Abstract
A fluorometric method allowing quantification of minute amounts of water in highly lipophilic solvents is proposed. This approach takes advantage of resonance energy transfer between 4-methylumbelliferone and Sudan I. The conditions required to observe effective energy transfer have been elucidated. The emission band of the donor molecule (4-methylumbelliferone) depends strongly on the water content—its position is affected by protolytic reactions occurring between the fluorescent dye and water in lipophilic solvent media. In consequence, the extent of overlap between the emission band of the donor and absorption peak of the acceptor scales with the water concentration. The sensitivity of this optical system can be considerably increased by addition of organic bases. The dependence of the emission band of 4-methylumbelliferone/Sudan I system on minute amount of water present in lipophilic solutions opens up possibility of determination of water content in these media at the unique wavelength, in a turn on mode. The presented system was successfully applied to determine water in model commercial oil samples. It was shown that using herein proposed method the increase of water contents in the sample can be observed as increase of the emission intensity, despite the native fluorescence of the vegetable oil samples. For these samples, a good correlation with golden standard Karl Fischer titration was obtained.
Highlights
Quantification of water in organic solvents is important for many applications, including industrial ones
To explore the possibility of observing emission maximum related to protolytic reaction between 4-MU and water in lipophilic solvents at other wavelengths than characteristic for 4-MU emission, interactions between different optically active species in the sample need to be taken into consideration
The approach proposed in this work is based on application of a reporter dye (4-MU) of proven sensitivity in Bturn on^ mode for increasing water contents in lipophilic solvents and the optical transducer (Sudan I)
Summary
Quantification of water in organic solvents is important for many applications, including industrial ones. The electrochemical approach is generally highly reliable; in some cases, especially for relatively small water concentrations and highly lipophilic, aprotic solvents, it may potentially be sensitive to, e.g., sample acquisition due to tendency of water to spontaneously accumulate close to more hydrophilic surfaces—e.g., glassware. This effect may result in inaccurate determination results (Dantan et al 2000; Gergen et al 2006). The Karl Fischer titration method is difficult to be implemented as a part of, e.g., industrial or technological process, or to be used to continuously monitor any occurrence of water in sample media. Different dyes have been proposed/tested in this respect, most of them were tailor made (synthesized) for water determination purpose, and in most cases, the increase of water contents in the sample was accompanied with
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