Introduction to Emission Spectra

Emission spectra are a fundamental tool in analytical chemistry that allows identification of chemical elements based on the light they emit when their atoms are excited. This phenomenon is based on the principles of quantum mechanics and atomic structure.

When atoms absorb energy (thermal, electrical, or luminous), their electrons can jump to higher energy levels. When returning to their ground state, they emit photons with specific energies that correspond to determined colors of the electromagnetic spectrum.

Theoretical Foundations

Atomic Structure and Energy Levels

Electrons in atoms occupy quantized energy levels. These levels are unique for each element, like an atomic "fingerprint." When an electron absorbs energy, it can jump to a higher level (excited state), but this state is unstable.

When returning to the ground state, the electron emits a photon whose energy is determined by Planck's equation:

Where:

• E = photon energy
• h = Planck's constant (6.626 × 10⁻³⁴ J·s)
• f = radiation frequency
• c = speed of light (3 × 10⁸ m/s)
• λ = wavelength

Relationship between Energy and Color

The photon energy determines its wavelength and therefore its color:

• Violet: 380-450 nm (higher energy)
• Blue: 450-495 nm
• Green: 495-570 nm
• Yellow: 570-590 nm
• Orange: 590-620 nm
• Red: 620-750 nm (lower energy)

Flame Tests

Technique Principle

The flame test is the simplest method to observe emission spectra. It consists of introducing a small amount of sample into a flame and observing the color produced. The thermal energy of the flame excites electrons in metallic atoms, producing characteristic light emission.

Characteristic Colors of Common Elements

• Lithium (Li): Intense crimson red
• Sodium (Na): Very intense yellow-orange
• Potassium (K): Violet (observable with cobalt blue glass)
• Rubidium (Rb): Violet-red
• Cesium (Cs): Blue-violet

• Calcium (Ca): Orange-red
• Strontium (Sr): Intense red
• Barium (Ba): Intense yellow-green

• Copper (Cu): Blue-green
• Boron (B): Intense green
• Thallium (Tl): Bright green

Experimental Procedure

1. Wire preparation: Clean the platinum wire with dilute hydrochloric acid
2. Verification: Introduce the clean wire into the flame to verify it produces no color
3. Sample application: Moisten the wire with the sample (solution or solid)
4. Observation: Introduce the wire into the oxidizing zone of the Bunsen burner flame
5. Recording: Note the observed color and its intensity
6. Cleaning: Clean the wire before the next sample

Factors Affecting Observations

• Sodium: Its presence in traces can mask other colors due to its intensity
• Cross-contamination: Inadequate wire cleaning
• Concentration: Too high can produce mixed colors

• Temperature: Must be sufficient for excitation
• Observation zone: The oxidizing zone is most suitable
• Exposure time: Some elements require more time

Advanced Emission Spectroscopy

Spectrometers

Spectrometers allow more precise analysis than simple visual observation:

• Line separation: Can distinguish very close spectral lines
• Quantitative measurement: Intensity proportional to concentration
• Invisible line detection: Ultraviolet and near infrared
• Simultaneous analysis: Multiple elements at once

Important Spectral Lines

• D1: 589.6 nm
• D2: 589.0 nm
• These lines are so intense they're used as references

• Hα (red): 656.3 nm
• Hβ (blue-green): 486.1 nm
• Hγ (violet): 434.0 nm
• Hδ (violet): 410.2 nm

• Green: 546.1 nm
• Blue: 435.8 nm
• Violet: 404.7 nm

Practical Applications

Qualitative Analysis

Emission spectroscopy allows:

• Elemental identification: Determine which elements are present
• Purity control: Detect metallic impurities
• Alloy analysis: Metal composition
• Mineral identification: Elements present in rocks

Quantitative Analysis

With appropriate equipment:

• Element concentration: Intensity proportional to concentration
• Trace analysis: Detect elements in very low concentration
• Quality control: In metallurgical and pharmaceutical industry

Astronomical Applications

Stellar spectroscopy allows:

• Stellar composition: Elements present in stars
• Stellar temperature: From the continuous spectrum
• Radial velocity: Doppler effect in spectral lines
• Magnetic fields: Zeeman effect

Environmental Applications

• Water analysis: Heavy metals and contaminants
• Emission control: Metals in combustion gases
• Soil analysis: Mineral content
• Forensic: Element identification in evidence

Fireworks: Chemistry of Color

Fireworks are a spectacular application of emission spectra:

• Red: Strontium (Sr) or lithium (Li) salts
• Green: Barium (Ba) or boron (B) salts
• Blue: Copper (Cu) salts
• Yellow: Sodium (Na) salts
• Orange: Calcium (Ca) salts
• Violet: Potassium (K) salts
• White: Magnesium (Mg) or aluminum (Al)

Color Mixtures

Pyrotechnicians can create intermediate colors by mixing salts of different elements, but must avoid chemical interferences that could extinguish desired colors.

Technique Limitations

Flame Test Limitations

• Only metallic elements: Mainly alkali and alkaline earth
• Limited temperature: Not all elements are sufficiently excited
• Interferences: Sodium can mask other elements
• Subjectivity: Visual observation can vary between people

General Limitations

• Sample matrix: Other components can interfere
• Oxidation states: Different states can give different spectra
• Concentration effects: Very high or very low concentration affects observation
• Environmental conditions: Humidity and temperature can influence

Comparison with Other Techniques

Absorption Spectroscopy

• Principle: Measures absorbed light, not emitted
• Advantage: Better for quantitative analysis
• Application: Elements in solution

Mass Spectrometry

• Principle: Separation by mass/charge ratio
• Advantage: Identifies isotopes
• Limitation: Requires sample ionization

X-ray Diffraction

• Principle: Interaction with crystalline structure
• Advantage: Identifies compounds, not just elements
• Application: Crystalline phase analysis

Historical Importance and Development

Historical Discoveries

• Kirchhoff and Bunsen (1859): Development of spectroscopy
• Element discovery: Cesium and rubidium by spectroscopy
• Periodic classification: Spectra helped order elements

Technological Development

• 19th century: Visual spectroscopes
• 20th century: Photographic and photoelectric detectors
• Present: CCD detectors and computerized analysis

Emission spectroscopy remains a fundamental technique in analytical chemistry, from educational laboratories to cutting-edge research and industrial applications.

Specific Accident Prevention

Burn Prevention

• Use tweezers to manipulate hot material
• Allow hot glass to cool before touching
• Mark hot surfaces
• Use thermal gloves when necessary

Cut Prevention

• Inspect glassware before use
• Dispose of broken glass in specific container
• Use appropriate techniques to insert tubes into stoppers
• Keep work surfaces clear

Poisoning Prevention

• Always work in well-ventilated area
• Use fume hood for toxic vapors
• Never pipette by mouth
• Read safety data sheets before using chemicals

Practical Safety Examples