Determination of acid-base equilibria of organic pollutants: the steady state fluorescence emission method

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Abstract

The determination of the pKa of pollutants that may undergo acid-base equilibria in the pH range of natural waters is of utmost importance. In fact, depending on the protonation state of the pollutant, an entirely different behavior in the environment is expected. Namely, the accumulation of these pollutants in biota, their photo and thermal degradation mechanisms, and the reactivity towards other natural or anthropogenic chemicals are strongly dependent on the form in which they are present. At least one of those chemical forms is generally scarcely soluble in water (often less than 10-7M), and the use of a highly sensitive detection method is required. An obvious choice is fluorescence emission. The resulting titration curves reflect not only the pKa of the ground state but also that of the excited state (pKa*). In certain cases the characteristic experimental titration curve reveals exclusively the value of pKa*. To avoid misinterpretations and to maximize the sensitivity of the method, a careful choice of emission and excitation wavelengths must be undertaken. Moreover, to gain accuracy in the pKa* values, the fit of the experimental data to theoretical predictive equations is essential. In this work the general equations that describe the shape of the steady state fluorescence emission titration curves are deduced. For the specific cases of exclusive excitation of acid or basic forms, the typical titration curves are displayed. The singular case of excitation at the absorption isosbestic point is also presented. The nature and concentration of the buffer used to control pH may strongly influence the experimental results. Disturbing effects avoidance is discussed. Two paradigmatic experimental examples are extensively analyzed, β-naphthol (2-hydroxy naphthalene) and Fuberidazole® (2-(2′-furanyl)-1H-benzimidazole).

Original languageEnglish
Pages (from-to)510-520
Number of pages11
JournalArchives of Environmental Contamination and Toxicology
Volume26
Issue number4
DOIs
Publication statusPublished - 1 Jan 1994

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