Extraction-spectrophotometric study on the chromium (VI)-2-hydroxy-5-bromothiophenol-hydrophobic amines-water-chloroform system and its analytical application

The present work is devoted to the study of the conditions for the interaction of chromium (VI) with 2-hydroxy-5-bromothiophenol (HBTP). In the presence of hydrophobic amines (Am), these compounds are extracted into the organic phase in the form of a Mixed-ligand complexes (MLC). Of the hydrophobic amines (Am), N, N-dimethylaminomethyl -4-methylphenol (Ap1) and N, N-dimethylaminomethyl -4-chlorophenol (Ap2) were used. To determine the state of chromium oxidation in MLC, two series of experiments were carried out. In the first series, Cr (VI) was used, and in the second series, Cr (III) was used, obtained by adding an additional reducing agent (SnCl2 or KI). It was found that the spectrophotometric characteristics of MLC Cr (VI) and Cr (III) are identical, that is, in interaction with HBTP, Cr (VI) were reduced to Cr (III). Extractibility of complexes was estimated in coefficient of distribution and extent of extraction. After a single extraction with CH3Cl 98.5% of chromium was extracted as an ion associate. The absorption maxima of the ternary Cr (III)-HBTP-Am complexes lie in the range of 465-470 nm. Molar coefficients of absorption make (2.92-3.28)×104 L mol–1 cm–1. Optimum conditions for the formation and extraction of MLC were found: {The maximum absorption of the extracted complex (in 5 ml of chloroform) is achieved when the pH of the aqueous phase is in the range 4.0-5.3, and the reagent concentrations (1.2-4.0)×10-3 mol / ml of HBTP and (0.92 - 4.0)×10-3 moles / ml Am. Our experiments are performed at pH = 4.5 in the presence of 1.6×10-3 mol / ml HBTP and 1.5×10-3 mol / ml at. The volume of the aqueous phase is 20 ml and contains 20 μg of Cr (III). Phase equilibrium is achieved after 90 s, but the extraction is carried out for 3 min in all experiments. Absorption of the work piece remains constant for 15 minutes}. It was found using the Nazarenko method that Cr (III) in the complexes was present in the form of Cr3+. The number of protons replaced by chromium in one HBTP molecule appeared to be one. Starik-Barbanel relative yield method, equilibrium shift method, crossed lines method and Asmus’ methods were employed to elucidate the composition of the complex. The results show that the molar ratio of Cr: HBTP: Am is 1: 3: 3. The effect of a diverse set of ions on the determination of chromium (III) with HBTP and Am is studied at pH 4.5 in the presence of 15 μg Cr (III) in 20 mL of an aqueous solution. The following ions (in n-fold excess) do not interfere: Cd (II) (435), Cu (II) (80), Ce (III) (125), Zn (II), Co (II) and HPO42- 1200, V (IV,V) and Ni (II) (130), Mo (VI) (10), Fe (II) (40), W (VI) (13), Mn (II) (30), Al (III), (120). A linear calibration graph drawn between absorbance and the metal ion concentration indicates that Cr (III) may be determined in the range 0.5-18 μg mL-1. Table 2 summarizes the calibration characteristics obtained with Cr (III)-HBTP-Ap1 and Cr (III)-HBTP-Ap2. The proposed method under the already established optimum conditions was applied for the determination of Cr (III) in alloys, in sewage water and Bottom sediments and in soils.