Synthesis and chromogenic properties of water-soluble hydrazones, spectrophotometric and analogue derivative spectrophotometric determination of trace amounts of iron with 2-pyridyl-3′-sulphophenylmethanone 2-pyrimidylhydrazone (PSPmH) and kinetic studies of the complex formation of iron(II) with PSPmH

Abstract

Three water-soluble hydrazones have been synthesised and their chromogenic properties and reactivities with metal ions investigated. One of them, 2-pyridyl-3-sulphophenylmethanone 2-pyrimidythydrazone (PSPmH), has been utilised for the spectrophotometric determination of iron. Iron(II) reacts with PSPmH to form a stable 1 : 2 (metal to ligand) complex having absorption maxima at 379 and 580 nm in the pH range 7.3–10.0. The apparent molar absorptivities of the complex are 4.79 × 10⁴ and 1.18 × 10⁴ l mol^–1 cm^–1 at 379 and 580 nm, respectively. A selective spectrophotometric method is proposed for the determination of iron at parts per million levels and has been applied successfully to the analysis of water samples. The sensitivity of the method can be increased significantly by employing analogue derivative spectrophotometry, iron determination at parts per billion levels being feasible. Further, the formation reaction of the iron(II)-PSPmH complex has been investigated by a stopped-flow spectrophotometric technique. The rate for the complex formation is proportional to each of the concentrations of iron(II) and PSPmH and independent of the hydrogen ion concentration. On this basis the 1 : 1 complex formation between Fe²⁺ and undissociated PSPmH is assumed to be the rate-determining step. The rate constant for the complex formation at an ionic strength of 0.2 and at 25 °C is 1.3 × 10⁵ l mol^–1. The activation parameters for the complex formation are as follows: E_a= 44.3 kJ mol^–1, ΔH^‡= 41.8 kJ mol^–1, ΔS^‡=–6.48 J K^–1 mol^–1 and Δ^‡= 43.9 kJ mol^–1.