Application of N,N′-ethylenebis(acetylacetoneiminato) Nickel (II) and Copper (II) Schiff Base Complexes as Acid–Base Sensing Materials and Indicators in Volumetric Titrations : Qualitative, Spectroscopic and Titrimetric Analyses

Abstract

The objective of the research described herein is to report the applicability of simple Schiff base complexes, [M(L)], where M = Ni(II), Cu(II); L = N,N′-ethylenebis(acetylacetoniminate), as sensing materials and indicators in qualitative, spectroscopic and volumetric determinations of acids and bases. In this regard, UV–Vis spectroscopic properties of [M(L)] in the presence of different acids (−0.25 < pKa < 5) were investigated, and acid–base titrations using these complexes as indicators were carried out. Alcoholic solutions of [Cu(L)] and [Ni(L)] are purple and orange-red, respectively, and both solutions become decolourized upon adding strong acids (pKa < 2). During this process, the intensity of the original d–d band (λ_max ~ 550 nm) decreases, and new anion-dependent CT (λ_max > 700 nm) and anion-independent CT (λ_max ~ 310 nm) bands, accompanied by three isobestic points, emerge. Addition of NaOH to the same solution, reproduces both the original colours and UV–Vis spectra, implying that the acid–base chemistry of [M(L)] is reversible. No such colour change occurs upon adding weak acids (pKa > 4), hence [M(L)] can be used as sensing materials for distinguishing strong acids from weak acids. In strong acid-weak base titrations, [M(L)] complexes are better indicators than methyl orange, as they give very sharp and clearly visible colour changes at the end point. The present paper reports, for the first time, the applicability of N,N′-ethylenebis(acetylacetoneiminato) nickel (II) and copper (II) Schiff base complexes as acid–base indicators in strong acid-weak base titrations where the pH at the equivalence point is ~4.