). The trajectory and polarization state of the incident electromagnetic radiation are modified by its interaction with such particles whose diameters are smaller than the wavelength of incident light. Induced dipole oscillations in Rayleigh scattering occur when particle diameters are up to about one-tenth of the wavelength of the scattered light (Ahn & Whitten, 2005). The dispersion intensity, I depends on several factors and is inversely proportional to the fourth power of the wavelength, λ: I α (1/λ ⁴) (Bender, 1952; Nave, 2016; Plumb, 1971). Because colloidal particle diameters are typically in the range 1–200 nm (Del Mazo-Vivar, 2016), when white light impinges on a colloid the shorter wavelengths (violet and blue) are dispersed more efficiently (i.e., radially scattered), whereas the longer wavelengths (orange and red) pass through the colloidal suspension with little or no dispersion. Such scattering is responsible for the beautiful sunsets and blue skies (Nave, 2016; Plumb, 1971).

Rayleigh scattering educational experiments are relatively scarce. One such experiment is aimed at observing the rotation of polarized light in an optically active medium, and the scattered light that tracks the directional radiation pattern of an oscillating dipole in an optically inactive medium (Avalos-Pecina et al., 1999). Another experiment allows direct determination of wavelength dependence of the Rayleigh scattering cross section (Chakraborti, 2007). A related experiment demonstrates that during fluorescence, the emitted wavelength is independent of the excitation wavelength, whereas the wavelength of Rayleigh scattered light increases with increasing excitation wavelength (Clarke & Oprysa, 2004).

In the present experiment, students reproduced this phenomenon at home with the contents of a kit sent to their homes by our school, together with easily available household items. They observed Rayleigh scattering by elemental sulfur indirectly electrogenerated by anodically-produced protons which acidify a thiosulfate solution. This in turn undergoes disproportionation into S(0) + S(IV). When shining light from a cell phone flashlight, the longer wavelengths pass through the solution, and the shorter wavelengths are radially scattered. The experiment should be useful for general, inorganic, environmental, green, and physical chemistry introductory lab courses, and can be performed either at school or at home. The simplicity of the required equipment and procedures should allow its implementation in high schools as well.

Experimental

Water electrolysis will be utilized to generate the necessary acidic medium for the thiosulfate disproportionation. Thiosulfate is obtained from an aquarium store in the form of a dechlorinator (typically an alkaline thiosulfate solution). In this case, we used Clorkill^® (a 0.12 M NaHCO₃, 0.63 M Na₂S₂O₃ solution). Due to the presence of bicarbonate, the basic medium generated by this salt should be initially neutralized to shorten the time required for the protons from water electrolysis to neutralize its basicity. The acidic medium for the required thiosulfate acidification will be provided by the anodic half-cell in the water electrolysis process.

Preparation and use of the electrochemical cell

Two 10-mL beakers (A and B) will contain the anolyte and catholyte, respectively. Add ∼5 mL of water to beaker A, followed by 10 drops of Clorkill^® and 3–4 drops of phenolphthalein indicator (1% solution). A pink coloration indicates that the solution is basic, mainly because of its NaHCO₃ content. To speed up the experiment, neutralize this basic solution by adding white kitchen vinegar dropwise up to the pink-to-transparent endpoint. Once this point is reached, 1–2 extra drops of vinegar may be added. Then, pour 10–15 mL of water in a 50 mL beaker and add a spatula-tip of table salt (preferably of the non-iodized type to prevent side reactions). Stir to dissolve. Transfer ∼5–6 mL of this solution to beaker B. Put in contact the contents of beakers A and B through a salt bridge made of an ∼8 to 10 cm piece of textile yarn (previously soaked for a couple of minutes inside the 50 mL beaker in the NaCl solution). See Figure 1A.

Figure 1:

Electrolysis set up.

(a) Before connecting the power source. (b) After connecting the power source. (Note some hydrogen bubbling in the cathodic side).

Connect a 6–8 cm long graphite lead (e.g., Steadtler Mars HB, 2 mm in diameter) to an alligator clip and immerse the lead into the solution of beaker A (that contains the Clorkill^®; this will be the anolyte). Do the same with another graphite lead for beaker B (the solution here will be the catholyte). Connect the alligator clips to an AC-to-DC adjustable voltage power adapter (e.g., Steren model ELI-035). Connect the positive end of the power source to the graphite lead in beaker A, and the negative to that in beaker B. Then, connect the adapter (at a nominal voltage setting of 12 V, which gives a true voltage of ∼25 V) to a regular home electricity outlet (i.e., 120 V). See Figure 1B. (A 9-V dry cell unfortunately does not yield the desired results within a reasonable timeframe).

Results and discussion

Protons from vinegar neutralize the excess basicity caused in Clorkill^® by the bicarbonate ions:

Then, the main electrolysis reactions are:

Oxidation:

Reduction:

Global reaction:

Reaction (2) furnishes the protons needed by the thiosulfate ions to disporportionate into S(0) and S(IV) species. The main reactions generally accepted are (Chakraborti, 2007; UMass-Amherst, 2011):

H₂SO₃ represents the equilibrium between SO₂ and H₂O. The Tyndall effect originated by the colloid produced by elemental sulfur can be observed with the aid of a common red laser. See Figure 2.

Figure 2:

Demonstration of the Tyndall effect. (Also note more hydrogen bubbles at the surface of the catholyte).

As elemental sulfur is produced, this and its polysulfide moieties in equilibrium grow and produce a slightly milky colloidal suspension that is ready to initiate Rayleigh’s scattering. When light shines on it, a bluish tone is observed from the wavelengths radially scattered while a yellowish tone is observed passing through. As particles grow larger, the unscattered light becomes more orange-reddish. See Figure 3.

Figure 3:

Rayleigh scattering in a colloidal suspension.

This phenomenon has been dramatically proven elsewhere with a more sophisticated set up by using 407-nm violet and 670-nm red diode lasers to monitor the growth of colloidal sulfur particles and examine reaction kinetics. Preferential scattering of violet light was observed at the beginning of the reaction (i.e., small particle sizes), whereas an intense red-light scattering occurred as particle size increased (Ahn & Whitten, 2005).

At the end of the course we analyzed key learning outcomes through a voluntary anonymous survey that was responded by 92% of the students (i.e., 11 out of 12). The main outcomes are now presented in Table 1 (Ibanez & Contreras-Ruiz, 2021).

Conclusions

Distance learning can be enhanced by performing live experiments at home. Electrolysis can provide basic or acidic media as required, depending on the compartment selected to this end. When an acidic medium is produced in a thiosulfate solution, a chemical disproportionation occurs, and the resulting sulfur colloidal suspension produces an easily observable Rayleigh scattering of white incident light.