Molten Alkali Metal Nitrates Research Articles

Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 2.—A thermodynamic investigation of the chloride, bromide and iodide systems

The complex formation between lead (II) ions and chloride, bromide and iodide ions in molten equimolar (K, Na) NO3 has been studied. The systems were investigated at four temperatures between 240 and 300 °C. The iodide system was only studied at one temperature, since I– is oxidized to I2 in melts rich in Pb(NO3)2. Halide-ion activities were measured in the systems (K+, Na+, Pb2+)–(NO–3, X–), X–= Cl–, Br– and I–, by means of Ag/AgX electrodes. The formation of PbX2, PbX+ and Pb2X3+ was observed. Special interest was focused on the formation of cationic complexes with anions as coordination centres. The thermodynamic parameters ΔH°mn and ΔS°mn have been evaluated for the chloride and bromide systems. The results are : ΔH°11=– 7.9 kJ mol–1, ΔS°11= 26.3 J K–1mol–1 for Pb2++ Cl –→ PbCl+; ΔH°11=–4.6 kJ mol–1, ΔS°11= 35.2 J K–1 mol–1 for Pb2++ Br–→ PbBr+; ΔH°21= 1.5 kJ mol–1, ΔS°21= 16.1 J K–1 mol–1 for PbCl++ Pb2+→ Pb2Cl3+; ΔH°21=–7.1 kJ mol–1, ΔS°21= 1.9 J K–1 mol–1 for PbBr++ Pb2+→ Pb2Br3+. The parameters are compared with previous literature data and recent results obtained from the lead (II) fluoride system in (K, Na)NO3(l).

Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 1.—A thermodynamic investigation of the fluoride system

The complexation of fluoride ions with lead (II) ions in molten equimolar (K, Na)NO3 has been investigated in the temperature range 240–280 °C. Complexes with formal compositions PbF2, PbF+ and Pb2F3+ were identified and the stability constants for these species were determined at five temperatures from fluoride activity measurement in the systems (K+, Na+, Pb2+)–(NO–3, F–) by means of a fluoride ion-selective electrode. ΔH°mn and ΔS°mn for the stepwise formation of PbmF2m-nn have been estimated, yielding : ΔH°12=– 7.0 kJ mol–1, ΔS°12= 38.3 J K–1 mol–1 for PbF++ F–→ PbF2; ΔH°11=– 15.3 kJ mol–1, ΔS°11= 30.0 J K–1mol–1 for Pb2++ F–→ PbF+; ΔH°21=– 5.9 kJ mol–1, ΔS°21= 1.9 J K–1 mol–1 for PbF++ Pb2+→ Pb2F3+. The thermodynamic results deviate from what is predicted by a quasi-lattice ideal configuration. These deviations are assumed to arise from coordination effects between lead (II) and nitrate and Pb–Pb interactions in Pb2F3+. The solubility of PbF2 in excess of Pb(NO3)2 in (K, Na)NO3(l) was measured at 280 °C. The complexation model derived from the e.m.f. measurements in dilute melts, expressing all activities in mole-fraction units, does accurately describe the solubility of PbF2 in the range CPbCF. In melts with an excess of fluoride over lead(II) the model fails though, probably because polynuclear, i.e. polyfluoric, species are formed.

Establishment of an Acidity‐Potential Diagram Through Use of an Electrode with a Solid Membrane Selective of Oxide Ions in Molten Alkali Metal Nitrate

The object of this study is to establish a practical acidity‐potential diagram in molten alkali nitrates at 475°C in order to delimit the zone of acidity of a molten bath in which glass protection would be ensured against surface alterations which may occur in the course of ion exchanges.

