Synthesis and chromogenic properties of water-soluble substituted 2-pyridyl-3'-sulphophenylmethanone 2-pyridylhydrazones and spectrophotometric and analogue derivative spectrophotometric determination of trace amounts of cobalt with 2-pyridyl-3'-sulphophenylmethanone 2-(5-nitro)pyridylhydrazone

Three water-soluble hydrazones, in which the 5-position of the pyridine ring in the hydrazine moiety of 2-pyridyl-3'-sulphophenylmethanone 2-pyridylhydrazone (PSPH) was substituted a methyl, chloro or nitro group, have been synthesised and their chromogenic properties and reactivities with metal ions investigated. The acid dissociation constants (K_a) of the synthesised hydrazones were determined spectrophotometrically. These values obeyed Hammett's law, the inductive effect of the substituents being found to reflect the pK_a values. Of the synthesised hydrazones, 2-pyridyl-3'-sulphophenylmethanone 2-(5-nitro)pyridylhydrazone (PSNPH) is the most sensitive for cobalt(II) determination. It reacts with cobalt(II) to form a 1:2 (metal to ligand) complex. This complex is rapidly oxidised above pH 2 to give a stable cobalt(III) complex with an absorption maximum at 496 nm. Beer's law is obeyed over the range 0.05-1.0 μg ml^-1 of cobalt and the apparent molar absorptivity of the complex is 5.69 × 104 l mol ^-1 cm^-1 at 496 nm. A sensitive and practical spectrophotometric method for the determination of trace amounts of cobalt with PSNPH is proposed and has been successfully applied to the determination of cobalt in steel samples. The method may be made more sensitive by employing an analogue derivative spectrophotometric technique.