Metal Flame Emission

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Metal Flame Emission TestingExpand

   In this lab, you will excite the electrons of six metal chloride salts with heat from a flame and measure the wavelength of light emitted by the relaxing electrons. The emitted light is in the visible spectrum, so you will see different colored flames for the different metals.

   At the end of the lab, you will measure the emission spectrum of a sparkler and use the data you collected from the metal salts to identify the metals in the sparkler. To help with this process, make a table in your lab notebook to record the wavelengths emitted from ambient light, the salts, and the sparkler.

   Table 1: Observed Wavelengths

   Run #	Source	Recorded wavelength(s)
   1	Room lights
   2	NaCl
   3	LiCl
   4	KCl
   5	SrCl2
   6	BaCl2
   7	CuCl2
   8	Sparkler

   Click Here to download Table 1

   During this lab, you will hold a salt-coated applicator at the edge of a Bunsen burner flame to excite the metal salts. If the applicator catches fire, immediately quench it in a beaker of water. For excitation, you will hold a detector near the flame and the sparkler to measure the wavelengths of the emitted light. Keep a safe distance to protect yourself and the detector. As some tasks are performed simultaneously, we suggest that you work in groups of four with each student completing tasks as needed.

   • Put on a lab coat, safety glasses, and nitrile gloves.
   • Set up the Bunsen burner in the fume hood.
   • Fill a 400-mL beaker with tap water and bring it to the hood. Place the salts, applicators, and tap water within easy reach on one side of the Bunsen burner.
   • Turn on the hand-held spectrophotometer and familiarize yourself with its components.
   • Create a new experiment and configure the spectrometer to measure emission intensity. Set the sample time to 200 ms and the number of readings at a given wavelength to 1. Record the settings and the experiment name in your lab notebooks.
   • Measure the spectrum of the room lights. Have one or two students hold the optical detector as close to the lights as possible without disconnecting the cables. Once ready, have a different student start the spectrometer. The spectrum should be clear, with minimal noise and no off-scale peaks. Note: If you have trouble, hold the probe in a nearly closed drawer or box to shield it from sunlight while pointing it at the room lights.
   • Immediately capture the spectrum when it looks good. Confirm that you have a high-quality spectrum before saving it. Record the wavelengths of the peaks in your data table.
   • Measure the emission spectra of the metal salts. Ignite the Bunsen burner and adjust the collar until the flame distinctly separates into an inner and outer flame.
   • Bring the spectrometer to the hood and arrange the cables so that the detector can be held about one inch from the flame.
   • Have one student prepare the spectrometer for the run. This student should have a clear view of the burner so they can begin acquiring data as soon as the salt enters the flame. Then, have another student hold the detector no closer than one inch from the flame. Note: Any closer risks damaging the detector. But you won't get a clear spectrum if the detector is too far away.
   • Have a different student take one of the applicators from the beaker of water and dip it into the tube of NaCl to coat the wet cotton with the salt. Then, carefully insert the salt-coated applicator about 1 mm into the outer flame, which will change color.
   • Start the spectrometer when the applicator enters the flame. Note: If the peak intensities are off-scale, adjust the detector position until all peaks are within detection limits. If the flame color change is hard to see, pick up more salt on the same applicator and try again until you see an intense color. If the spectrum is noisy or weak, try pointing the detector at a different part of the flame.
   • When you see a good spectrum, capture and save it. Everyone should review the spectrum to ensure that it is of good quality. Write down the emission wavelengths in your table.
   • Submerge the NaCl applicator in your wastewater beaker. Always use a different applicator for each salt.
   • Follow the same procedure to test the remaining five salts, being careful to avoid cross-contamination between the salts. Once you have tested the last salt and recorded the wavelengths, set up data collection for the sparkler.
   • At the instructor's hood, light the sparkler with the Bunsen burner and place it handle down in the Erlenmeyer flask. Remember, the sparks may be very hot. Turn off the gas to extinguish the Bunsen burner.
   • As one student holds the detector close to the sparkler, have another student start data collection. Position the detector to get as many peaks in scale as possible without losing the less intense peaks.
   • Capture and save the spectrum once it looks good. Remember to write down the wavelengths in your table. Note: If the sparkler is long-lived, collect a second spectrum and either choose the best one or keep them both for the analysis.
   • Let the sparkler burn out in the hood while you send the collected data to everyone in the group and shut down the data acquisition system.
   • Bring the copper, barium, and strontium tubes to the waste hood and thoroughly rinse the tubes with deionized water into the designated waste container.
   • Flush the sodium, potassium, and lithium salts down the drain and rinse the remaining test tubes. Put away the test tubes and the other equipment.
   • Once the ashes and sparkler handle have cooled, dump them in the trash.
2. ResultsExpand
   • First, plot the data for each run with intensity on the y-axis and wavelength on the x-axis.
   • Compare the sparkler spectrum to the background spectrum from the room lights. Identify which small peaks may be from the room lights.
   • Compare the metal spectra to the sparkler spectrum. Sodium appears to be the source of the peak at about 589 nm. There are no peaks matching the lithium spectrum, but potassium matches the large peak at 770 nm.
   • The sparkler spectrum doesn't seem to have strontium peaks. However, barium is a good match for several peaks in the sparkler spectrum. Lastly, no copper seems to be present in the sparkler. Thus, we conclude that the sparkler was burning sodium, potassium, and barium when the spectrum was acquired.