Sulphonated alizarin fluorine blue: an improved reagent for the positive absorptiometric determination of the fluoride ion

Abstract

Alizarin fluorine blue, [3-NN-di(carboxymethyl)aminomethyl]-1,2dihydroxyanthraquinone, has been modified by the introduction of a sulphonate group into the 5-position. The complex formed by lanthanum(II1) with this new compound demonstrates a reaction towards fluoride similar to that shown by the complexes formed by lanthanum and cerium(II1) with the original alizarin fluorine blue but with the added advantage of increased solubility. This allows a metal to reagent complex ratio of 2 : 1 to be used, which gives considerable increase in sensitivity towards fluoride.ALIZARIN fluorine blue { [3-NhT-di (carboxymethyl) aminomet hyl] -1, Z-dihydroxyanthraquinone; AFB} has been used for the determination of fluoride ions in a great variety of matrices since 1959.1-3 The complex formed by lanthanum with this reagent (A,,,, 500 nm) reacts with fluoride to give a ternary complex (A,,,.567 nm) rather than undergo decomposition to give alizarin fluorine blue and lanthanum fluoride. While the reagent is highly effective at the final concentration level of 5 x M, the insolubility of the lanthanum complex in water at the operating pH of 4.5 limits the range of fluoride concentrations that can be examined. This problem can be largely overcome by operating with mixed aqueous organic solvents or at a higher pH, or both, but undesirable spectral shifts then occur, which reduce the difference between the absorption spectra given by the alizarin fluorine bluelanthanum and alizarin fluorine bluelanthanumfluoride complexes. Additionally, even at 5 x 10-5 M concentration, the presence of a buffer system that possesses mildly complexing anions such as acetate or succinate appears to be necessary in order to hold the complexes in solution and produce results of satisfactory reproducibility. Such buffers compete with alizarin fluorine blue for the metal ion and reduce sensitivity, although this effect is not serious. Numerous thorough studies of the influence of solvent, pH and reagent to metal ratio on the efficacy of the reaction have been r e p ~r t e d . ~-l ~ For many years improved versions of alizarin fluorine blue have been sought but without success. 14-16 Recently, however, we have succeeded in synthesising the alizarin fluorine blue analogue of alizarin-5-sulphonic acid, the structural formula for the potassium salt of which {potassium sulphonate; AFBS) 3-[NN-di(carboxymethyl) aminomethyl] -1,Z-dihydroxqant hraquinone-5is shown below.