New highly selective and sensitive extraction-photometric method for the determination of nickel

The interaction of nickel with azomercaptophenols (AMP, H2L) {1-(2-pyridylazo)-2-hydroxy-4-mercaptophenol (PHMP), 1-(5-chloro-2-pyridylazo)-2-hydroxy-4-mercaptophenol (CHPMP), 1 -(5-bromo-2-pyridyl-azo)-2-hydroxy-4-mercaptophenol (BPGMF), 1-(5-iodo-2-pyridylazo)-2-hydroxy-4-mercaptophenol (IPGMP)} in the presence of aminophenols (AF): 2-(N,N-dimethylaminomethyl)-4-methylphenol (AFi), 2-(N,N-dimethylaminomethyl)-4-chlorophenol (AF2). The synthesized compounds (PHMP, HPMP) were characterized by IR and NMR spectroscopy. AMP - yellow-green crystalline substances, poorly soluble in water, in acids, partially soluble in alkalis, acetone, well soluble in chloroform. Depending on the acidity of the medium, AMP can exist in the form of three forms: H2R, HR^-, HR^2-. AMP and its derivatives form colored complexes with nickel, which are in soluble in nonpolar organic solvents. The charge of the complexes was established by the method of electro-migration of ions and by the method of electrophoresis. Colored complexes are anions. When amines are introduced into the system, the transition of anionic complexes to the organic phase in the form of a mixed ligand complex (RLC) is observed. The best extractants were chloroform, dichloroethane and carbon tetrachloride. A single extraction with chloroform yields 97.1-98.9% nickel(II) in the form of RLA. The optimal range of acidity, at which the optical density is maximum and constant, is at pHop. 2.4-6.5 (pNob. 0.7-8.4). RLC Ni(II) with AMP and AF are stable in aqueous and organic solvents and do not decompose for three days, and after extraction for more than a month. The maximum optical density is reached within 8-12 minutes. The optimal condition for the formation and extraction of these compounds is (1,12-2,34) х 10 ³mol/l concentration of AMP and (6.3-8.4)x10^-4mol/l - AF. An increase in the aqueous phase by 20 times relative to the organic phase does not affect the completeness of the extraction. The maximum analytical signal during the complex formation of nickel with AMP and AF is noticeable at 605-648 nm. AMP is maximally absorbed at 515-530 nm. During complex formation, a bathochromic shift of the light absorption maximum by 110-118 nm is observed. The stoichiometry of the studied complexes was established by the methods of equilibrium shift and relative yield. In the composition of RLC, 1 mole of nickel accounts for 2 moles of AMP and AF. It was established by the Nazarenko method that the complexing form of nickel is Ni²⁺. In this case, the number of protons displaced by it from one H2L molecule turned out to be equal to 2. The performed calculations showed that the RLC in the organic phase do not polymerize and are in the monomeric form (γ=0.94-1.07). A thermogravimetric study of the [Ni(PHMP)2](AP2)2 complex showed that its thermal decomposition proceeds in three stages: at 60-120°C water escapes (mass loss - 2.14%), at 340-390°C decomposes AP (mass loss 38.48%), and at 490-510°C, PHMP decomposes (mass loss 59.45%). The end product of the thermolysis of the complex is NiO. RLC Ni(II) extracts obey the basic law of light absorption at concentrations of 0.518 pg/mL. The determination of Ni(II) with AMP and AF is not interfered with by ions of alkaline, alkaline earth elements and REE. The interfering effect of ions is eliminated by changing the pH of the medium, using masking agents and using extraction. Comparison of the analytical capabilities of the studied reagents and hydrophobic amines shows that the contrast and sensitivity of the reaction decreases in the series CPGMP -BPGMP - IPPGMP - PHMP. The results of studies on the formation and extraction of RLA Ni(II) with AMP and AF, the physicochemical and analytical characteristics of these compounds served as the basis for the development of new methods for the extraction-photometric determination of Ni(II) in various natural materials. Contribution of the authors: the authors contributed equally to this article. The authors declare no conflicts of interests.

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