The stability constants of analogs microbial iron chelators, gallic acid (catechol), salicylhydroxamic acid (hydroxamate), complexes of trace metals such as AI(III), Cr(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II), have been measured potentiometrically.
In potentiometric titrations, experimental results were treated with well known computer program “BEST”. The results were refined by this program.
The stiochiometric ratios of different species formed during complex formation were determined. Concentrations of the present in aqueous solution at different pH were also studied by these methods.
For thermodynamic study, pH titrations of metal galic acid and metal salicylhydroxamic acid complexes were performed at different temperatures (30oC, 35oC, 40oC, 45oC, 50oC). The data obrained from pH titrations were utilized for calculation of B values by computer program Best. The change in B balues at different temperatures were examined and enthalpy and entropy values were determined for ML, ML2, and ML3, complexes of gallic acid and salicylhydroxamic acid.
Some of complexes showed absorbance in the visible range such as Cr(III), Mn(II), Fe(III), Co(II), Ni(II), and Cu(II). They were studied spectrophotomeltrically. Mole ratio method and job’s methods were applied. For this purpose spectra of these complexes were recorded on UN/visible spectrophotometer and thus max were selected. It was found that generally transition metals complexes have 1:3 metal ligand ratio while nontransition metals have a ratio of 1:2.
Stability constant values of metal gallic acid complexes and metal salicylhydroxamic acid complexes were compared. These values were found to be larger in case of gallic acid complexes. Stability constant values were also compared with iron gallic acid and iron salicylhydroxamic acid complexes.
The stability constants values of dispositive ions followed the Irving William order of stability. Tripositive ions AI(III) and Cr(III) have comparable values with the Fe(III) of complexes.
It was found that transition elements having 2+ oxidation state, i.e Ni and Cu have high enthalpy and entropy values. AI(III) has comparable values with iron complexes which are even higher than Cu(II) complex.For ML3 complexes it was found that Fe(III) and Cr(III) can form stable complexes having high H and S values. Competition reactions of iron complexes were performed with other metal ions whose stability constants were close to iron siderophore complexes.