Influence of pH, surfactant and synergic agent on the luminescent properties of terbium chelated with benzoic acid derivatives in aqueous solutions

The luminescent properties of terbium chelated with o- and p-hydroxybenzoic and o- and p-aminobenzoic acids were investigated. The four benzoic acid derivatives have equivalent triplet-state energy levels and the efficiency with which terbium luminescence is sensitized is related to the ability of the acid to bind the lanthanide ion. When only the carboxylic group is dissociated, in neutral or slightly alkaline solutions, terbium luminescence is moderate. The luminescence is appreciably enhanced when the -OH or -NH2 groups are in the ortho position with respect to the carboxylic group, but vanishes almost completely when EDTA is added to the solution. Formation of a ternary chelate between terbium, EDTA and the acid is possible only with o-hydroxybenzoic acid when the phenol group which is adjacent to the carboxylic group is also dissociated. This arises in alkaline solutions in the presence of cetyltrimethylammonium chloride and contributes to an increase in sensitivity of one order of magnitude.