B. Tremillon

Oxoacidity reactions in molten LiCl+KCl eutectic (at 470°C): Potentiometric study of the equilibria of exchange of O2− between Al(III) systems and carbonate and water systems

Abstract The reactions between aluminium chloride and oxide anion were studied in molten LiCl+KCl eutectic at 470°C. For that purpose, a pO2− indicator electrode made with a membrane in yttria-stabilized zirconia has been used and tested by means of a carbonate ion whose dissociation constant, 10−2.15 atm, was determined. Then, the electrode has allowed us to obtain the formation constant of AlO+ (solvated by Cl−) and the solubility product of Al2O3 (S): 1010.7 and 10−27.4, respectively (molality scale). The equilibrium constant of the following system: 2HCl (g)+O2− agH2O (g)+2Cl− has also been determined: 10−9.93 atm mol kg−1. The conditional solubility of alumina in LiCl+KCl melt is discussed.

Electrochemical studies of technetium at a mercury electrode

Abstract The electrochemistry of technetium was studied by polarography, cyclic voltammetry and coulometry in chloride and sulfate media as a function of pH in the range 1.5–13. Compounds of Tc(III) and Tc(IV) are produced by reduction of pertechnetate, and the system Tc(III)/Tc(IV) was investigated in acidic media. The potential—acidity diagram of technetium is described for two total pertechnetate concentrations. Evidence for the dismutation of Tc(III) below pH 4 is discussed.

Conditional solubility versus pO2− of cerium(III) oxide in molten equimolar NaCl+KCl at 727°C⋆

Abstract Reactions between cerium trichloride and oxide ions were studied in NaCl+KCl (1/1) at 1000°K, by potentiometry with a calcia-stabilized zirconia membrane electrode. Titration curves clearly demonstrated the existence of soluble cerium oxychloride (CeO+) and precipitated cerium oxide (Ce2O3), with respective dissociation constants 10−11 and 10−30 (molality scale). The corresponding conditional solubility diagram {log S (CeIII)=f(pO2−)} is presented and discussed.

Electrochemistry in molten NaAlCl4 (at 210°C): Identification of dissolved oxide-containing species, solubility of alumina in terms of pCl− and properties related to H2O in this melt

Abstract It has been shown that a nickel electrode is an indicator of the presence of oxide ions dissolved in chloroaluminate melts. From quantitative interpretation of electrochemical results (voltammograms and potentiometric measurements), O 2− has appeared to behave, in terms of the chloro-acidobasicity concept, as a strong dibase, through a solvolytic reaction giving AlOCl 2 − as solvated form. AlOCl 2 − itself is a weak base, according to the system: AlOCl 2 − AlOCl+Cl − , pK B /mol kg 1- =4.35 (at210°C) Thus O 2− behaves cumulatively as a tribase. AlOCl is the solvated form of O −II in very acidic melts. A saturation effect by Al 2 O 3 appeared when the concentration of O −II was increased. The solubility of Al 2 O 3 in chloroaluminate melts (one third of total O −II solubility) was determined from the electrochemical results obtained. A minimum of 0.13% in weight (solubility of O −II =4×10 −2 mol kg −1 ), is observed for pCl − =4.5 that is in a mixture containing 51.4 moles per cent of AlCl 3 . The solubility is increased by adding either more AlCl 3 (formation of AlOCl) or NaCl (formation of AlOCl 2 − ). Its variation versus pCl − has been expressed quantitatively. Through a study of the system HCl/H 2 O, it was pointed out that hydrolysis of chloroaluminate melt does not occur to a great extent in basic media. On the contrary, elimination of oxide ions through H 2 O formation by reaction with HCl was shown to be possible. A quantitative relation, which binds the total concentration of O −II , pCl − and the respective partial pressures of gaseous HCl and H 2 O in equilibrium with the melt, is given. A treatment in order to carry out this purification in these melts appears necessary in order to prevent many dissolved metallic ions from precipitating under the form of oxides.

Industrial aluminas and hexahydrated aluminum chloride chlorinations by phosgene in LiClKCl eutectic melt at 470°C

Abstract The variations of pO 2− during the chlorination by phosgene of industrial aluminas and hexahydrated aluminium chloride were observed by means of an yttria-stabilized zirconia electrode. The curves obtained showed that chlorinations carried out in a continuous flow reactor, take place rapidly (30 min with a phosgene rate of flow of 157 cm 3 min −1 and a simple bubbler). Transformation of oxychloride to tetrachloroaluminate is the slow step; its specific rate constant k s and its order α have been determined ( k s = 0.19 mol −1 kgs −1 ; α = 2). The latter value is interpreted by postulating the involvement of the complex Al 2 OCl − 5 . The catalytic effect of the presence of aluminum chloride on the chlorination of the alumina has also been illustrated.

