Abstract
Morin (5, 7, 2′, 4′-tetrahydroxyflavonol) forms a stable chelate with thorium(IV). The chelate showed a constant fluorescence intensity (λex 415nm, λem 505nm) in acidic solutions, pH 2.2. The stoichiometric composition of the chelate was 1:1 (thorium:morin). In the presence of arsenic(V), the chelate dissociated, owing to the formation of a more stable complex of thorium arsenate. By using the experimental result that the fluorescence intensity of thorium-morin chelate decreased in proportion to the concentration of arsenic(V), 0.01-1ppm of arsenic(V) could be determined. In the reaction, arsenic(III) did not form a thorium arsenite complex. Accordingly, arsenic(V) in the mixture of arsenic(III) and arsenic(V) was accurately determined by the fluorescence quenching method.
