Gleb Mamantov

Advances in Molten Salt Chemistry

Ionic Mobilities (A. Klemm). Introduction. Internal Mobilities in Single Salts. External Mobilities of Single Salts and Mixtures. The Nature of External Mobilities. External Mobilities and External Transport Numbers. Analytical Methods. Volumetric Methods. The Streaming Potential. Experimental Results and Discussion. Internal Mobilities of Mixtures. Analytical Methods Based on External Mobilities. Analytical Methods with Checked Ionic Concentrations in a Column. The Occurrence of Migrating Boundaries. Mobility Ratios from the Observation of Migrating Boundaries. EMF Methods. Experimental Results and Discussion for Isotopes. Experimental Results and Discussion for Nonisotopic Additive Binary Mixtures. Cationic Self Diffusion Coefficients of Binary Nitrate Mixtures. References. Aluminum Electrolysis Electrolyte and Electrochemistry (J. Thonstad). Introduction. The Electrolyte. Phase Equilibria. Thermodynamic Properties. Solubility of Aluminum and Aluminum Carbide. Measurements of Aluminum Solubility. Vapor Pressure of Dissolved Metal. Electrochemical Properties of Dissolved Metal. Nature of the Dissolved Metal. Solubility of Aluminum Carbide. Physico-Chemical Properties. Density. Surface Properties. Viscosity. Electrical Conductivity. Transference Numbers. Diffusivity. Electrode Reactions. Reference Electrodes. Double Layer Capacity. The Cathode Reaction. The Anode Reaction. Anode Effect. Current Efficiency and Energy Efficiency. Current Efficiency in Commercial Aluminum Cells. Energy Efficiency. Future Trends. Inert Electrode Materials. Aluminum Chloride Electrolysis. The Electrolyte. Cell Reaction and Current Efficiency. Conclusions. References. The Chemistry and Electrochemistry of Magnesium Production (G.J. Kipouros, D.R. Sadoway). Introduction. Electrolytic Methods of Magnesium Production. Cell Feed Preparation. The Electrolyte. Industrial Electrolysis Cells. Mechanism of Magnesium Electrodeposition. Thermochemical Methods of Magnesium Production. Carbothermic Reduction. Metallothermic Reduction. Other Methods of Reduction. Flux Chemistry. Future Outlook. References. Organic and Organometallic Reactions in Molten Salts and Related Melts (R.M. Pagni). Introduction. Aluminum Chloride-Containing Molten Salts and Melts. The Scholl Reaction. Ene Reaction. Electrophilic Aromatic Substitution. Rearrangements and Isomerizations. Final Comments on A1C1 3 -Containing Melts. Pyridine Hydrohalide Molten Salts. Cleavage of Ethers. Cyclization Reactions. Deacylation. Dealkylation with Rearrangement. Aromatization. Isomerization. Synthetic Applications. Ammonium and Phosphonium Salt Melts. Tetra-n-butylammonium Fluoride. Phosphonium Salts. Tetraalkylammonium Tetraalkylborides. Nitrate and Nitrite Containing Melts. Alkali and Alkaline Earth Salts. Alkali and Alkaline Earth Halides. Alkali Thiocyanate Melts. Alkali Metal Carboxylates. Alkali Metal Hydroxide. Zinc, Copper (I and II), Iron (III) and Tin (II) Chloride Melts. Desulfurization of Alkyl Sulfides.

Molten Salts—Characterization and Analysis

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Electrochemical Studies of Titanium in Molten Fluorides

