UV-Visible Spectroscopy and Colorimetry: Principles and Applications

Expert reviewed • 23 November 2024 • 5 minute read

Introduction

UV-Visible spectroscopy and colorimetry are fundamental analytical techniques used to determine the concentration of colored compounds in solution. These methods are essential tools in modern chemical analysis, particularly in the analysis of inorganic compounds.

Principles of UV-Visible Spectrophotometry

UV-Visible spectrophotometry involves exposing samples to electromagnetic radiation in two specific ranges:

  • Ultraviolet (UV) radiation: 10-400 nm
  • Visible light: 400-700 nm

The technique is based on a simple principle: when radiation passes through a sample, molecules can absorb specific wavelengths of light, causing electrons to transition to higher energy levels. This absorption is selective and depends on the molecular structure of the compound.

Key Components of a UV-Vis Spectrophotometer

  • Light source (UV and visible)
  • Monochromator
  • Sample holder (cuvette)
  • Detector
  • Data processor

The Beer-Lambert Law

The fundamental relationship governing absorption spectroscopy is the Beer-Lambert Law:

A=εclA = \varepsilon c l

Where:

  • AA = Absorbance
  • ε\varepsilon = Molar extinction coefficient
  • cc = Concentration
  • ll = Path length

Colorimetry: Principles and Applications

Colorimetry is a specific application of spectrophotometry that focuses on visible light absorption. It relies on the relationship between a compound's color and its complementary absorbed wavelength.

Color Wheel and Absorption

A solution's color is complementary to the wavelength it absorbs. For example:

  • Red solutions absorb green light (490-560 nm)
  • Blue solutions absorb orange light
  • Yellow solutions absorb violet light

Practical Application: Iron Analysis

Here's a worked example of determining iron concentration in a tablet:

Given:

  • Sample mass: 0.200 g
  • Final volume: 200 mL
  • Measured absorbance: 0.6105

The reaction involved:

Fe(aq)3++SCN(aq)−−>[FeSCN](aq)2+{Fe^{3+}_{(aq)} + SCN^-_{(aq)} -> [FeSCN]^{2+}_{(aq)}}

Calculation steps:

  • From calibration curve at A = 0.6105: c=5.5×10−5 mol L−1c = 5.5 \times 10^{-5} \text{ mol L}^{-1}

  • Calculate moles of Fe³⁺: n=cV=5.5×10−5 mol L−1×0.2 L=1.1×10−5 moln = cV = 5.5 \times 10^{-5} \text{ mol L}^{-1} \times 0.2 \text{ L} = 1.1 \times 10^{-5} \text{ mol}

  • Calculate mass of iron: m=55.85 g mol−1×1.1×10−5 mol=6.14×10−4 gm = 55.85 \text{ g mol}^{-1} \times 1.1 \times 10^{-5} \text{ mol} = 6.14 \times 10^{-4} \text{ g}

  • Calculate percentage: Percentage=6.14×10−40.200×100=0.307%\text{Percentage} = \frac{6.14 \times 10^{-4}}{0.200} \times 100 = 0.307\%

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Atomic Absorption Spectroscopy: Principles and Applications

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