Synthesis of meso-tetra-(3,5-dibromo-4-hydroxylphenyl)- porphyrin and its application to second-derivative spectrophotometric determination of lead in clinical samples

Abstract

A new very sensitive and selective chromogenic reagent, meso-tetra-(3,5-dibromo-4-hydroxylphenyl)porphyrin [T(DBHP)P], was synthesized and studied for the determination of trace lead in detail. In 0.10 mol l^–1 NaOH medium, lead reacts with T(DBHP)P to form a 1∶2 yellow complex, which gives a maximum absorption at 479 nm; 0–0.48 µg ml^–1 Pb(II) obeyed Beer’s law. The molar absorptivity of the complex and Sandell’s sensitivity are 2.5 × 10⁵ l mol^–1 cm^–1 and 0.000812 µg cm^–2, respectively. Second-derivative spectrophotometry is better than conventional spectrophotometry in sensitivity and selectivity, and its limit of quantification, limit of detection and relative standard deviation are 0.70 ng ml^–1, 0.21 ng ml^–1 and 1.0%, respectively. Ca (3250-fold), Mg (2000-fold), Sr (1000-fold), Ba (750-fold), Al (1000-fold), Bi (500-fold), Fe (2000-fold), Co (750-fold), Ni (1000-fold), Cu (750-fold), Zn (1250-fold ), Cd (2500-fold) and Ag (550-fold) do not interfere with the determination of lead. The chromogenic system is remarkably superior to other reagents, especially porphyrin compounds. The influence caused by oxygen in air or in solution can be easily eliminated by adding Na₂SO₃. The reaction is very stable, the stability constant of the complex being 1.2 × 10⁴⁵. The chromogenic reaction is completed within 1 min at room temperature when 8-hydroxylquinoline is used as catalyst. The proposed method has been applied to the direct determination of trace lead in clinical samples. The accuracy and precision are both very satisfactory.