The purpose of this work is to develop a highly selective method for the photometric determination of iron (III) in environmental objects, in particular, in oil and oil products using hydroxyhalogenthiophenol (H₂R) - 2-hydroxy-5-chlorothiophenol, 2-hydroxy -5-bromothiophenol and 2-hydroxy-5-iodothiophenol and hydrophobic amines - 1,10-phenanthroline, α,α'-dipyridyl, diphenylguanidine o-, m-, p-toluidine, o-. m-, p-xylidine. It has been established that the reagent is a dibasic acid and, depending on the acidity of the medium, can be in molecular (H₂R) and anionic (HR^- and R^2-) forms. For 2-hydroxy-5-chlorothiophenol: pK₁=5.1; pK₂=10.6; 2-hydroxy-5-bromothiophenol: pK₁=5.05; pK₂=10.4; 2-hydroxy-5-iodothiophenol: pK₁=5.0; pK₂=10.2. The dependence of the ionization constants (pKa) H₂R on the ionic strength of the solution pK_n= f(μ) in the range μ = 0 – 1 is linear and is described by the equations: 2-hydroxy-5-chlorothiophenol: pK₁=5.2762 – 0.5506√μ, pK₂=10.7289 – 0.4028√μ; 2-hydroxy-5-bromothiophenol: pK₁ = 5.2643 – 0.6697√μ, pK₂=10.5341 – 0.4191√μ; 2-hydroxy-5-iodothiophenol: pK₁=5.1483 – 0.4634√μ; pK₂=10.3621–0.5066√μ. The effect of the concentration of reacting components, time, and temperature on the formation of mixed-ligand complexes was studied, and their stoichiometry was determined by various methods. It has been established that an extractable compound is formed with a ratio of components of 1:2:2. The complex includes Fe³⁺ and Fe(OH)²⁺ ions. The charge of iron (III) complexes with H₂R was determined by electromigration and ion-exchange chromatography. The molar absorption coefficient of the complexes is (3.0-4.5)×10⁴ (λ = 545–595 nm). The calibration curve is linear in the iron (III) concentration range of 0.40–22.5 mg/ml. The detection limit for Fe(III) at Р = 0.95 is 0.011–0.017 mg/ml. The developed technique was applied to determine the trace amounts of iron in oil and oil products

doi.org/10.32737/0005-2531-2023-3-126-133