Spectroscopy {UV-visible spectroscopy} can find solution substance concentrations by measuring visible or ultraviolet light absorbance.
purposes
UV-visible spectroscopy can detect functional groups, bonds, and spatial configurations and so identify molecules. It can find equilibrium point, pH, or pK by comparing absorbance at two wavelengths. UV-visible spectroscopy is simple and cheap.
purposes: visible
Visible light can detect colored molecules, in concentrations down to 0.01 M, using standard curve for calibration.
technique
Extinction coefficient or molar absorptivity depends on outer-shell-electron transition energies and probabilities. Tables show values for most substances. Method can also use integrated absorption coefficient or oscillator strength. If molar absorptivity is high, sensitivity is high. Wavelength is maximum-absorbance wavelength. Bandwidth is broad, because energy is low. Path length through solution is typically one centimeter. Air absorbs light of less than 200-nanometer wavelength, so UV light path has vacuum. Glass absorbs all UV light, so containers are quartz.
theory: transition metal
Transition-metal ions have incomplete d orbitals, with three at lower energy and two at higher energy, which have electronic transitions in visible light. Central transition-metal ions can have two sets of low-energy d orbitals, with no symmetry center from metal-ion bonding orbitals to ligand antibonding orbitals or from metal-ion antibonding orbitals to ligand antibonding orbitals. Then intensity is low, because vibrations can also cause such transitions. High intensity is if charges transfer from ligand to ion, or vice versa.
theory: double bonds
Ultraviolet light can detect molecules with carbon-oxygen double or triple bonds or with carbon-carbon conjugated bonds. Unconjugated double bonds involve UV light. Pi-bond electrons jump to antibonding pi orbital to make UV light.
If double bond conjugates, electron delocalization causes small jump and visible light. Intensity is high.
Indicator color changes are large, because proton gain or loss changes conjugation. More conjugation makes longer wavelengths.
Molecules with lone electron pairs can make UV or visible light by jumping to antibonding pi orbitals, typically forbidden visible-light transitions.
theory: no light
Closed-shell electrons and sigma-bond electrons do not give UV or visible light. Hydroxyl, amine, and halogen groups do not give UV or visible light, but they can affect intensity or shift chromophore wavelengths.
Physical Sciences>Chemistry>Analytical Chemistry>Spectroscopy
5-Chemistry-Analytical Chemistry-Spectroscopy
Outline of Knowledge Database Home Page
Description of Outline of Knowledge Database
Date Modified: 2022.0224