Determination of the complexation constants of Pb(II) and Cd(II) with thymol blue using spectrophotometry, SQUAD and the HSAB principle

This paper presents the results concerning the determination of the formation constants of the complexes between thymol blue, TB, and the metal ions Pb(II) or Cd(II). The experimental procedure was carried out in the presence of a nitrogen atmosphere at 25 °C. The spectrophotometry data obtained were processed through the software SQUAD to calculate the complexation constants of the metal-indicator and to establish an adequate base of the models which considered the structure of the indicator, and the actual metal species in the aqueous solutions. For the Pb(II)–TB–H2O system the log K value calculated for the PbTB complex was 5.591 ± 0.057 while for the Cd(II)–TB–H2O system, the log K value of the CdTB complex was 5.099 ± 0.008. Also, supporting theoretical chemistry results on the chemical hardness of TB molecule were performed to enable establishment of a relative prediction scale of the TB complexation constants ranking in the framework of the Principle of Hard and Soft Acids and Bases or HSAB Principle.