Chromotropic acid, a fluorogenic chelating agent for aluminium(III)

Complexation of aluminium(III) with the fluorogenic ligand chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonic acid) has been revisited with the aim of using enhancement of the fluorescence intensity as an analytical tool. Complexation at the optimum pH approximately 4 was shown to lead to a 1:1 complex with a stability constant log beta110=18.4 +/- 0.7. The fluorogenic effect was thoroughly investigated. Nearly selective excitation of the chelate rather than the ligand could be achieved at wavelengths longer than 360 nm. For analytical purposes the main interfering ion was Ga3+. The strongest competing ligand was shown to be citric acid. Competitive complexation by acetate or formate ions can also make their use in a buffer at the usual concentration, 0.2 mol L(-1), questionable, whereas a 10(-2) mol L(-1) formic acid buffer was shown to be a good alternative. The calibration plot showed that the dependence of response on Al(III) concentration was linear up to 500 microg L(-1); the detection limit was 0.65 microg L(-1) (3 SDblank, n=10, SD= +/- 1.4% at 10 microg L(-1) and +/- 0.8% at 100 microg L(-1)). The analytical procedure was successfully applied to several samples of tap water and the results were in good agreement with those from AAS determination.