Elucidating atomic emission and molecular absorption spectra using a basic CD spectrometer: a pedagogical approach for secondary-level students

2.1 Student participants

The activities were conducted at Nossa Senhora do Carmo School (CNSC). This school is situated in Nova Cruz, a town within the wild region of Rio Grande do Norte state. A cohort of 28 students (12 boys and 16 girls), in their first year of secondary education, participated. Their ages ranged from 14 to 16 years.

2.2 Materials

Sodium chloride (>99.5 %), copper (II) sulphate pentahydrate (>98 %), and potassium permanganate (>99 %) were used. All the standard solutions and samples were prepared using distilled water.

Constructing the CD spectrometer necessitated specific materials, namely a shoebox, a recordable CD, masking tape, and two razor blades. In the absorption experiment, a plastic cuvette was employed as the sample holder, while the emission experiment utilized a spray bottle as the injector and a Bunsen burner.

2.3 Procedure

The complete construction process of the CD spectrometer is detailed in the Experimental Procedure section of the Supplementary material (SM). Constructing a CD spectrometer is quite straightforward, and students are able to do so in approximately 1 h. As students formed groups of four people to carry out the experiment, including the production of homemade spectrometers. An example of these devices is shown in Figure 1.

Figure 1:

A shoebox transformed by the students into a CD spectrometer.

The external measurements of the shoebox were not the same for all groups, varying according to the materials each group brought from home. Only the measurements of the location where the cuvette is placed should be fixed, as the cuvette has a size of 1 cm × 1 cm. In front of the shoebox shown in the figure, the CD diffraction grating (3 cm × 4 cm) functions as a simple diffraction grating, and the optical entrance slit is located at the back of the box.

The students followed safety protocols by wearing protective gear while handling chemicals, and were advised to exercise caution when using sharp tools. To ensure their safety, they maintained a secure distance from the Bunsen burner during the flame test. As for the other students, it is important for them to maintain a minimum distance of 1.5 m from the flame. Additionally, the students directly involved in conducting the experiment were instructed to maintain a distance no closer than 0.4 m from the flame.

In the case of atomic emission spectroscopy, the students prepared two working solutions, namely, a 0.1 mol dm⁻³ sodium chloride solution and a 0.5 mol dm⁻³ copper (II) sulphate pentahydrate solution, both in distilled water. The experimental setup, as depicted in Figure 2, was arranged on the laboratory bench. To ensure reproducibility, the distance between the flame and the mobile phone was fixed at approximately 0.4 m.

Figure 2:

Experimental setup for atomic emission spectroscopy. The distance between the camera and the sample was fixed at 0.4 m.

To accomplish this, a two-person procedure was conducted: one person sprayed the sample while the other captured the image, with the images being acquired using a 12-megapixel digital camera integrated into a Samsung Galaxy S21 FE smartphone. First, students captured the spectrum of a fluorescent lamp to determine the location at which the spectrum will appear in the image (Figure 3). Following this, they utilized the open-source software Gimp (http://www.gimp.org/downloads/) to crop the spectra and were instructed to annotate the position and size of the crop for use in cropping other spectra.

Figure 3:

Region of the image obtained by the CD spectrometer where the spectrum appears.

The visible spectrum covers approximately the region between 400 and 700 nm, which is between blue (400 nm) and red (700 nm). Therefore, it was important for the students to note the position and size of the image containing the fluorescent lamp’s spectrum, as these data were used to plot the spectra. The images should be with blue on the left side and red on the right side. If they are the opposite, the student should rotate the image 180°.

Subsequently, the students proceeded by spraying distilled water into the flame Bunsen burner and recorded a video (Blank) of the experiment using the mobile phone. This process was repeated twice for each trial. The same procedure was then carried out using the prepared working solutions.

The molecular absorption spectra were acquired using solutions of copper (II) sulphate pentahydrate and potassium permanganate at concentrations of 0.5 mol dm⁻³ and 0.004 mol dm⁻³, respectively. The students prepared the solutions and transferred them into plastic cuvettes. Subsequently, two students conducted the experiment: one student used the flashlight on their mobile phone as a light source positioned on one side of the CD spectrometer, while the other student captured the image using another mobile phone. The experimental setup, including the arrangement of the components, can be observed in Figure 4.

Figure 4:

Experimental setup for molecular absorption spectroscopy. The distance between the flashlight and the spectrometer was set at 0.1 m.

The procedure commenced by capturing duplicate photographs of the distilled water cuvette. Subsequently, the same steps were repeated with the working solutions. Finally, the students identified the region in the spectrum displaying the most intense signal and cropped it. The selected region was then saved in JPEG format, with the calibration image serving as a reference.