The Raman Spectrum of Mercury (II) Iodide in High Temperature Solvents

Abstract The Raman spectrum of mercury(II) iodide was observed in cesium nitrate at 430°C and in meta- and para-terphenyl at 235 and 240°C, respectively. A single strong polarized line was found at 148. 5 cm−1, width at half height 15 cm−1 in cesium nitrate, and a single line at 154. 0 cm−1 width at half height 7 cm−1 in the terphenyls. The observed spectra are not consistent with interactions of the solute and the solvents involving bonds of highly covalent character, but do not exclude other interactions. In the course of an investigation on the nature of mercury (II) iodide species in solution in molten alkali metal nitrates, by its distribution between the salt melts and terphenyl melts1, 2, it became of interest to study by means of Raman spectroscopy the mercury (II) iodide species formed. Several such studies at lower temperatures have already been made: in the gas phase3, in a krypton matrix4, in alcohols5, tributy phosphate6 and dioxame7, and in molten mercury(II) iodide8, chloride9, and bromide9. The reason for looking at the Raman spectrum in yetfurther media was the suggestion made on the basis of thermodynamic and kinetic data2,10 that the mercury(II) iodide species in the alkali mitrate melts are solvated by nitrate anions, and that possiby the mixed anion terrahedral species HgI2(NO3)2 is formed. Recent Ranan sepctrophotometric data on mixed halide anionic complexes of mercury11 identified prominent lines of the spectrum of the species HgBrnI2– 4–n, including HGBr2I2 2–, so that a comparison could be made. The solubility of mercury (II) iodide in molten alkali metal nitrates is rather small, expect for cesium nitrate12, Where the solubility should be sufficent for the Raman spectrum to be recorded. Also, it was comcluded from vapor pressure osmometric data in aromatic solvents that demiric species of mercury(II) halides (even a trimeric species of the iodide) are found, in which the mercury has a distorted octahedral coordination, with three halogen atoms bonded to a mercury atom in the dimer13. In terphenyl melts, the solubility

Effect of pressure on the diffusion coefficient of silver ions in molten alkali metal nitrates

The interdiffusion coefficient of silver ions (DAg+) in molten NaNO3, KNO3, RbNO3 and CsNO3 has been measured at temperatures in the range 623–723 K and at pressures up to 1 kbar (1 bar = 105 N m–2), using the chronopotentiometric method. The volume ΔVD=–RT(∂ ln DAg+/∂p)T was evaluated for each system, at several temperatures. For a given solvent, ΔVD was independent of temperature within experimental error; at a given temperature, ΔVD increases as the size of the solvent cation increases. In each case, the activation energies for interdiffusion were derived, at constant pressure and also at constant volume. The results are compared with the corresponding volume and energy terms for electrical conductivity in the molten alkali metal nitrates.

A thermogravimetric study of acidic and basic solutes in two molten alkali metal nitrate eutectics

The reaction of ten oxyanions and oxides, acting as Lux—Flood acids have been studied in both lithium nitrate—potassium nitrate and sodium nitrate—potassium nitrate eutectics and the stoichiometries of the reactions established. A series of acids arranged in order of decreasing strength has been drawn up, which is very similar for both eutetic solvents. Five Lux—Flood bases have also been studied in these two solvents. Sodium carbonate proved to be very stable to 500°C. Sodium peroxide revealed significant differences in stability and solubility in the two solvents, while lithium, sodium and potassium hydroxides showed differences in the two melts and also considerable differences between each other, suggesting that little cation exchange takes place in these solutions.

Infrared Emission Spectra of Molten Salts

Infrared emission spectra of molten alkali metal nitrates and chlorates were recorded over the frequency interval from 50 to 3000 cm−1 employing a Fourier transform spectrometer. Emission spectra of molten mixtures of NaNO3 and NaClO3 also were measured, and the ν3 mode of NO−3 ion was observed with solutions containing about 0.1 mole % NaNO3. Far infrared spectra of the melts revealed a broad band which progressively shifted to a lower frequency as the mass of the cation increased. The emission spectrum of each salt at a given temperature was recorded in fewer than 2 min and was of comparable or of better quality than previously reported infrared spectra.

Interaction of gases with ionic melts. Solubility of carbon dioxide and ammonia in molten alkali-metal nitrates

Solubilities of carbon dioxide (pressure 1 bar) and ammonia (pressure 0.1–0.4 bar) were determined in several (Li,K) and (Na,K) nitrate solvents in the temperature range 470–670 K and the relevant thermodynamic parameters calculated. The dissolution processes appear to be largely influenced by the solute–solvent interaction energy.

