Aqueous solutions of cobalt(II) nitrate with 1 : 15, 1 : 25, and 1 : 40 mole ratios at 298 and 323 K were studied by the X-ray diffraction. As the solution concentration decreases, ion triads (1 : 15) pass into contact ion pairs (1 : 25) and then to independently hydrated ions (1 : 40). The structural variations are accompanied by increase in a number of water molecules in the second coordination sphere of the cations and in the hydration shell of the anions. Heating produces destruction of ion triads in the 1 : 15 solution and ion association in the 1 : 40 solution. The structure of all the solutions at 323 K is characterized by a system of contact ion pairs. The structure of concentrated aqueous solutions of electrolytes attracts considerable attention of researchers involved in the fundamental and applied science. Correlations of the trend in structure formation in aqueous electrolyte systems with the chemical nature of ions and solution concentration have been revealed [1]. Recently polythermal studies of such solutions have gained special importance [234]. To obtain statistical data on quantitative characteristics of aqueous electrolyte solutions and to extend the body of experimental information, in the present work we performed an X-ray diffraction study of aqueous solutions of cobalt(II) nitrate at various mole ratios and temperatures. The Co + ion is a complex-forming ion, and a series of X-ray researches have been devoted to its complex formation in solutions of cobalt(II) chlorides and bromides. The aim of the present work was to study structural aspects of ion hydration in solutions, and, therefore, we chose nitrate ions as counterions not inclined to complex formation.In early works [5, 6] on the basis of measuring viscosity of aqueous CoSO 4 solutions it was noted that Co 2+ ions reinforce water structure in a wide concentration range. Kinetic studies on oxygen exchange between nitrate ions and water molecules in aqueous solutions of Co(NO 3 ) 2 at concentrations close to the boundary of complete hydration revealed a twolayer hydrate surrounding [7,8]. In the crystal hydrates Co(NO 3 ) 2 . 2H 2 0 and Co(NO 3 ) 2 . 4H 2 0, cobalt ions are surrounded by six oxygen atoms at distances of 0.205130.2124 nm, of which two atoms belong to nitrate ions (0.206030.2097 nm) and the other two in the first case and four in the second, to water molecules [9]. The octahedral coordination of water molecules around the cation in concentrated aqueous solutions of cobalt(II) halides [10312] and sulfate [13] was revealed by X-ray diffraction analysis, the Co 2+ 3 OH 2 distances varying within 0.20730.216 nm. The formation of contact ion pairs by counterions in CoCl 2 solutions of various concentrations was established [11,14]. Aqueous solutions of CoCl 2 : nH 2 O (n = 6, 4, 2) at various temperatures (298, 313, 328, and 343) were studied by X-ray diffraction. In octahedral cobalt(II) complexes, the Co3O and Co3Cl distances were found to be 0.211 and 0.240 nm, respectively [15]. The number of Co3Cl contacts in saturated...
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