**2.3 Direct potentiometric method**

264 Mass Transfer - Advanced Aspects

Smolenski, 2010; Smolenski et al., 2008a, 2008b, 2009] to those of actinides [Fusselman et al., 1999; Morss, 2008; Osipenko et al., 2010, 2011; Roy et al., 1996; Sakamura et al., 1998; Serp et al., 2004, 2005a, 2005b, 2006; Serrano & Taxil, 1999; Shirai et al., 2000] hence separation between these groups of elements is very difficult. For this reason, a good knowledge of the

The goal of these investigations is to determine the electrochemical and thermodynamic properties of some fission products (Tm and Yb), their mass transfer, and behavior in different fused solvents using transient electrochemical techniques, and potentiometric

The solvents LiCl (Aldrich, 99.9%), NaCl (Aldrich, 99.9%), KCl (Aldrich, 99.9%), and CsCl(Aldrich, 99.9%) were purified under vacuum in the temperatures range 293-773 K. Then the reagents were fused under dry argon atmosphere. Afterwards these reagents were purified by the operation of the direct crystallization [Shishkin & Mityaev, 1982]. The calculated amounts of prepared solvents were melted in the cell before any experiment

Dry lanthanide trichlorides (LnCl3) were obtained by the way of well-known method

• First, the crystalline hydrate (LnCl3⋅nH2O, where *n* is 4.5-5.0) was prepared by direct interaction of Ln2O3 (Tm2O3 OST 46-205-81 TuO-1 and Yb2O3 IbO-L TU 48-4-524-90)

• Second, dry LnCl3 was prepared by using the operation of carbochlorination of crystalline hydrate during heating in CCl4 stream vapor in horizontal furnace. The obtained lanthanide chlorides (LnCl3) were kept into glass ampoules under atmosphere of dry argon in inert glove box. Ln3+ ions were prepared by direct addition of anhydrous

The experiments were carried out under inert argon atmosphere using a standard electrochemical quartz sealed cell using a three electrodes setup. Different transient electrochemical techniques were used such as linear sweep, cyclic, square wave and semiintegral voltammetry, as well as potentiometry at zero current. The electrochemical measurements were carried out using an Autolab PGSTAT30 potentiostat-galvanostat (Eco-

The inert working electrode was prepared using a 1mm metallic W wire (Goodfellow, 99.9%). It was immersed into the molten bath between 3 - 10 mm. The active surface area was determined after each experiment by measuring the immersion depth of the electrode. The counter electrode consisted of a 3 mm vitreous carbon rod (SU - 2000). The Cl–/Cl2 electrode is the most convenient reference electrode because it can be used for the direct thermodynamic calculations. It standard construction is the following. The quartz tube with porous membrane in the bottom and molten solvent in it has the graphite tube for chlorine gas introduction into the system. The chlorine gas is bubbling through the melt during the

basic properties of REE in the proposed separation media is very important.

method (*emf*).

**2. Experimental** 

[Korshunov et al., 1979].

[Revzin, 1967] in two steps:

with HCl acid solution.

LnCl3 to the fused electrolytic bath.

experiment [Smirnov, 1973].

**2.2 Transient electrochemical technique** 

Chimie) with specific GPES electrochemical software (version 4.9).

**2.1 Preparation of starting materials** 

The potentiometric study was carried out using an Autolab PGSTAT30 potentiostat-galvanostat (Eco-Chimie) with specific GPES electrochemical software (version 4.9). The electrochemical techniques were used such as potentiometry (zero current) and coulometry methods.

The electrochemical set-up for potentiometric investigations is shown in Fig. 1. The inert working electrode was prepared using a 5 mm vitreous carbon rod (SU - 2000) which was located in BeO crucible with the investigated melt. It was immersed into the molten bath between 3 - 5 mm. During the experiments Ln3+ ions were electrochemically reduced to Ln2+ ions up to ratio Ln3+/Ln2+ equals one. The counter electrode consisted of a 3 mm vitreous carbon rod (SU - 2000) which was placed in quartz tube with porous membrane in the bottom with solvent melt and located in vitreous carbon crucible (SU - 2000) with pure solvent without lanthanide chlorides*.* The Cl–/Cl2 electrode was used as reference electrode.

1 – Pt/Pt-Rh thermocouple; 2- Cover of thermocouple; 3- Section; 4- Capsule of chlorine electrode; 5- Nickel screen; 6- Alumina tube; 7- Cl- /Cl2 reference electrode/counter electrode; 8- Getter of zirconium; 9- Current contact; 10- Vitreous carbon working electrode; 11- Quartz test-tube with cover; 12- Beryllium oxide crucible; 13- Vitreous carbon crucible; 14- Investigated salt system; 15- Asbestos diaphragm.

Fig. 1. Experimental set-up for potentiometric study

The total lanthanide concentrations were determined by taking samples from the melt which were dissolved in nitric acid solutions and then analysed by ICP-MS. The concentration of the reduced form of lanthanides was determined by volumetric method.

Electrochemistry of Tm(III) and Yb(III) in Molten Salts 267

0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s 0.75 V/s

> 0.08 V/s 0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 v/s



Fig. 4. Linear sweep voltammograms of fused NaCl-KCl-CsCl-YbCl3 salt at different sweep

At low frequencies a linear relationship between the cathodic peak current and the square root of the frequency was found. Under these conditions the system can be considered as reversible and equation 1 can be applied [Bard & Faulkner, 1980]. The number of electrons exchanged was close to 1. The same results were obtained in NaCl-KCl, NaCl-KCl-CsCl and

Potentiostatic electrolysis at potentials of the cathodic peaks for all systems studied did not show the formation of the solid phase of tungsten surface after polarization. There is no plateau on the dependences potential – time. Also the working electrode did not undergo any visual change. X-ray analysis of the surface of the working electrodes after experiments

rates at 873 K. [Yb(III)] = 7.45·10-2 mol kg-1. Working electrode: W (S = 0.36 cm2)

Fig. 3. Linear sweep voltammograms of fused NaCl-KCl-YbCl3 (3.79·10-2 mol/kg) for the reduction of Yb(III) to Yb(II) ions at different sweep potential rates at 973 K. Working


electrode: W (surface area = 0.27 cm2)

0


also show an absence of formation of solid phase.




**I/A**

CsCl media.






**I/A**



0
