SOLVENT DONICITY INFLUENCE IN THE FLUORESCENCE EMISSION OF THE LITHIUM 1,4-DIHYDROXYANTHRAQUINONE SYSTEM - IMPROVEMENT IN THE QUANTIFICATION OF LITHIUM/

The fluorescence emission from the lithium/1,4-dihydroxyanthraquinone system shows a great enhancement in the presence of certain water-misc ible solvents. This is justified from the donicity (nucleophilic prope rties) of the solvent that facilitates the solvation of the lithium ca tion in solution and the stabilization of an nondissociated ion-pair b etween the solvated-lithium cation and the 1,4-dihydroxyanthraquinonat e anion. A very sensitive analytical method was proposed for the spect rofluorimetric determination of lithium based on its reaction with 1,4 -dihydroxyanthraquinone (quinizarin) in a dimethylsulfoxide medium (90 %) and in presence of sodium hydroxide. The fluorescence is measured a t an excitation wavelength of 602 nm and an emission wavelength of 670 nm and it is stable at 25 degrees C at least 6 h. The calibration cur ve is linear over the concentration ranges of 2-40 mu g/l of lithium i n an aqueous matrix and 3-50 mu g/l in a serum matrix; the RSD's in th e determination of 20 mu g/l of Li+ were 2.6% and 3.2%, respectively. The proposed procedure was satisfactorily applied to the determination of lithium in drugs, dietetic products and human serum.