2.4 Data treatment

Only through the images can the distinctions between atomic and molecular spectra be effectively illustrated to students. Should the teacher consider incorporating programming skills, a script based on the open-source software R, provided by the R Foundation for Statistical Computing, is accessible in the Supplementary material (SM) to plot the spectra in the “Program Script in R” section. The R Foundation for Statistical Computing software can be acquired from http://www.r-project.org/ (R Core Team, 2023).

The algorithm presented in the supplementary material (SM) script was divided into two parts. The first part was used to obtain the emission spectra. In this case, the images were converted to the RGB (Red-Green-Blue) channels, and then the maximum values of these three channels were summed and called Intensity. The intensity of the spectrum was obtained by subtracting the metal intensity from the blank intensity, according to equation (1).

The second part of the script was used to obtain the absorption spectra. In this case, the Beer–Lambert law was applied in equation (2). The Beer–Lambert law relates the attenuation of light to the properties of the material through which the light is traveling and the concentration of the material.

The implementation of the algorithm provides students with a unique opportunity to observe and comprehend the underlying equations governing atomic emission and molecular absorption spectra. By engaging with algorithmically generated data, students can seamlessly correlate the outcomes of the practical experiment with theoretical principles. This approach not only enhances their understanding of spectroscopy but also facilitates a deeper connection between the empirical observations and the fundamental concepts articulated in the curriculum.

3.1 Atomic emission spectra

Figure 5 displays the discernible spectral lines corresponding to sodium and copper, illustrating the emission of light within a narrow frequency range. Due to the brief lifetime of excited states and the discrete injection of the sample via spraying, the students captured videos during this phase of the experiment. Subsequently, they selected the frame showcasing the most prominent lines, i.e., the frame exhibiting the highest signal intensity and proceeded to crop the image accordingly. To find the spectral region, the students also recorded a video capturing the emitted bands from a fluorescent lamp (Figure 3), maintaining a consistent distance of 0.4 m throughout the process. This additional video served as a reference to ensure accurate calibration within the spectral domain.

Figure 5:

Atomic emission spectra for sodium (A) and copper (B) obtained from images captured by students (left).

As is widely recognized, the flame colors observed in chemical reactions are a direct consequence of electronic transitions occurring within the energy levels of atoms (Barrow & Caldin, 1949). Specifically, the vibrant orange-yellow hue of sodium flames arises from the reversion of excited electrons from the [Ne]3p¹ level to the [Ne]3s¹ level. The emission line associated with sodium atoms has been measured at a wavelength of 589.6 nm, refer to Figure 5A (Juncar et al., 1981). Conversely, the emission spectrum of copper, as depicted in Figure 5B, exhibited the detection of the 578.2 nm line, while the lines at 510.5 nm, 515.3 nm, and 521.8 nm manifested as a band (Zhao & Horlick, 2006). In the case of copper, the most intense line stems from a transition between the [Ar]3d¹⁰4p¹ level and the [Ar]3d⁹4s² level. Conversely, the band emerges from transitions originating from the [Ar]3d¹⁰4d¹ level to the [Ar]3d¹⁰4p¹ level (Shenstone, 1948). Consequently, the distinct blue-green color observed in copper flames can be attributed to the multiple electronic transitions occurring within the visible range of the electromagnetic spectrum.

3.2 Molecular absorption spectra

Lastly, the students conducted molecular absorption spectroscopy as part of their experimentation. In this particular experiment, two mobile phones were employed: one served as the light source, utilizing the flashlight feature positioned at a distance of 0.1 m from the CD spectrometer, while the other phone was designated for capturing spectral photos. The experimental configuration is presented in Figure 4. As before, a fluorescent lamp was utilized for calibration purposes to ensure the depiction of the spectra. Figure 6 exhibits the resulting spectra obtained from aqueous solutions of copper (II) sulphate pentahydrate and potassium permanganate.

Figure 6:

Molecular absorption spectra for copper (II) sulphate pentahydrate (A) and potassium permanganate (B) aqueous solutions obtained from images captured by students (left).

In an aqueous solution of copper sulphate, the Cu²⁺ ions undergo coordination with water molecules, resulting in the formation of the octahedral complex [Cu(H₂O)₆]²⁺. The oxidation state of copper in this hexaaqua complex is +2, indicating an electronic configuration of [Ar]3d⁹. The characteristic blue color exhibited by the [Cu(H₂O)₆]²⁺(aq) species can be elucidated through the application of crystal field theory (CFT). According to CFT, the presence of water molecules causes a splitting of the d-orbitals into two distinct groups with differing energy levels (refer to Figure 7). This energy gap between the d-orbitals corresponds to the energy of photons within the visible light spectrum. Consequently, when white light traverses a solution containing [Cu(H₂O)₆]²⁺ ions, photons of red light are absorbed, resulting in a blue-colored solution (complementary color). This absorption process, commonly referred to as d-d transition, gives rise to the spectrum depicted in Figure 6A.

