Spectrophotometric determination of lithium ion using a water-soluble octabromoporphyrin in aqueous solution

Abstract

A water-soluble porphyrin, (2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin; H 2obtpps 4−) was synthesized and developed for the determination of lithium ion in aqueous solution. The octabromo groups lower the basicity of the porphyrin by their electron-withdrawing effect, and enable the porphyrin to react with the lithium ion in alkaline solution to form the lithium complex along with a shift of absorption maximum: λ max/nm (log ε/mol −1 dm 3 cm −1) of the lithium porphyrin are 490.5 nm (5.31) and 734 nm (4.36). Sodium and potassium ions did not react with the porphyrin. The equilibrium constant for the reaction Li ++Hobtpps 5−⇌[Li(obtpps)] 5−+H + was found to be 10 −8.80 and the conditional formation constant of the [Li(obtpps)] 5− at pH 13 is 10 4.21. The above results were applied to the determination of lithium ion in aqueous solution. The interference from transition and heavy metal ions was masked by using N, N′-1,2-ethanediylbis[ N(carboxylmethy)glycinato]magnesium(II) ([Mg(edta)] 2−) solution. Absorbance at 490 nm was measured against a blank solution. A calibration graph was linear over the range of 0.007–0.7 μg cm −3 (1×10 −6–1×10 −4 mol dm −3) of lithium(I) with a correlation factor of 0.967. Lithium ion less than ppm level was determined spectrophtometrically in aqueous solution. The proposed method was applied to the determination of lithium in human serum and sea water samples.