The Schiff bases of salicylaldehyde and aminopyridines as highly sensitive analytical reagents

The analytical applicability of heteroaromatic Schiff bases, 2-(3-pyridylmethyliminomethyl)phenol (compound 1), 2-(2-pyridyliminomethyl)phenol (compound 2), 2-(2-amino-3-pyridyliminomethyl)phenol (compound 3), N,N′-bis(salicylidene)-2,3-pyridinediamine (compound 4), N,N′-bis(salicylidene)-2,6-pyridinediamine (compound 5) and 2-(2-amino-4-methox-ymethyl-6-methyl-3-pyridylmethyliminomethyl)phenol (compound 6), was proved. The investigation included a study of the characteristics that are essential for solvent extraction and for the spectrophotometric and spectrofluorimetric determinations of copper. Despite structural similarities, the investigated Schiff bases exhibited different behaviour in solution. Compounds 2–5 adopted predominantly the enolimino form in most solvents, whereas compound 1 was partially present as ketoamine in the solutions of polar solvents. Compound 6 was found to be the tautomeric mixture of cyclic diamine and enolimine in most solvents. All the investigated Schiff bases were capable of quantitatively extracting transition metals into chloroform. In the case of Cu(II), compound 4 proved to be an effective extractant in the range 5 < pH < 12, compounds 1 and 6 were effective in the range 6 < pH < 12, whereas compounds 2, 3 and 5 were effective only in the alkaline medium above pH 9. The spectrophotometric determination of Cu(II) after extraction was very sensitive and selective with regard to Cd(II) and Pb(II). Highest sensitivity was achieved with compound 4 (ϵ430 = 2.1 × 104 mol−1 1 cm−1). This characteristic was favourable compared to other well-known copper reagents. Since the fluorescence of Schiff bases was quenched by Cu(II), Mn(II), Fe(III), Ni(II) and Cr(III), the sensitivity as well as the selectivity were further enhanced using the spectrofluorimetric procedure. Additional advantages of these compounds to other reagents for copper(II) are simple preparation and a detailed knowledge of their equilibria.