These groups give it a capacity of complexation with various bivalent MIs. This indicator is similar to xylene orange and has an N 2O 6 coordination sphere geometry. The MTB has nine active functional groups (four carboxylic acids, two phenols, two amines and one sulfonyl (cf. Here, we considered 3,3′-Bisthymolsulfonephthalein (Methylthymol blue MTB) as a representative of thymol containing metallochromic indicators.
in measuring acidic or association constants or when applying methods like the ion-increment procedure for the determination of free MI concentrations. These are necessary for some applications e.g. Physiochemical knowledge about MI-metallochromic indicator complexes can provide a deep insight into the determination of active chemical contributions of each chemical species at their complexation systems, coordination sites, coordination numbers and molecular geometries of MI-metallochromic indicator complexes. MI-metallochromic indicator complexes can form organometallic dyes, which have different colors and distinguished spectral features from that of the intact metallochromic indicators 14. Furthermore, they are employed as active modifiers on different nanocomposites and liquid membrane electrodes for measuring MIs 7, 8, 9, 10, 11, 12, 13. The metallochromic indicators are used as equivalence point identifiers in chelometric titrations, as spectrophotometric reagents to form stable complexes in spectrophotometric/fluorimetric quantification of MIs and as preconcentration agents in separation of trace quantities of heavy MIs 3, 4, 5, 6.
Metallochromic indicators are broadly employed in chemical analysis 1, 2. However, the Fe II complexes always show octahedral geometry. The correlation between experimental and computed wavelengths revealed that the 1:1 complexes of Zn II and Cu II prefer square pyramidal geometries. However, its higher energy transitions are a mixture of LLCT, MLCT, and metal-to-metal charge transfer (MMCT) characters. For the Fe 2(MTB) complex, the lowest energy transition of has an LLCT character. However, the transitions in the UV–Visible spectrum of the Fe(MTB) complex have LLCT and metal-to-ligand charge transfer (MLCT) characters. The majority of transitions observed in the UV–Visible spectra of the Zn(MTB) and Cu(MTB) complexes have ligand-to-ligand charge transfer (LLCT) characters. However, the formation of successive 1:1 and 2:1 complexes are suggested for Fe II by the molar ratio and MCR-ALS. The results of molar ratio and multivariate curve resolution alternating least squares (MCR-ALS) revealed the formation of successive 1:2 and 1:1 complexes (MI:MTB) for the Zn II and Cu II systems. Quantum chemical calculations revealed that electronic transitions in the UV–Visible spectra of MTB have mixed n → π* and π → π* characters. The complexes were characterized using UV–Visible absorption spectroscopy combined with soft/hard chemometrics methods and quantum chemical calculations. In this study, we determine the acidity constants of methylthymol blue (MTB) and association constants of its complexes with the Zn II, Cu II, and Fe II metal ions (MIs), through theoretical and experimental means.