Solubility of H2 and CO in molten alkali-metal nitrates

The solubility of H2 and CO often used as fuel gases in high-temperature fuel-cells has been studied in the molten equimolar mixture (sodium + potassium) nitrate at temperature values ranging between 500 and 600 K. In both cases the plots log(KH/mol cm–3 bar–1) against K/T yielded straight lines whose least-squares equations are log KH=–5.39–768 K/T and log KH=–5.98–980 K/T for hydrogen and carbon monoxide respectively. The heats of solution and the standard entropies are calculated and compared with those of other systems. Both gases react slightly with the solvent, by H2+ NO–3→ NO–2+ H2O and CO + NO–3→ NO–2+ CO2, which gave rise to “unusual” solubility kinetic curves. The experimental results are discussed in terms of the Uhlig–Blander theory.

Laser raman studies of the iodate ion in molten nitrate solutions

Raman spectra of the iodate ion in molten alkali metal, silver(I), and thallium(I) nitrates are reported. These spectra are found to be consistent with the existence of a pyramidal IO 3 −1 ion. Comparison of the spectral features of the IO 3 −1 and NO staggered3 −1 ions indicates that the nature and magnitude of the cation-anion interactions are remarkably similar. There is evidence which suggests that the IO 3 −1 ion may be approaching planarity in these melts.

Solubility of metallic oxides and the free energy of solvation of oxide ion in molten alkali metal nitrates

Solubility of metallic oxides and the free energy of solvation of oxide ion in molten alkali metal nitrates

The behaviour of several oxoanions dissolved in molten alkali-metal nitrates

The behaviour of several oxoanions (HSO4-, HSO3-, NO2-, ClO4-, ClO3-, SO42-, SO32-) dissolved in a molten NaNO3-KNO3 eutectic at 300�C was studied by means of an oxygen electrode. NaHSO4 and NaHSO3 were found to decompose very rapidly to form S2O72- or S2O52-, and water. NO2-, SO42-, and SO32- were found to dissociate to a measurable extent to form O2- and the equilibrium constants were determined for these dissociations. SO42- and SO32- appear to dissociate according to the equations SO42- ←→ SO3 + O2 And SO32- ←→ SO2 + O2 in which SO3 and SO2 remain solvated in the melt under the conditions used. ClO4- was found to be completely stable in the melt, but ClO3- appears to form an adduct with O2-. Tentatively assuming that the reaction is ClO3- + O2-←→ ClO43- we have found the equilibrium constant to be 3.57 x 103.

Dipole Correlations in Molten Alkali Metal Nitrates and Sodium Nitrite

The far infrared spectra of some solid and molten alkali metal nitrates and sodium nitrite have been measured at various temperatures. The dipole moment correlation functions obtained from these spectra by Fourier transformation are presented and are connected with a solid state formalism. These dipole correlation functions give a measure of the lifetime of the correlated motions in the liquid phase. The lifetimes for the molten salts are of the order of 10−12−10−13 sec. High anharmonicity and large fluctuations characterize the liquid phase.

The reaction of SO 2 with molten nitrates and nitrites

Sulfur dioxide was observed to react rapidly with molten alkali metal nitrates and nitrites. The overall reaction of SO 2 with nitrate produced sulfate and NO 2. Reaction of SO 2 with nitrite resulted in sulfate, NO, NO 2, and N 2O as major products. Reaction schemes are suggested to account for the observed products.

The Oxygen Electrode in Molten Alkali Metal Nitrates: II . Standard Potentials as a Function of Na+/K+ Ratio

not Available.

Determination of Stability Constants in Molten Alkali Metal Nitrate Solvents

MOST determinations of stability constants in alkali metal nitrate solvents have concerned the halide complexes of silver1. Recently the cryoscopic2, solubility3, polarographic4, galvanostatic5, spectroscopic6 and e.m.f.7 techniques have been used for investigating the formation of the halide complexes of various metals (for example, cadmium) in these solvents. A silver–silver bromide electrode has been used to study the bromo-complexes of cadmium and lead7. An attempt8 to use the silver–silver chloride electrode in a similar way to study chloro-complexes was not successful.