Synthesis and complexing properties of N-sulphonated 2-(2-thiazolylazo)-5-aminophenols and their application to the determination of iron in river waters

Abstract

Five N-sulphoalkyl derivatives of 2-(2-thiazolylazo)-5-aminophenol were synthesised and their analytical potential as water-soluble chromogenic reagents was studied spectrophotometrically. The acidity constants and complexing reactions with 3d type metal(II) ions were examined, and the relationship between their properties and the position of the substituents is discussed. These sulphonated derivatives react sensitively with 3d type metal(II) ions to form brownish water-soluble complexes and iron(II) complexes show a characteristic absorption band in the near-infrared region. By utilising this absorption, a simple and selective spectrophotometric method for the determination of iron has been developed. Among the reagents, 2-(4-methyl-2-thiazolylazo)-5-[N-(sulphopropyl)amino]phenol is the most suitable and the iron(II) complex has an absorption maximum at 745 nm, with a molar absorptivity of 2.82 × 10⁴ l mol^–1 cm^–1. The optimum pH for iron complex formation lies between 5.0 and 10.7 and Beer's law holds up to 1.6 p.p.m. of iron. Many ions commonly present in natural waters can be tolerated and the proposed method has been applied to the determination of iron in river waters and Japanese sakés with satisfactory results.