Spectroscopy Absorbance Practice Exercises

Absorbance and Transmittance

• Absorbance (A) is related to transmittance (T) by the equation A = -log T
• A percent transmittance of 50% is equivalent to a transmittance of 0.500, which corresponds to an absorbance of 0.301
• If the absorbance is 1.27, the transmittance is 0.054, which corresponds to a percent transmittance of 5.4%

Molar Absorptivity

• Molar absorptivity (ε) is a measure of the absorption of a substance at a given wavelength
• The equation A = εlc can be used to calculate the molar absorptivity of an analyte, where A is the absorbance, ε is the molar absorptivity, l is the pathlength of the sample cell, and c is the concentration of the analyte
• A 5.00 × 10^(-4) M solution of an analyte has an absorbance of 0.338 at a wavelength of 490 nm, which corresponds to a molar absorptivity of 676 cm/M

UV-Vis Molecular Absorption Spectrometry

• Many types of molecular species absorb ultraviolet and visible radiation, which can be used for qualitative and quantitative analyses
• Molecular absorption by these species can be used to measure the absorption or emission of electromagnetic radiation by a chemical substance
• Electromagnetic radiation (EMR) is a form of energy that consists of perpendicular oscillating electric and magnetic fields, which travel in a bundle of energy called a photon

Electromagnetic Radiation (EMR)

• EMR has several forms, including X-ray, microwaves, UV, and visible light
• EMR has several properties, including amplitude, wavelength, frequency, and wave number
• Amplitude is the maximum vertical displacement of the wave from its normal position
• Wavelength is a measure of the distance between two identical peaks or troughs in a wave
• Frequency is the number of waves that pass a fixed point per second, and is measured in Hertz (Hz)
• Wave number is the number of waves in a unit length or distance per cycle, and is a reciprocal of the wavelength

Absorption of Light

• If a sample absorbs all wavelengths in the visible region of the spectrum, it will appear black
• If a sample absorbs none of the wavelengths in the visible region of the spectrum, it will appear white
• The color of an object is due to the wavelengths transmitted or reflected, while the other wavelengths are absorbed

Types of Absorbing Species

• Organic compounds can absorb light due to electronic transitions, including π → π* and n → π* transitions
• Saturated compounds containing atoms with unpaired electrons can absorb light due to n → σ* transitions
• Unsaturated systems with π electrons or n electrons can absorb light due to electronic transitions

Chromophores

• Chromophores are unsaturated organic functional groups that absorb in the ultraviolet or visible region
• Most absorption bands of organic compounds are due to electronic transitions

Inorganic Compounds

• Metal-ligand complexes can absorb light due to charge-transfer transitions
• d-d transitions can occur between d orbitals of metal split by the presence of a ligand field
• These transitions usually absorb in the visible region of the spectrum

Applications of Molecular Absorption Spectrometry

• The determination of an analyte's concentration based on its absorption of ultraviolet or visible radiation is a common quantitative analytical method
• Many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum
• If an analyte does not absorb UV/Vis radiation, it can often be reacted with another species that is strongly absorbing