L2 Beers law

Introduction to Spectroscopy

• Course: CH4303 Analytical Chemistry 1

• Topic: Introduction to Spectroscopy & Beer's Law

• Instructor: Dr. Soumya Mukherjee

Reference Materials

• Key Text: Quantitative Chemical Analysis (Ed. 9) by Daniel C. Harris

• Available on CH4303 BrightSpace site

Key Concepts in Spectroscopy

Molecular Absorption

• Beer's Law Summary: Essential for understanding molecular absorption.

Properties of Electromagnetic Radiation

Wave Properties

• Described in terms of sinusoidal waves

• Does not require a medium for transmission

Particulate Properties

• Absorption and emission explained via photons, which are discrete particles.

Chromophores

• Definition: The part of a molecule that absorbs light.

• Visible light absorption results in perceived color; the observed color is complementary to the absorbed light.

  • Wavelength (nm) Color vs. Complementary Color:

    • 400 - 435: Violet -> Green-Yellow

    • 435 - 480: Blue -> Orange

    • 480 - 500: Green-Blue -> Red

    • 500 - 560: Green -> Red-Violet

    • 560 - 580: Yellow-Green -> Violet

    • 580 - 595: Yellow -> Blue-Violet

    • 595 - 650: Orange -> Blue

    • 650 - 750: Red -> Blue-Green

Molecular Absorption Process

• Interaction Highlights:

  • Photon absorption occurs when energy (hn) matches energy difference (ΔE) between ground and excited states.

  • Transition results in an excited state (M*), while most molecules remain in ground state at room temperature.

  • Diagram: M + hn ⟹ M*

Quantum Theory Implications

• Energy gap calculation:

  • ΔE = 6.626 x 10^-19 J = hn, where h = 6.626 x 10^-34 Js

  • Wavelength related formulas:

    • n = 1 x 10^15 s^-1

    • λ = 300 nm at given energy gap.

Relaxation and Energy Loss

• M* relaxes back to ground state irreversibly, typically in 10^-8 s, mostly releasing energy as heat.

• Events: Possibility of fluorescence/phosphorescence with re-emission of light.

Spectrophotometry Overview

Experiment Setup

• Key Components: Light source, wavelength selector, detector, cuvette (sample cell).

  • Intensity of light measured before and after passing through the sample solution.

Absorption of Light Mechanics

• When light passes through a solution, its intensity decreases:

  • Transmittance (T) is defined as the fraction of incident radiation transmitted by the solution:

    • T = P/P0, expressed as %T.

Absorbance Calculations

• Absorbance (A) defined as:

  • A = -log10 T = log(P0/P)

• Beer's Law Equation:

  • A = ecb, where:

    • A: absorbance

    • e: extinction coefficient (M^-1cm^-1, varies with wavelength)

    • c: concentration (M)

    • b: path length (cm)

Sample Preparation and Calibration

• Process for creating accurate and precise standards using stock solutions and serial dilution.

• Absorption spectra display energy at different wavelengths.

Experimental Considerations

Comparing Solutions

• To measure absorption accurately, compare the light intensity through the analyte solution with that through a solvent in an identical cell.

Spectrophotometer Functionality

• A spectrophotometer can produce a beam of monochromatic radiation that can shift across different wavelengths.

• Important for obtaining absorbance profile across the spectrum.

Absorbance and Concentration Relationship

• Illustrates the correlation as per Beer's Law.

• Example calculations for absorbance and transmittance at known concentrations.

Limitations of Beer's Law

• Conditions for Application:

  • Monochromatic light

  • Dilute solutions (≤0.01 M)

• Failure Points:

  • Concentration-dependent equilibria where absorbing species interact.

Types of Deviations in Beer's Law

Chemical Deviations

• Occur due to a reaction involving analytes at high concentrations, e.g., weak acid behaviors.

Instrumental Deviations

• Arise from polychromatic radiation affecting absorbance measurements.

Conclusion

• Understanding and using Beer's Law is critical in quantitative analytical chemistry, with attention required to limitations and proper calibration methods.