Emission Spectroscopy

Introduction

Excite selected metallic ions in a flame test and observe their characteristic emission spectra. Then identify these metallic ions by the distinctive colors they emit.

Concepts

• Absorption/emission
• Flame tests
• Diffraction grating
• Emission spectra

Materials

Safety Precautions

Barium chloride is severely toxic by ingestion. Hydrochloric acid is highly toxic by ingestion or inhalation; severely corrosive to skin and eyes. Zinc metal dust could be flammable; dust may be present at the bottom of your bottle of mossy zinc. The metal can contains sharp edges; handle with care. The metal can will become hot from the burner flame; allow it to cool before handling. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Label the side of each Petri dish bottom with one of the metallic ions used. (Ba²⁺, Ca²⁺, Li⁺, K⁺, Na⁺, Sr²⁺)
2. Place approximately 3 grams of each metallic chloride into the appropriate Petri dish bottom.
3. Add 25 mL of 1 M hydrochloric acid to each of the Petri dish bottoms.
4. Using a clean stirring rod, carefully stir each solution until most of the solid has dissolved. Do not use the same stirring rod in each solution unless it is cleaned between each use.
5. Place a cover on each of the Petri dishes. (Only the first Petri dish cover should have a hole cut into it.)
6. Through the hole in the Petri dish cover, add 3–4 pieces of mossy zinc to the first of the Petri dishes. Observe the reaction between the zinc and hydrochloric acid.
7. Place a Bunsen burner on top of the Petri dish (see Figure 1). Make sure the air intake valve is open.
   {13356_Procedure_Figure_1}
8. Place the specially cut metal can over the burner. The tip of the burner should stick out of the hole in the top of the can (see Figure 2).
   {13356_Procedure_Figure_2}
9. Attach the Bunsen burner hose to the gas jet.
10. Light the burner and adjust the flame so that the burner is operating efficiently.
11. Turn off the lights and observe the flame color.
12. Observe the emission spectrum by looking at the flame through a Flinn C-Spectra^®. Observations may also be made by viewing the flame through a piece of cobalt glass.
13. Repeat steps 5–12 with the other metallic salts in the Petri dishes. Note: Before proceeding to step 6, replace the Petri dish cover with the cover having a hole in it.
14. Try showing your students “unknown” salt solutions and have them identify the metallic ion from its characteristic color.

Student Worksheet PDF

13356_Student1.pdf

Lab Hints

• Difficulty in color identification is common. Calcium, lithium and strontium are the most difficult. Calcium will be the most orange and lithium will be the most red—strontium is red-orange and the color is in between the other two colors.
• The yellow color of sodium will always be a dominant color and will tend to obscure your results if you are not careful. A good example is to aspirate distilled water into the flame. No color should be evident from pure distilled water. Next rinse your hands in distilled water and aspirate some of that water into the flame. The salt present in the perspiration on your hands should be enough to show the characteristic yellow color of sodium ions.
• Hold the cobalt glass close to your eye during observation. Cobalt glass will absorb the yellow color of sodium but will not absorb the violet color of potassium. This gives the opportunity to clearly see the color for potassium ions. This may provide the opportunity to talk about light filters. Cobalt glass is only good for absorbing the yellow color of the sodium spectrum—it is not suitable as a filter for bright light or sunlight.

Teacher Tips

• Metallic ion identification is done by many means (e.g., spectroscopy, chromatography, precipitates formed after reaction with acids, gas evolution). Flame tests are simply one identification tool and many factors may influence their usefulness.
• Metallic ions will always generate the same color upon excitation but the color intensity will vary with the strength of the solution. Weak solution—less intense color; strong solution—more intense color.
• It is difficult to prevent the metal can from rusting due to its exposure to hydrochloric acid fumes.
• If two metals are present, the one in greater concentration will obscure the color of the one in lesser concentration. Or the strength of one color may obscure another. If, for example, a mixture of lithium chloride and potassium chloride is used, the violet flame of potassium may be obscured by the scarlet-crimson color of lithium.

