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A water-soluble porphyrin (2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin ; H2obtpps4-; H2P4-) was synthesized and developed for the determination and separation 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 lithium ion in alkaline aqueous solution to form the lithium complex along with a shift of absorption maxima; λmax (log ε/mol-1 dm3 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+ + HP5- ⇌ [LiP]5- + H+ was found to be 10-8.79 and the conditional formation constant of the [LiP]5- at pH 13 is 104.21. The [LiP]5- can be extracted into chloroform as an ion-pair complex with tetrabutylammonium ion (X+) and the extracted X5LiP dissociates to X4LiP− and X+ in chloroform. The extraction constant for the reaction of [LiP5-]a + 5[X+]a ⇌ [X4LiP−]o + [X−]o was found to be (8.4 ± 0.7) × 1012 mol-4 dm12, where subscripts of a and o denote chemical species in aqueous and organic phases, respectively. The above results were developed for the determination of lithium in serum, sea water and hot spring water samples at a range of 0.07–0.7 mg dm-3 (1 × 10-5 - 1 × 10-4 mol dm-3). The interference of heavy metal ions was masked by N,N‘-1,2-ethanediylbis[N-(carboxylmethyl)glycinato]magnesium(II) ([Mg(edta)]2- or H4edta if sample contain magnesium(II) ion.