Electrochemical Properties of Molybdenum in Alkali Chloride Melts
Authors: Ivanov Alexander B. , Volkovich Vladimir A. , Likhachev Pavel Yu. , Kamalov Robert V. 
Published: February 2020

Fused chlorides can be employed as working media for molybdenum electrorefining and for depositing molybdenum coatings. Existing literature contains limited data on the electrode potentials of molybdenum in alkali chloride based melts, i.e. NaCl-KCl equimolar mixture, LiCl-KCl and NaCl-CsCl eutectics (1–3). In the present work electrode potentials of molybdenum were determined in the melts based on individual alkali chlorides (LiCl, NaCl, KCl, RbCl, CsCl), NaCl-KCl equimolar mixture and NaCl-CsCl and LiCl-KCl-CsCl eutectics.
The measurements were performed using the emf method. The potentials were measured at zero current vs. silver chloride reference electrode. The system was considered at equilibrium if the potential remained constant within ±0.5 mV and did not show the tendency to a monotonous change. Molybdenum ions were introduced to the melt by anodic dissolution of the metal or by dissolving potassium hexachloromolybdate(III). Molybdenum concentration in the melt was determined by chemical analysis after each experiment.
On the results of the experiments the formal standard electrode potentials of molybdenum were determined in fused LiCl-KCl-CsCl, NaCl-KCl, NaCl-CsCl, LiCl, NaCl, KCl, RbCl and CsCl. The results obtained are shown in Fig. 1. Temperature dependencies of molybdenum formal electrode potential (obtained by the least squares fit) are described by the following equations:

E*
Mo3+/Mo
= (–1,727+9,0·10–4
T) V (NaCl-CsCl) T = 793–1023 К

E*
Mo3+/Mo
= (–1,327+4,0·10–4
T) V (NaCl-KCl) T = 973–1123 К

E*
Mo3+/Mo
= (–1,274+5,0·10–4
T) V (LiCl-KCl-CsCl) T = 680–973 К
From the results of emf measurements in individual alkali chlorides (LiCl, NaCl, KCl, RbCl, CsCl) the following dependence of molybdenum formal electrode potential on radius of alkali metal cation was obtained at 1123 K:

E*
Mo3+/Mo
= (0,2387÷r – 1,0198) ± 3·10–2 V
Gibbs free energy change of the formation of molybdenum trichloride in alkali chloride melts was determined from the results of electrochemical measurements:
ΔG
MoCl3(melt)
= (–483,5 + 228,3·10–3
T) ± 3 kJ/mol (NaCl-CsCl)
ΔG
MoCl3(melt)
= (–384,2 + 129,2·10–3
T) ± 4 kJ/mol (NaCl-KCl)
ΔG
MoCl3(melt)
= (–368,8 + 148,9·10–3
T) ± 4 kJ/mol (LiCl-KCl-CsCl)
Electronic absorption spectra of molybdenum(III) ions were measured at 1123 K in individual molten salts LiCl, NaCl, KCl, RbCl, CsCl and at 673–1073 K in the low melting LiCl-KCl-CsCl eutectic and molar absorption coefficients determined, Fig. 2. Resolution of the spectral curves into individual bands allowed determination of the energies of the electronic transitions. From these results the main spectroscopic parameters of MoCl6
3– complex ions were calculated, i.e. ligand field splitting parameter Δ, Racah parameters B and C. From the found values of B the nephelauxetic parameter β = В/В0, characterizing ionicity of Mo–Cl bond in МоCl6
3– was calculated.

lgD = (–3702,3÷T – 0,9) ± 0,3 cm2·s–1

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