Answers to Questions

1. Name the metal ion present in each Petri dish and the flame color it produced.

   Petri Dish 1—The ion present was sodium, Na⁺. It produced a yellow flame.
   Petri Dish 2—The ion present was strontium, Sr²⁺. It produced a red-orange flame.
   Petri Dish 3—The ion present was copper, Cu²⁺. It produced a green flame.
   Petri Dish 4—The ion present was lithium, Li⁺. It produced a bright red flame.
   Petri Dish 5—The ion present was potassium, K⁺. It produced a violet flame.

2. When an element is placed in a burning solution, the element’s electrons absorb energy and move to an “excited” energy state, different from its normal ground state. Explain how this creates colored light.

   An excited electron must eventually return to its ground state. When it does so, it emits a form of energy, light.

3. What is the name for the spectrum of specific wavelengths produced by exciting an element?

   The spectrum of wavelengths for each excited element is called a line spectrum.

4. Why is this spectrum different for every element?

   The spectrum differs from element to element because each element has a particular group of electrons and energy levels. Therefore, the wavelengths of light are different because the energy states the element has electrons falling from and to are different.

Discussion

If an element can be placed in solution and that solution aspirated into (sucked into) a burning flame, the element’s electrons will absorb energy. This process is sometimes called “exciting” the electrons. As the excited electrons return to their normal or “ground” state, energy is emitted in the form of electromagnetic radiation. Simply stated—if an electron is excited via heat, that excited electron will emit light as it returns to its ground (non-excited) state (see Figure 3).

Every element emits a characteristic light. Just as a fingerprint is unique to each person, the color of light emitted after excitation of an element is unique to that element. Only a few elements give off a characteristic light in the visible region of the spectrum. The visible region of the spectrum is that which is visible to the human eye (400–700 nm). For most elements, the characteristic color is detectable only in the ultraviolet or infrared region of the spectrum.

The emission of a characteristic color (electromagnetic radiation) as the excited electron returns to its ground state has provided remarkable tools to the analyst in the form of analytical instrumentation (e.g., emission spectrograph, quantometer, flame spectrophotometer). This activity roughly replicates the process used in these very sophisticated instruments. The very specialized instruments enable the analyst to detect:

• What is present (Qualitative Analysis)
• How much is present (Quantitative Analysis)

Barium, calcium, lithium, potassium, sodium and strontium are examples of elements that display a characteristic emission in the visible region of the spectrum. These metallic ions can be identified via the flame test.

The most common flame test method for these elements is to use a platinum wire loop. The loop is scrupulously cleaned by immersion in acid. It is then held in the flame of the burner. If the loop is clean, no elemental color will appear in the flame. Next the loop is placed in a sample of a reagent such as strontium chloride. A few crystals of the reagent will cling to the loop. The loop is again placed in the burner flame and the characteristic color of excited strontium (bright red-orange) will appear. Any other contaminants present will influence the results. 

Even better results can be obtained by this new inexpensive method. A burner is placed on top of a Petri dish which has a hole cut into its top. The Petri dish contains a metal ion dissolved in hydrochloric acid and zinc metal. The zinc metal reacts with the acid to evolve hydrogen gas (Equation 1). As the gas escapes, it carries with it some of the metallic ions. The gas and ions are drawn into the air ports of the burner, and the flame causes the excitation of the electrons in the ions. As the electrons return to their ground state, the flame becomes the color of the element’s characteristic light. The color will last several minutes which allows for viewing the flame through the Flinn C-Spectra^®, spectroscope or cobalt glass.

The specially cut metal can is placed over the Bunsen burner to direct the hydrogen gas up through the burner. The can prevents the interference of air drafts.

References

Special thanks to George McKelvy, chemistry teacher, Woodward Academy, Atlanta, GA, who provided Flinn Scientific with the instructions for this activity.