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  • P-ISSN 1225-0163
  • E-ISSN 2288-8985

A thermodynamic analysis on thermochromism of fluoran dyes

Analytical Science and Technology / Analytical Science and Technology, (P)1225-0163; (E)2288-8985
2009, v.22 no.2, pp.159-165



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Abstract

The thermochromism of fluoran has been examined. The DCF exists as a colorless lactone in aprotic solvents. However, the DCF exists in the form of an equilibrium mixture of a colored zwitter-ion and a colorless lactone in protic solvents. When an acid is added to the solution, the DCF exists an equilibrium mixture as a colorless lactone and a colored cation even in aprotic solvents. In order to understand the interaction between the DCF and the solvent, absorption spectra of the DCF in various solvents were measured. The thermodynamic parameters of the DCF have also been investigated. From the variation of absorbance with temperature, the standard enthalpy changes ΔH⁰ of the equilibrium between the lactone and the zwitter-ion in various solvents have been determined. The standard enthalpy change ΔH⁰ is approximately -2.0 kJ/㏖ in protic solvents. In acidic solution, the standard enthalpy change is measured to be to zero in protic solvents within the experimental error. When the carboxylic group is protonated in acidic solution, a poor interaction between the dye and the solvent is expected.

keywords
thermochromism, functional dye, equilibrium constant, enthalpy


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