Spectrophotometric study of the system Hg(II)–thymol blue–H 2O and its evidence through electrochemical means

Abstract

The detailed analysis of the experimental spectrophotometric data obtained from solutions containing the acid–base indicator thymol blue (TB) and mercury(II) (Hg(II)) coupled with data processing by means of the squad program, a chemical model was determined that includes the formation of complexes indicator-metal ion (HgTB and HgOTB), dimer species (H 3TB 2 and H 4TB 2) and monomer species (HTB and TB). The values of the overall formation constants (log β) were calculated for the chemical equilibria involved: TB+Hg↔HgTB log β=16.047±0.043, TB+Hg+H 2O↔HgOHTB+H log β=7.659±0.049, 2TB+4H↔H 4TB 2 log β=31.398±0.083, 2TB+3H↔H 3TB 2 log β=29.953±0.084 and H+TB↔HTB−log β=8.900. To compliment the present research, the values of the absorptivity coefficients are included for all the species involved, within a wide range of wavelengths (250–700 nm). The latter were used subsequently to carry simulations of the absorption spectra at various pH values, thus corroborating that the chemical model proposed is fully capable to describe the experimental information. Voltammetric study performed evidenced the formation of a complex with a 1:1 stoichiometry Hg(II):TB.