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Nuclear magnetic resonance studies of some electrolyte solutions. Haque, Rizwanul

Abstract

The equilibrium constant for the dissociation of the BeF₄˭ ion in aqueous solutions of the ammonium salt has been computed by using the intensities of the fluorine nuclear magnetic resonance signals to indicate the concentrations of the three species at equilibrium. The simple dissociation may be represented as; BeF₄˭⇢ BeF₃¯+ F¯ (i) KE = 8.0 ± 0.9 x 10¯²moles (Kg)¯¹at 33°C. The equilibrium was measured over the temperature range 0-50°C but no appreciable temperature coefficient was observed. Variation of the concentration of BeF₂ solution allows the identification of further resonances, which have been associated with the unionized BeF₂ and a possible existence of a BeBeF₄ species. Chemical shift changes in the ⁹Be nuclear resonance positions are minimal. Some suggestions are made regarding the kinetics of fluorine exchange in reaction (i). The large changes in ¹¹B-¹⁹F coupling constant and ¹⁹F chemical shift of BF₄¯ ion at different concentration in aqueous and mixed solvents (acetone/water, dioxane/ water and D.M.S.O./water) have been interpreted in terms of changes in the hydrogen bonding of water to the BF₄¯ ion. The unique behavior of chemical shift and coupling constant in D.M.S.O./water has been explained by preferential solvation of BF₄¯ ion by the protolysis product of D.M.S.O. in water. The concentration dependence of the fluorine chemical shift of diamagnetic salts of fluoroanions of the type F¯, BF₄¯ and SiF₆˭ have been studies and the molal chemical shift has been correlated with the size and the charge of the cation. A displacement of the fluorine chemical shift to low field was observed for F¯, BF₄¯, BeF₄˭, PF₆¯ and SiF₆˭ aqueous solutions with paramagnetic cations Cu⁺⁺, Ni⁺⁺, Co⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺ and Mn⁺⁺. These shifts are explained on the basis of ion-pair formation between cation and fluoroanion. The molar shifts have been interpreted in terms of a contact interaction between unpared electron and ¹⁹F nucleus. The contribution due to Pseudo contact shift, which arises due to anisotropy in g-factor is probably small. The changes in ¹⁹F line width with the concentration of paramagnetic ion have been discussed in the light of exchange between freee and ion-paired fluoro-anion. Further evidence regarding the ion-pair formation in f luoro-borates has been obtained from ¹¹B chemical shift and line width measurements. ¹⁹F N.M.R. spectra of HF₂¯ ion of Na⁺, K⁺ and NH₄⁺ cations have been studied, and the equilibrium constant for the reaction HF₂ ⇢ HF + F¯ (ii) have been measured for the potassium salt. Preliminary investigations of different gallium fluoride complex in mixtures of AgF and Ga(ClO₄)₃ have been made

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