Classroom Flame Spectroscopy
A group of students and I put the PLOTS spectroscope to use observing the spectra from solutions of common metal salts that were excited in the flame of a bunsen burner. We calibrated the spectroscope using a flouro in the classroom and identified lines in the spectra of Fe (III), Ca(II), Na(I), Cu(II) and Ba(II).
This was an investigation into whether the spectroscope could be used in an educational setting and how hard it would be to observe and identify elements based on their spectra.
We used a modified bunsen burner to excite the metal salt solutions which were sprayed into a 2 litre soft drink bottle which fed the spray into the venturi of the bunsen burner. The spectroscope was simply held in a retort stand level with the flame. The solutions were approximately 1M concentration.
We noticed that copper was particularly hard to see in the spectroscope and a wider single slit at the front of the spectroscope would help by letting more light enter. We experimented with different sized slits and noticed that while we lost a little wavelength resolution, the results were still visually good. We had to re-calibrate each time we moved the slit however.
The slits were made from aluminium foil that was sandwiched between two microscope slides. Many slits were made so it was a simple matter of moving the slide and sticking it down over the main aperture into the spectroscope.
The spectroscope was our first attempt and used a cheap 8MP webcam, 3cm autofocus and cost $5. We used DVD plastic and a spare box covered in contact adhesive. The fittings are only held in place using blu-tak, including the DVD over the lens. We only worked in the visible spectrum.
We made a video of our efforts.
I would imagine extending this activity confidently for a middle school class. I would expect to be able to provide the students with the laptops and spectroscopes and have them calibrate the spectroscopes using the simple CFL calibration that is available in spectral workbench (offline). I would then have the students record the identified lines in the spectra of some know solutions. I would then ask the students to identify an unknown that contained up to two salt solutions that they had already identified.
For senior students, I would envisage using the NIST database http://physics.nist.gov/PhysRefData/ASD/lines_form.html to identify unknown solutions.
We noticed that using too greater amount of a solution contaminated the apparatus and the residue took a long time to disappear.
We put the spectroscope quite close to the flame and did on one occasion melt it slightly.
The experiment can be done in a lit classroom, but it is better with the lights off. The spectroscope consistently found stray reflections from the flouro lights. Ideally a black backdrop would be preferable behind the bunsen.
A simple capture button would enable the students to save their spectra for printing in a report. A colour printer would be excellent.
An API being proposed is to have sample spectra displayed on the analysis screen, at the same scale as that being captured by the spectroscope, to give immediate feedback and provide qualitative analysis of the spectra being observed.
My students enjoyed participating in the exercise. It would take some practice and good classroom management to do it with a large group of students. Compared to merely "burning" the chemicals in a bunsen burner and if you are lucky, using a handheld ($60) optical spectroscope to see some lines, this apparatus gives quantitative data that is equal to many classroom spectrometers costing thousands of dollars.
We also observed some discharge tubes and got some marvelous looking spectra from Ne, Hg, Cd and Na. I would imagine setting up an exercise where the students observed the spectra from a sodium lamp and then compared it with an "unknown" sample of common salt captured using the flame rig on the spectroscope.