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    STUDIA CHEMIA - Issue no. 2 / 2009  
         
  Article:   THE COMPARATIVE STUDY OF THE INFLUENCES OF THE EXTERNAL FACTORS ON THE UV-VIS ABSORPTION SPECTRA OF SOME POTENTIALLY TAUTOMERIC AZOCOUPLING PRODUCTS.

Authors:  IOAN PANEA, MIRELA PELEA, LUMINIŢA SILAGHI-DUMITRESCU, IOAN BÂLDEA, MARIA COROŞ. 		 
       
         
  Abstract:  The influences of the pH'', water content, sample concentration, temperature and time on the UV-Vis absorption spectra of four potentially tautomeric azocoupling products 1- 4 have been comparatively studied in ethanol-water mixtures. The pH'' influence can be explained by the involvement in two steps deprotonation equilibria of each dye 1- 4. These equilibria are characterized by the pKa'' values. The three species involved in the two deprotonation steps of each dye 1- 4 exhibit different visible absorption maxima. The hydrazone structures 1b- 4b of the initial neutral species of the dyes are supported by IR and 1H-NMR data. The presence of other tautomeric forms (e.g. 1a- 4a) has been not proved. The comparative study of the effect of other external factors on the spectra with that of pH'' has shown that: i)- the influence of either water content, sample concentration or temperature, which is characterized by the presence of isosbestic points, may be explained by the involvement of the dyes in the first step acid ionization; ii)- the influence of either water content, sample concentration, temperature or time, when isosbestic points do not appear, might be explained by the superposition of the acid ionization equilibrium of the dyes with other phenomena. Thus, to account for the effect of water content and time on the spectra of the dye 1, the superposition of a disaggregation process and acid ionization equilibrium has been assumed.


Keywords: azacoupling products, UV-Vis spectra of azacoupling products



  
         
     
         
         
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