Spectrophotometric and analogue derivative spectrophotometric determination of trace amounts of iron using sulphonated 5-(3,4-dihydroxyphenyl)-10,15,20-triphenylporphine

Abstract

Sulphonated 5-(3,4-dihydroxyphenyl)-10,15,20-triphenylporphine (SDHP), a water-soluble porphyrin with the ability to complex with metal ions not only inside but also outside the porphine ring, has been synthesised. The analytical applications of the porphyrin, its chromogenic properties and reactivity with metal ions have been investigated. It was found that SDHP forms complexes with metal ions classified as so-called hard acids, such as aluminium(III), iron(III) and tungsten(VI), and also with those classified as soft or border-line acids, such as cadmium(II), copper(II) and lead(II), all these complexation reactions proceeding rapidly at room temperature. As an example, the iron(III)-SDHP system has been studied in detail spectrophotometrically. SDHP reacts with iron(III) to form a 1 : 1 (metal : ligand) complex with a Soret band at 430 nm at pH 4 and a 1 : 2 complex with a Soret band at 415 nm at pH 7. Spectrophotometric and analogue derivative spectrophotometric methods are proposed for the determination of iron at ng ml^-1 levels utilising these complexations. The apparent molar absorptivities were 1.09 × 10⁵ and 5.37 × 10⁵ mol^–1 cm^–1 for the methods utilising the 1 : 1 and 1 : 2 complex formations, respectively.