A facile, rapid and sensitive spectrophotometric method for the determination of zinc is performed, based on complexation reaction between the metal ion and 1,5-diphenyl-3-acetylformazan (I) 1-(o-carboxyphenyl)-3-acetyl-5-acetylformazan (II), 1-(o-carboxyphenyl)-3-acetyl-5-phenylformazan (III), and 1-(o-carboxyphenyl)-3-acetyl-5-m-tolylformazan (IV) in the presence of cationic surfactant cetylpyridinium chloride (CPC). The important analytical parameters and their effects on the reported system are investigated. Zinc reacts with the reagents (I-IV) and CPC in the ratio 1:1:2 (metal:reagent:CPC) in the pH range 8.5, 7.5, 5.5 and 6.5 to form a ternary complex with an absorption maximum 616, 656, 672 and 599 nm, respectively. The reaction was extremely rapid at room temperature, and the absorbance value remains unchanged for at least 1 week. The apparent stability constant of the complex were found to be 13.1 9.2, 11.4 and 12.3, and adheres to Beer's law for 0.05-3.50 microg per 10 ml of zinc. For more accurate analysis, Ringbom optimum concentration range was found from 0.08 to 3.20 microg per 10 ml of zinc. The apparent molar absorptivity, Sandell sensitivity, detection and quantification limits are also calculated. Taking a constant concentration of metal ion and determining its concentration in the presence of large number of foreign ions tested the effect of foreign ions. The method was applied for determination of zinc in serum, human hair and pharmaceutical formulations, where excellent agreements between reported and obtained results were achieved. The relative standard deviation was better than 1.67%.
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