Electromagnetic Spectrum and Spectrophotometry

Electromagnetic Spectrum

• Includes gamma ray, X-ray, ultraviolet (UV), visible, infrared (IR), microwave, and radio wave radiations.
• Ranges from very short wavelengths (gamma and x-rays) to very long wavelengths (microwaves and radio waves).
• UV radiation has shorter wavelengths (200-380 nm) than violet light and is not visible to the human eye.
• IR radiation has longer wavelengths than the red light and is not visible to the human eye.
• The visible region of the spectrum extends from 380 nm to 780 nm.
• The human eye can only detect colors within this wavelength range because it's called "visible".

Spectrophotometry

• White light can split to produce different colors or wavelengths of visible light which appear as different colors to our eyes: red, orange, yellow, green, blue, indigo, and violet.
• When all wavelengths or colors of the visible light are transmitted together, the light appears white.
• When all wavelengths or colors of the visible light are absorbed, it appears black.
• When white light is passed through an object, the object will absorb certain wavelengths, leaving the unabsorbed wavelengths to be transmitted.
• The residual transmitted wavelengths are seen as color.

Complementary Colors

• The absorbed color is different from the actually observed color.

Cut-off Wavelengths of Some Common Solvents

• Water 190 nm
• Ether 205 nm
• Ethanol 207 nm
• Methanol 210 nm
• Chloroform 247 nm
• Carbon tetrachloride 257 nm
• Benzene 280 nm
• Acetone 331 nm

Important Terms in Spectrophotometry

• Chromophore: An unsaturated functional group responsible for absorbing UV and/or visible light (200-800 nm) and imparting color to the molecules.
  • Contains double or triple bonds like -C=C, -C=O, -N=N, -C=N
  • Responsible for -and n-electronic transitions
• Auxochrome: A saturated group with unshared electrons like -OH, -NH2, etc.
  • Does not absorb radiation on its own but enhances absorption by chromophore when attached to it.

Absorption Spectrum

• A plot of the absorbance (A) against the wavelength (λ).
• Each substance has a characteristic absorption spectrum that depends on its structure.
• It has a characteristic shape which shows the wavelength of maximum absorbance (λmax).
• Two parameters define an absorption band:
  • Its position (λmax) on the wavelength scale (depends on chemical structure)
  • Its intensity on the absorbance scale (depends on concentration)
• λmax is characteristic for each molecule according to its structure and types of transitions.
• It can be used for:
  • Identification of a chemical substance
  • Quantitative measurement

Spectral Shifts of the Absorption Spectrum

• Bathochromic Shift (Red Shift): The shift of λmax to a longer wavelength due to:
  • Substitution with certain groups like OH and NH2
  • Solvent effect (e.g., polarity of solvent)
  • Presence of two or more chromophores in conjugation.
• Hypsochromic Shift (Blue Shift): The shift of λmax to a shorter wavelength due to:
  • Substitution and/or solvent effect
  • Removal of conjugation.
• Hyperchromic Effect: An increase in the intensity of absorption due to the presence of an auxochrome.

Spectrophotometry - Examples

Phenol

• In acid medium (phenol): λmax = 270 nm
• In alkaline medium (phenate anion): λmax = 290 nm
• The spectrum of phenol in alkaline medium exhibits bathochromic shift and hyperchromic effect due to the formation of conjugated double bonds, increasing the delocalization of the  electrons, and making the electrons more energetic (absorb longer wavelengths).

Aniline

• In alkaline medium: λmax = 280 nm
• In acid medium: λmax = 254 nm
• The UV spectrum of aniline in an acid medium shows hypsochromic shift and hypochromic effect.
• This blue shift is because of the protonation of the amine group, making the pair of electrons unavailable for the quinonoid conjugated structure formed in alkaline medium.

To overcome spectral changes with pH change, solutions must be buffered at specific pH.

Solvent Effect

• The solvents may have a strong effect on the position of λmax due to its effect on the energy of transition
• Less polar solvents (e.g., hydrocarbons) interact less strongly with the solute than do polar solvents (e.g., water and alcohols).

π-π* Transitions

• Dienes (Compounds that contain two double bonds):
  • π-π* bands of dienes are not shifted by any change of solvent polarity.

Visual Methods (Visual Colorimetry)

• Used for measuring colored solutions only.
• Standard series method:
  • The test solution is matched with a series of standards similarly prepared.
  • The concentration of the unknown solution is equal to that of the standard solution whose color is matched exactly.
  • Example 1: The determination of copper with ammonia by forming a blue color.
  • Example 2: The determination of iron (III) with thiocyanate anion by forming blood red color.
  • Use matched Nessler cylinders to avoid path length effects.

Instrumental Methods

Spectrophotometers and Colorimeters:

• Spectrophotometer:
  • Has a wider wavelength range than a colorimeter (200 nm in the UV region to 1000 nm in the IR region).
• Essential parts of a spectrophotometer:
  • Stable light source of radiant energy
  • Monochromator (wavelength selector)
  • Sample compartment
  • Detector
  • Signal readout meter (recorder)

Light Source

• The light source provides the light that passes through the sample solution.
• The light source should deliver highly intense, continuous, constant and uniform radiation which covers the range required.
  • For UV measurements: Hydrogen or Deuterium discharge lamp (gives radiations from 190 - 375 nm).
  • For visible measurements: Tungsten lamp (gives radiations from 350 - 1000 nm).

Types of Spectrophotometers

• Single-Beam Spectrophotometer
  • All the light from the monochromator goes to the sample and then to the detector.
  • Advantages: Relatively inexpensive and simple.
  • Disadvantage: The reference point (blank) has to be reset frequently when measuring a series of samples over a period of time.
• Double-Beam Spectrophotometer
  • The light is split by a beam splitter into two paths of equal intensity.
  • One path goes through the reference compartment, and the other goes through the sample compartment.
  • Both beams are either directed to the same detector or each has its own detector.
  • The signal for the absorption of the reference cell is automatically subtracted from the sample cell, giving a net signal corresponding to the absorption of the components in the sample solution.
  • Advantages:
    • Stability of the readings over time.
    • Ability to simultaneously correct for any solvent effects.
    • Compensate for fluctuations in the source, detector or electronics.
  • Disadvantages:
    • Expense
    • Less sensitivity to measure highly absorbing samples (compared to single beams).

Deviation from Beer's Law

• Real Deviation:
  • Occurs at high concentrations.
  • Due to molecular interaction and association, which alters the ability of the species to absorb at a definite wavelength.
• Instrumental Deviation:
  • Irregular Deviation: Unmatched cells, unclean handling, and unclean optics.
  • Regular Deviation:
    • Slit width control: Wide opening of the slit results in unspecific wavelength passing through.
    • Stray Light: Any radiation of wavelength other than those which are absorbed. Also includes light that reaches the detector without passing through the sample.
• Chemical Deviations:
  • pH effects
  • Solvent interactions
  • Temperature effects
  • Time factor of colored solutions (color may fade due to oxidation, reduction, hydrolysis, or other reactions).
  • Photo effect (some substances like vitamin K undergo degradation when exposed to light).

Explore the various types of electromagnetic radiation, including gamma rays, X-rays, and visible light, in this quiz. Learn about their wavelengths and how they interact with human perception. Dive into the principles of spectrophotometry and the behavior of light as it relates to color.