Emission Spectroscopy

In this experiment we’ll be observing the emission spectra of atoms. We’ll begin by looking at the emission spectra produced by gas discharge tubes to determine the precision of the spectrometer. These tubes contain a small amount of pure gas (hydrogen or helium), which is excited by passing high energy electrons through the length of the tube using a high voltage power supply. The excited gas atoms emit photons as their electrons return to the ground state. The spectrometer measures the wavelength of the emitted photons, which can be related to the energy difference between the ground and excited states of the gaseous atoms. You will compare the wavelengths observed from the helium discharge tube with literature values to determine the precision of the spectrometer.

For hydrogen, the wavelengths of the emission spectrum can be determined by the Rydberg equation:

1/l = R(n₁^-2- n₂^-2) (eq. 1)

where l is the wavelength of the observed transition, R is the Rydberg constant (1.096776x10^-7 m^-1), and n₁ and n₂ are integers representing the quantum levels and n₂> n₁. The wavelengths of hydrogen emission you observe from the spectrometer can be compared to those predicted by the Rydberg equation, to determine the accuracy of the theory.

Next we’ll be looking at the emission spectra of some simple metal cations. Salt solutions of the metal cations will be heated in a burner flame, producing hot, gaseous ions. The high temperature of the flame causes electrons of the ions to be excited to higher energy states. As these excited ions rise out of the flame and cool, the excited electrons return to the ground state, emitting a photon in the process. You will measure the characteristic wavelengths of several known metal solutions, then use these values to identify the metals in an unknown sample.

You can also use a ‘pocket spectroscope’ (essentially just a diffraction grating) to actually visualize the emitted lines, just like our good friend Oliver Sachs. The grating disperses the incoming light into its component wavelengths, which can be resolved as lines because atoms emit light of characteristic discrete energies based on the energy difference between the ground and excited state of the atom.

Materials:

Hydrogen and helium discharge tubes and voltage source
Ocean Optics spectrometer with fiber-optic cable
Hand-held spectroscope
Nicrome wire
Bunsen burner
Watch glass
LiCl, NaCl, KCl, MgCl₂, CaCl_2,SrCl₂ 0.5M solutions
Unknown salt solutions, A and B

Procedure:

Emission of hydrogen and helium: Make sure the Ocean Optics spectrometer (OOS) is connected to LoggerPro and is in Emission mode. Turn on the high voltage source to light the helium discharge tube. It may require slight adjustment until the tubes are glowing steadily. Align the OOS optical sensor until the lines on the monitor are at their maximum height. Record the wavelengths and signal intensities of each line. Look at the tube using your pocket spectroscope and record what you see. Do the same for the hydrogen discharge tube.

Emission of metal cations: Set up a Bunsen burner on a ring stand with the OOS sensor at least an inch from the flame. Place a small amount (a few drops) of salt solution on the watch glass, dip your wire into it and put it in the burner flame. Adjust the position of the OOS sensor until the lines are at maximum height (this may take a couple tries). Once you think you’ve found the optimal position, take a snapshot of the spectrum and record the wavelength and intensities of each line. Also look at the flame with your spectroscope and record your observations (you may want to do this before using the probe to get a rough idea of the line positions). Repeat the procedure with all of the known and unknown salts (remember to record the unknown number!). Make sure you use the wire designated for each solution! Residue from contaminated wires will produce false lines in your spectrum - it doesn’t take much! This is also true for dirty watch glasses, so rinse well after each solution.

Data workup and report considerations:

Use your helium measurements to determine the accuracy of the spectrometer. The commonly accepted values for transitions in the visible range are in the table below.

For each line, find the deviation of your experimental value from the literature one:

Deviation = ½experimental value - theoretical value½

For each deviation, find the percent relative deviation:

Compare your measured hydrogen emission lines to those predicted by Rydberg and Balmer (use eq.1 where n₁=2). Find the deviations and percent relative deviations of these values and comment on the accuracy of the theory based on your results.

Use your observations for the known metal solutions to determine which cations are in the unknown solutions. Describe your reasoning in detail, using any data and observations to support your conclusions, including what you know about the accuracy of the spectrometer.

Known helium emission lines in the visible range

Wavelength (nm)	Line color	Relative intensity	Wavelength (nm)	Line color	Relative intensity
728.1	Red	3	492.2	green	5
706.5	Red	7	471.3	blue	4
667.8	Red	10	447.1	violet	10
587.6	Yellow	100	438.8	violet	3
501.6	Green	10	402.6	violet	7

Desert course (optional): Change the setting on the sensor to Absorption mode. Collect the absorption spectra of the metal ions in solution. What is the relationship between the absorption and emission spectra?