The electrochemical reduct ion of T i ( IV) in mol ten L i F N a F K F [46.5-11.542.0 mole per cent ( m / o ) ] at 500~ was s tudied by means of l inear sweep vo l tammet ry , chronopotent iometry , and chronoamperomet ry . The reduct ion of T i ( IV) to T i ( I I I ) was found to proceed revers ib ly at both p l a t inum and pyroly t ic graphi te electrodes; fu r ther reduct ion of T i ( I I I ) to the me ta l was shown to be a revers ib le process involving a l loy format ion wi th the p la t inum electrode. S t anda rd electrode potent ia ls for the processes, T i ( IV) + eT i ( I I I ) and T i ( I I I ) + 3e = Ti, are --0.058 and --1.798V, respect ive ly (both vs. a unit mole f ract ion Ni ( I I ) / N i e lec t rode) . The electrochemical oxidat ion of T i ( I I I ) at unsheathed p l a t inum and sheathed glassy carbon electrodes was s tudied in mol ten LiF-BeF~-ZrF4 (65.629.4-5.0 m/o ) at 500~ Evidence for a revers ib le e lectrode react ion was obta ined from vol tammetr ic and chronopotent iometr ic studies. A s tandard poten t ia l of +0.197V (vs. a unit mole fract ion N i ( I I ) / N i e lectrode) was de te r mined for the process T i ( I I I ) : T i ( IV) + e. The more anodic value of the s tandard potent ia l of the T i ( I V ) / T i ( I I I ) couple in LiF-BeF2-ZrF4 in comparison to L i F N a F K F reflects the h igher ac id i ty (Lewis concept) of the LiF-BeF2-ZrF4 melt . In this p a p e r we descr ibe e lect rochemical s tudies of t i t an ium solute species in mol ten L i F N a F K F [46.511.5-42.0 mole per cent ( m / o ) ] and LiF-BeF2-ZrF4 (65.6-29.4-5.0 m / o ) . In teres t in mol ten fluorides stems from the i r impor tance in nuclear reactor technology and use in product ion of a luminum and fluorine, e lect rodeposi t ion of re f rac tory metals, format ion of corros ionres i s tan t diffusion coatings, and p repa ra t ive e lectrochemical fluorination. Most of the studies of e lectrode react ions in mol ten fluorides have been ra the r recent. The status of this subject th rough 1968 has been summar ized by Mamantov ( I ) . Recent reviews of mol ten sal t e lec t rochemis t ry (2-4) contain much more informat ion per ta ining to mol ten fluorides than did ear l ier reviews on the same topic (5-8). The e lec t rochemis t ry of t i t an ium has been studied to a much grea ter ex ten t in mol ten chlor ides than in any other mol ten ha l ide system. The usual solvent is LiC1-KC1 eutectic. The resul ts of vo l t ammet r ic studies in this solvent have been summar ized in two rev iews (6, 7). In polarographic studies of the reduct ion of T i ( I I I ) in mol ten LiC1-KCI eutectic at 400~176 two reduct ion steps, T i ( I I I ) to T i ( I I ) and T i ( I I ) to Ti, were observed. Baboian, Hill, and Bai ley (9) have repor ted E ~ va lues for the revers ib le couples, Ti ( I I I ) / T i ( I I ) and Ti ( I I ) /T i , vs. the P t ( I I ) / P t reference electrode in the LiC1-KC1 eutectic at 450 ~ and 550~ Flengas (10) was able to measure the s tandard potent ia l of the T i ( I V ) / T i ( I I I ) couple in addi t ion to the above two couples in NaC1KC1 at 700~ Senderoff (11) has rev iewed the methods of obtaining t i t an ium meta l by electrolysis of mol ten hal ide solutions. Barksda le (12) has discussed the commercia l product ion of t i t an ium including the electrolyt ic methods in fused salt baths. However , l i t t le information exists regard ing the na ture of the e lectrode reactions involved in these e lectrolyt ic processes. Never theless, the two reviews (11, 12) serve to point out the impor tance of lower -va len t species to the over -a l l cur ren t efficiency, the na ture of deposi ts obtained, and the cell design considerations. Most of the e lectrolyt ic * Electrochemical Society Act ive Member .

Electrochemical Studies of Uranium and Thorium in Molten LiF ‐ NaF ‐ KF at 500°C

Electrochemical studies of U(IV) in molten Lif--NaF-KF (46.5-process at a platinum electrode. The first step is complicated by disproportionation of U(III) to regenerate U(IV); the second step involves formation of uranium metal. Standard electrode potentials with respect to a unit mole fraction Ni(II)/Ni electrode are estimated for the U(IV)/U(III) and U(III)/U couples as --1.41 and -- 1.81V, respectively. These values must be considered tentative because of complications in the electrode process. The voltammetric oxidation of U(IV) at platinum and pyrolytic graphite electrodes in LiF--NaF--KF occurs at ~+ 1.3V vs. a Ni(II) (saturated)/Ni reference electrode. The results point to the disproportionation of electrochemically generated U(V). In voltammetric studies of Th(IV) in molten LiF--NaF--KF at 500 deg C, a reduction wave is obtained at nickel and tungsten electrodes, with a peak potential of --2.02 V (vs. a Ni(II) (saturated)/Ni reference electrode). Analysis indicated that Th(IV) is reversibly reduced to the metal with alloy formation between the deposited thorium and the nickel. A standard electrode potential for the Th(IV)/Th couple in this melt is calculated to be -2.13V vs. a unit mole fraction Ni(II)/Ni electrode. (auth)

U.V.‐Visible and Electron Spin Resonance Spectroelectrochemical Studies of Sulfur Oxidation in AlCl3 ‐ NaCl (63/37 m/o) Melt

The electrooxidation of sulfur in molten (63‐37 mole percent) has been investigated by means of u.v.‐visible and electron spin resonance spectroelectrochemistry in the temperature range 132°–250°C. The electrode reaction sequence for the oxidation of to that was proposed previously based solely on electrochemical results has been modified to include several other intermediates, such as , , , and S (II).

