Spectroscopic monitoring of titrations and kinetics

When the emphasis is on measurement of the intensities rather than the energies of transitions, the spectroscopic investigation is usually called spectrophotometry. Provided at least one of the species involved has a suitable absorption band, it is usually straightforward to carry out a ‘spectrophotometric titration’ in which the extent of reaction is monitored by measuring the concentrations of the components present in the mixture. The measurement of UV–visible absorption spectra of species in solution also provides a method for monitoring the progress of reactions and determining rate constants. Techniques that use UV–visible spectral monitoring range from those measuring reactions with half-lives of picoseconds (photochemically initiated by an ultrafast laser pulse) to the monitoring of slow reactions with half-lives of hours and even days. The stopped f low technique (Fig. 8.14) is commonly used to study reactions with half-lives of between 5 ms and 10 s and which can be initiated by mixing. Two solutions, each containing one of the reactants, are mixed rapidly by a pneumatic impulse, then the flowing, reacting so lution is brought to an abrupt stop by filling a ‘stop-syringe’ chamber, and triggering the monitoring of absorbance. The reaction can be monitored at a single wavelength or successive spectra can be measured very rapidly by using a diode array detector. The spectral changes incurred during a titration or the course of a reaction also provide information about the number of species that form during its progress. An important case is the appearance of one or more isosbestic points, wavelengths at which two species have equal values for their molar absorption coefficients (Fig. 8.15; the name comes from the Greek for ‘equal extinguishing’). The detection of isosbestic points in a titration or during the course of a reaction is evidence for there being only two dominant species (reactant and product) in the solution.

Figure 8.14 The structure of a stopped-flow instrument for studying fast reactions in solution.

Figure 8.15 Isosbestic points observed in the absorption spectral changes during reaction of HgTPP (TPP tetraphenyl porphyrin) with Zn²⁺ in which Zn replaces Hg in the macrocycle. The initial and final spectra are those of the reactant and product, which indicates that free TPP does not reach a detectable concentration during the reaction. (Adapted from C. Grant and P. Hambright, J. Am. Chem. Soc. 1969, 91, 4195.)

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المزيد

مواضيع ذات صلة

Thermal analysis

X-ray fluorescence elemental analysis

CHN analysis

Atomic absorption spectroscopy

Ionization and detection methods

X-ray absorption spectroscopy

Photoelectron spectroscopy

Absorption spectroscopy

Neutron diffractionNeutron diffraction

Single-crystal X-ray diffraction

Powder X-ray diffraction

X-ray diffraction