Figure 7:

The origin of the colors in [Cu(H₂O)₆]²⁺ and [MnO₄]^– complexes. When a complex is exposed to the light of the proper energy (hv), an electron can be excited to a higher energy orbital through the process of light absorption.

Potassium permanganate aqueous solution has an intense purple color. [MnO₄]^– is a complex ion that contains an Mn(VII) ion at its center. Then, as the Mn is in its highest oxidation state, the electronic configuration of Mn is [Ar]3d⁰4s⁰. Since there are no d electrons, the solution colour of the [MnO₄]^– ion cannot be explained by d-d transitions. Instead, the electronic transition involved is from an orbital of the oxygen atom to empty d orbitals of the Mn⁷⁺ ion (see Figure 7). This type of electronic transition is known as the charge transfer process. Then, the observed spectrum in Figure 6B is not due to d-d electronic transitions, but to ligand-to-metal charge transfer (LMCT). In general, the resulting d–d transitions for transition metal ions are relatively weak as compared to the charge transfer transitions because of the Laporte selection rule. Therefore, the copper sulphate solution concentration in the experiments was 125 times higher than the potassium permanganate solution to overcome the weaker d-d transition associated with copper (II) complex absorbance. Similar spectra of these two solutions can be observed in the textbook Principles of Instrumental Analysis (Skoog et al., 2017).

3.3 Learning outcomes

Following the completion of the activity, the students engaged in a thorough discussion regarding their observations and attempted to address the questions presented to them in a questionnaire prepared by the chemistry teacher (SM). The initial query aimed to elucidate why atoms do not generate a spectrum of bands. Consequently, several groups reasoned that electrons within atoms can solely undergo electronic transitions, thereby resulting in spectral lines, as per the Bohr atomic model studied in class. Other groups mentioned that band spectra are exclusive to molecules due to the diverse energy levels associated with chemical bonds. The subsequent questions probed why atoms selectively absorb and emit specific colors rather than all wavelengths. In response, the students provided similar explanations, emphasizing that atoms possess distinct electronic structures, and consequently, the energy required for electron-level changes is likewise specific and limited to particular wavelengths. Another question explored why molecules yield a spectrum of bands instead of individual lines. Eighty-six percent of the students (24) effectively differentiated between an atomic spectrum and a molecular spectrum. Through their experimental endeavors, the students significantly enhanced their comprehension of these phenomena, with a majority of groups recognizing that the variation in energy within the chemical bonds of molecules directly influences the energy of electronic transitions, thereby resulting in the production of a range of band spectra rather than discrete lines.

The questionnaire served as a means to assess the students’ perspectives regarding the incorporation of cell phones in fostering scientific comprehension, as well as their perception of the enhanced engagement resulting from the integration of these devices. Ninety-three percent of the students (26) expressed that integrating mobile devices to enhance scientific understanding successfully captured their attention, leading to increased levels of engagement. Overall, the students concurred that utilizing cell phones heightened their interest in the lessons and facilitated the sharing of their experiences with individuals beyond the confines of the classroom.

The general consensus among the students was that the experiment was quite intriguing and could enhance their understanding of spectroscopy, with 96 % of the class (27 students) identifying it as an excellent activity for introducing this subject. However, despite the positive perception of the activity, only 75 % were able to provide consistent explanations regarding electronic, vibrational, and rotational transitions.

Additionally, the educator implemented Novak’s research by incorporating mind maps into the instructional approach (Bretz, 2001). Students from Nossa Senhora do Carmo School were tasked with constructing mind maps to ascertain the impact of these tools on meaningful learning. An illustrative mind map, translated from the student’s native language to English, is available in the Supplementary material section. Upon analyzing the mind maps created by students, in summary, the experiment outlined in the mind map achieved valuable learning outcomes. By constructing a spectrometer from recyclable materials, students gained hands-on experience in spectroscopic techniques and developed an understanding of atom and molecule behavior. Cell phone cameras as detectors provided practicality and accessibility, while the inclusion of flame tests and the study of molecular absorption expanded students’ knowledge in these domains. The experiment’s results and positive student perception demonstrate its efficacy in promoting student engagement and cultivating interest in scientific exploration.