Spectroelectrochemistry in Melts: Applications to Molten Chloroaluminates

Spectroelectrochemistry is an effective approach to studies of redox chemistry in solutions. uv-visible spectroelectrochemical studies in molten chloroaluminates (AlCl/sub 3/--NaCl mixtures) were initiated using a silicon vidicon rapid-scan spectrometer. Electrodes made from glassy carbon foam and metal grids are used in optically transparent thin layer cells. The solute species of interest are the oxidation products of sulfur and some of the Group IVB--VIB transition metal ions. The apparatus and the initial results are described.

Electrochemical and spectroscopic studies of sulfur in aluminum chloride-N-(n-butyl)pyridinium chloride

The behavior of sulfur in aluminum chloride-N-(n-butyl)pyridinium chloride (AlCl/sub 3/-BPC) was studied using Raman spectroscopy and electrochemical techniques. In basic (BPC-rich) melts, sulfur can be reduced to sulfide, probably in the form of an AlSCl-like species. No oxidation to positive oxidation states of sulfur is observed in basic melts. In acidic (AlCl/sub 3/-rich) melts, sulfur can be oxidized to S(I) and eventually to S(IV), which is only stabl in the melt for short periods of time. No reduction to sulfide-like species or formation of low oxidation states is observed in acidic melts.

A reinvestigation of the electrochemical behavior of Nb(V) in AlCl[sub 3]-NaCl[sub SAT] and related melts

The electrochemistry and spectroelectrochemistry of Nb(V) in molten sodium chloroaluminate saturated with NaCl at 178 C were re-examined. It was determined that previous studies involved solutions of high oxide content. The reduction of NbCl[sub 5] in oxide-free melts was found to proceed via four steps at short times and low concentrations. At higher concentrations and long times, the reduction sequence was complicated by a chemical reaction which followed the initial reduction step. The effect of temperature on the electrochemistry of Nb(V) in sodium chloroaluminate and fluorochloraluminates was examined; temperature had a marked effect on the behavior of Nb(V) in these melts, but no significant differences were observed between melts with and those without fluoride. Attempts to produce electrolytically niobium metal from Nb(V) in these metals met with limited success.

Spectroelectrochemical Study of the Formation of the Radical Cation and Dication of Perylene in Molten Antimony(III) Chloride

The oxidation of perylene (PeH) in aprotic melts containing primarily was studied spectroelectrochemically. In molten containing at 100°C, PeH is oxidized at 0.26V relative to the , saturated reference electrode to form a stable solution of the radical cation PeH+.The optical absorption spectrum (500–900 nm) of this cation has five maxima with the highest at 545 nm . In molten containing at 100°C PeH is spontaneously oxidized to PeH+. by reduction of . In this melt electrochemical oxidation of PeH+ to form the dication PeH2+ at 100°C occurs at 0.97V relative to the , saturated reference electrode. The optical absorption spectrum (400–800 nm) of PeH2+ consists of two overlapping bands with a peak at 533 nm .

Electrochemical and Spectroscopic Studies of Tungsten Species in the AlCl3 ‐ NaClsat Melt

The electrochemical behavior of KWCl{sub 6} in the AlCl{sub 3}-NaCl{sub sat} melt at 175 to 200 C was investigated using multiple electrochemical methods: cyclic, normal pulse, square wave voltammetries, chronoamperometry, and exhaustive electrolysis. Electronic spectroscopy and x-ray diffraction methods were employed to characterize the products formed at different potentials. The melt was treated-with CCl{sub 4} to remove oxide impurities. It was found that the reduction sequence of W(V) was dependent on the scan rate, i.e., at fast scan rates three main redox couples were observed, while additional redox couples appeared in low scan rate voltammograms. To reach a better understanding of the mechanism, some low oxidation state tungsten compounds (K{sub 2}WCl{sub 6} and W{sub 6}Cl{sub 12}) were also examined. The reduction of W(V) to tungsten metal involves several intermediate oxidation states and cluster species, such as W(IV), unstable W(III), dimers W(IV, III) and W(III,II), [W{sub 6}Cl{sub 8}]{sup 4+} and its one-electron reduction product. Partial characterization of some of the intermediate reduction products has been achieved.