A fluorescence quenching study of the complexation of salicylic acid mono- and dianions with the copper(II) ion in aqueous solutions

Fluorescence quenching of the anions of sodium salicylate and sulfosalicylic acid by the Cu2+ ion in water has been studied. The Stern-Volmer (SV) curves for all salicylic acid derivatives are concave. Using the modified Stern-Volmer equation, the values of the SV constant K = 1420 +/- 70 and 470 +/- 20 l/mol were obtained for salicylic (SA) and sulfosalicylic acid (sulfo-SA), respectively. It has been concluded that static quenching (complexation) takes place. The proportions of fluorescing available for quenching were determined to be 0.53 +/- 0.02 and 0.66 +/- 0.03 for SA and sulfo-SA, respectively. Tentative qualitative interpretation of the difference in the complex formation constants between sodium salicylate and sulfosalicylic acid is given in terms of the concept of difference in the accessibility of the quenching sites of salicylate ions to copper ions owing to the difference in the magnitude of interaction with water molecules.