Characterization of a Single Sample by Combining Thermodynamic and Spectroscopic Information in Spectral Analysis

• Published in: Analytical chemistry (Washington) 1996-05, Vol.68 (10), p.1706-1710
• Main Authors: Nygren, Jan, Andrade, José Manuel, Kubista, Mikael
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Equilibrium; Exact sciences and technology; General, instrumentation; Thermodynamics
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac951147p

We have previously shown that chemical equilibria can be characterized spectroscopically, even when the component spectral responses are unknown and overlapping, by exploiting appropriate thermodynamic relations (Kubista, M.; Sjöback, R.; Albinsson, B. Anal. Chem. 1993, 65, 994−998). Application of this strategy requires sets of samples that differ in a physical property, such as pH or total concentration. In this work, we show that a similar strategy can be applied to characterize a single sample. Utilizing the van't Hoff relation, which describes the dependence of the equilibrium constant on temperature, we show that spectra recorded at different temperatures can be deconvoluted into contributions from the individual components. To illustrate the approach, we characterize two monomer/dimer equilibria, thiazole orange in aqueous solution and benzoic acid in n-heptane, by studying the effect of temperature on the absorption spectrum. We determine the spectral responses of the monomer and dimer species, their concentrations as a function of temperature, and the enthalpy change upon dimerization. These are the first examples ever where the spectral responses of the components in a single sample are determined without making assumptions about spectral overlap and without reference spectra.