Acid-base and UV properties of some aminophenol ligands and their complexing ability towards Zn2+ in aqueous solution

The UV–VIS and the acid–base behaviour of three aminophenol ligands (m-aminophenol, 4-amino-2- hydroxytoluene, 2-amino-5-ethylphenol) was studied by spectrophotometry at t=25 °C and I=0.004 mol L−1 in aqueous solution. The stability constants for their Zn2+ complexes were determined and compared by potentiometry at I=0.1 mol L−1 inNaCl andUV–VIS spectrophotometry at I=0.004mol L−1. The results showed, in the pHrange between 2 and 7, a veryweak complexing ability towards the Zn2+ ion for all aminophenols under study. For all the systems, at higher pHs, the strongest species are ML(OH)2 − and especially M2L(OH)3 0 . Stability constant values show that the interactions between Zn2+ and aminophenols under study are rather weak. For example, regarding the formation equilibriumM(OH)2 0 +L−=ML(OH)2 −,we found logK=4.6, 4.3 and 4.8, for maminophenol, 4-amino-2-hydroxytoluene, 2-amino-5-ethylphenol, respectively, at t=25 °C and I=0.1 mol L−1 in NaCl. Speciation profiles for all the systems were also calculated. In this investigation the spectrophotometric technique was used as a validating method of the speciation model calculated on the basis of potentiometric evidence and also to obtain reliable formation constant values. Suggested values, obtained by taking into account the results of both the techniques, were calculated at I=0, 0.1mol L−1 and t=25 °C.