Dissociation and solubility variation versus pO2- of scheelite CaWO4 in molten NaCl−KCl (At 1000 K)

Abstract The conditional solubility of scheelite CaWO4 in molten NaCl−KCl (1:1) has been studied either in oxobasic or oxoacidic media. In the former case it is increased by formation of sparingly soluble CaO according to: Ca2+ + O2CaO(s), pKsCaO = 10800/T − 5.8 (molality scale) In the later case, WO42- behaves as an oxobase according to the following equilibria: WO42−WO3 (s) + O2− pK0 = 10.0 at 1000 K 3 WO42−W3O102− pK3 = 12.7 at 1000K The latter equilibrium favours an increase in the WVI solubility when pO2- is increased, for instance by injection of HCl. The whole set of results has been summarized by a conditional solubility diagram of scheelite versus pO2-.

Effect of the addition of fluoride on the conditional solubility of alumina in LiCl-KCl eutectic melt

Abstract Owing to its well-known high complexing power toward Al 3+ ion, fluoride ion is able to increase the solubility of alumina in alkali chloride melts. To determine the extent of this effect, the formation of aluminium(III) fluoro-complexes was studied potentiometrically in LiCl-KCl eutectic at 470°. But the sodium fluoride addition appeared to produce not only the complexation effect but also a mineralization effect on alumina. So, the thermodynamical stability of alumina formed in this melt by precipitation from aluminium chloride with carbonate ion (oxide anion donor) depends on the fluoride ion concentration. These two effects explain the solubility variation of alumina in the LiCl-KCl eutectic + NaF mixtures. A pF − - p O 2− diagram, which represents the stability area of the various aluminium (III) species is established, and leads to some conclusions concerning the electrowinning of aluminium from molten chloride melts. The cumulative formation constants of the aluminium(lII) fluoro-complexes (AlF 3−i i ) have been obtained, whose values are the following: 2.5 ± 0.4, 4.7 ± 0.6, 5.7 ± 0.5, 7.5 ± 0.4, 8.0 ± 0.5, 9.0 ± 0.6, respectively for i = 1, 2, 3, 4, 5 and 6. It has been shown that oxyfluoride species such as AlOF 1− j i does not exist. The solubility products of gamma and alpha-alumina have been determined and are equal to 10 −42.9 and 10 −44.0 respectively (all the constants are given in the molality scale). They differ widely from the solubility product of the alumina obtained in the absence of fluoride ion, ie 10 −27.4 .

Use of quinone systems as electrochemical pH indicators in anhydrous hydrogen fluoride solvent: I. Electrochemical behaviour of quinones in HF and in HF-water mixtures

Abstract The redox behaviour of eleven quinones or hydroquinones has been studied by means of voltammetry and controlled-potential coulometry. In anhydrous hydrogen fluoride, these compounds with the exception of chloranil and quinalizarine, are reducible in two monoelectronic reversible steps with the intermediate formation of semiquinones. In hydrogen fluoride+water mixtures, the transition was observed between this particular behaviour and the classical one-step bielectronic reduction generally observed in aqueous or acidic media. The results obtained are compared with those previously reported in acidic or aprotic solvents. The corresponding mechanisms are discussed and a scheme is proposed to account for the unusual processes occurring in anhydrous hydrogen fluoride. Teflon-coated electrodes designed for electrochemical measurements in AHF are described, together with Voltalef vacuum lines used for the storage, handling and disposal of AHF and HF+H2O mixtures.

Use of quinone systems as electrochemical pH indicators in anhydrous hydrogen fluoride solvent: II. Potentiometric acid-base titrations in anhydrous HF

Abstract The use of quinone compounds as potentiometric indicators for acidity measurements in anhydrous hydrogen fluoride (AHF) is described. The correct response of such indicators to pF variations and their stability in very acidic media were verified. A comparison is made with the hydrogen electrode which suffers from some drawbacks in AHF; the interest of the quinone acidity indicators in thus pointed out. Acid-base titrations are performed with various acidic and basic solutes and the corresponding titration curves are determined by means of quinonic indicators and a glassy carbon electrode. Values of the basicity constants of the considered solutes are deduced from these titration curves and discussed together with the value (10−11.5) of the ionic product of AHF. An acidity scale is established in this solvent: no strong acid appears to exist; even antimony pentafluoride cannot be considered as a strongly acidic compound.

Properties of arsenic in molten potassium tetrachlorogallate at 300°C: Formation of gallium arsenide

Abstract Redox properties of As(III) species have been studied in molten KGaCl4. The plot of equilibrium potential of As vs acidity is interpreted in terms of reaction AsCl4− + 3 e = As + 4 Cl−. X-Ray analysis of electrodeposits shows that only As is formed under potentiostatic conditions. Conversely, gallium arsenide is observed under galvanostatic conditions, for which the electrode potential becomes very negative.