Conductivity, spectroscopic and thermogravimetric studies in some low melting eutectics

Eweka, Imasuen Emmanuel (1991) Conductivity, spectroscopic and thermogravimetric studies in some low melting eutectics. University of Southampton, Doctoral Thesis.

Abstract

Thirteen eutectics (viz: [1] NaNO₃ - KNO₃ - NaNO₂ (m.p. 143 ± 2^oC), [2] LiNO_3 - KNO_2 (108^oC), [3] LiNO_3 - KNO_3 (124^oC), [4] LiNO_3 - NaNO_3 - KNO_3 (118^oC), [5] SnCl_2 - NaCl (186^oC), [6] SnCl_2 - KCl (210^oC), [7] NH_4Cl - CuCl (140^oC), [8] KI - CdBr_2 (204^oC), [9] acetamide - KNO_3 (77.3^oC), [10] butyramide - NaNO_3 (104 ± 2^oC), [11] acetamide - KI (55^oC), [12] acetamide - NaI (50^oC), [13] acetamide - LiI (41^oC) were chosen on the basis of criteria. Of these the last four (composition, 5.5 mole % NaNO₃, 13 mole % KI, 12 mole % NaI and 13.5 mole % LiI respectively) are new and their melting points were determined by visual observation and cryoscopy. Whilst those of the first nine were redetermined cryoscopically. The densities of [11] to [13] were measured over a temperature range and shown to vary linearly with temperature. [9] and [10] were measured at specific temperatures. Electrical conductivity was measured, usually from 30^oC below to 30^oC above the melting points, over complete cooling and heating cycles. Conductivity plots had quite similar shapes for all the eutectics studied, and showed a sharp rise or fall in conductivity before the melting points, i.e. premelting with hysteresis of varying magnitudes between rising and falling temperature curves and a continuation of `liquid like' conductivities below the melting points, (more noticeable for cooling measurements). The differences compared with the analogous measurements on single component salts were explained on the basis of the larger interfacial area between solid phases (and hence disorder) in the eutectics compared to their component salts. Thermogravimetric analysis of Ni(II), Co(II), Cr(III), Cr(IV), Cu(II) and Fe(III) salts in NaNO₃ - KNO₃ - NaNO₂ eutectic showed the anion reacting to be nitrite. Finally spectroscopic studies on solutions of Ni(II), Cr(III) and Cr(VI) in NaNO₃ - KNO₃ - NaNO₂, of Ni(II) in acetamide - NaI, of Cr(III), Ni(II), Co(II), Cr(VI) Cu(II) and Fe(III) in acetamide - KI, Ni(II), Cr(VI), Cr(III), Co(II), Cu(II) and Fe(III) in acetamide - KNO₃ and of Cr(III), Cr(VI), Ni(II) and Co(II) in butyramide - NaNO₃ eutectics, showed that in all these melts Ni(II) and Cr(III) were octahedrally coordinated, in some cases by mixed ligands. A distorted octahedron was found for Cu(II) in molten acetamide - KNO₃, while Cu(II) and Fe(III) were reduced to Cu(I) and probably Fe(II) respectively, liberating iodine in acetamide - KI. Fe(II) probably formed an unstable mixed Cl^-/acetamide/NO₃^- tetrahedral complex in acetamide - KNO₃. At moderate temperatures (i.e. 60 - 75^oC), Co(II) was octahedrally coordinated, possibly also by mixed ligands, in molten acetamide - KI, while at higher temperatures an octahedral - tetrahedral equilibrium was proposed. Above 90^oC the major Co(II) species present was the tetrahedral complex [Co(acet.)(I)₃]^1-. However, in acetamide - KNO₃ and butyramide - NaNO₃, Co(II) was always octahedrally coordinated.

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