**5.1 Choice of different types of halides as the electrolyte**

It is possible to use different types of halides as the electrolyte: chlorides, bromides and iodides. All these salts are hygroscopic and their treatment by the corresponding hydrogen halide (HCl, HBr, HI) is necessary to avoid water marks. If one type of halide unsuitable due to the exchange reaction, it can be replaced by another.

For the study of the Ni-Cu (Geyderih et al. 1969), measurements of EMF were made from two reference electrodes

$$\text{(-) Ni(sol)} \mid \text{KCl} + \text{LiCl} + \text{NiCl}\_2 \mid \text{Ni}\_x\text{Cu}\_{\text{(l.x)}} \text{ (sol) (+)}$$

$$\text{(-) Cu(sol)} \mid \text{KCl} + \text{LiCl} + \text{CuCl} \mid \text{Cu}\_x\text{Ni}\_{\text{(l.x)}} \text{ (sol) (+)}$$

In both cases there is a reduction of the second metal; the electrodes of pure metals are covered with crystals of the second metal. So, there are two oxidoreduction reactions:

$$\text{NiCl}\_2 + 2\text{ Cu (from alloy)} \rightarrow \text{Ni} + 2\text{CuCl}\_2$$

$$2\text{ CuCl} + \text{Ni (from alloy)} \rightarrow 2\text{ Cu} + \text{NiCl}\_2$$

$$(1/\text{nB})\text{ B (in A}\_3\text{B}\_{\text{(l.o)}}) + (1/\text{nA})\text{ A}^{n\*} \rightarrow (1/\text{nA})\text{ A} + (1/\text{nB})\text{B}^{n\*} \tag{\text{II}}$$

The replacement of the chloride on iodide electrolyte permits to determine the thermodynamics of the Cu-Ni system.

Electrochemical Cells with the Liquid Electrolyte

**5.2 Exchange reactions** 

If we study the cells of the type:

in the Study of Semiconductor, Metallic and Oxide Systems 87

The interaction between the electrodes via the electrolyte is one of the most important problems. In the electrolyte, the A and B elements are characterized by different potentials against a reference electrode (Hladik, 1972). We must choose an electrolyte which causes a potential difference of A and B as large as possible. The more the difference is between of the electrode potentials, the smaller is the exchange reaction between the component B and the An+ ions in the melted electrolyte. Information about a possible exchange reaction between electrodes can be found from the electrode potentials for different halide melts (Hladik, 1972). This set of the chemical potentials characterizes activity of metals against each other for the system in study for giving electrolyte. The more metal is electronegative the more it is chemically active. In particular, in the set of the electrochemical potentials each metal replaces in an electrolyte all metals with lower potential. And, in turn, it will be replaced in the same electrolyte by metals with greater potential. So, the metal with the most negative potential, in the given electrolyte, replaces all those with more positive potentials. If we study the binary system Zn-Sb or In-Sb by the potentiometric method we do not see any problem of exchange reactions. So we can note that if the difference of the electrochemical potential reaches 0.4 V, the exchange reactions do not exist. And opposite, this problem appears for the

systems Zn-In and In-Sn if the difference of the electrochemical potential is 0.19 V.

systems. The speed of the exchange reaction depends on:

2. the difference in alloy composition,

4. the temperature in the cell.

electrolyte,

1. the difference of electrochemical potentials of the elements,

We will consider some binary systems based on elements of Table 2 and 3.

(-) Zn Zn2+ in an electrolyte ZnxIn(1-x) (+)

(-) In In+ in an electrolyte InxSn(1-x) (+)

the exchange reactions take place easily when the concentration of the second element has reached 90%. The continuous drop of the EMF of the cells with alloys x 0.1 is observed if the duration of the experiment is over several weeks (Vassiliev et al., 1998b; Mozer, 1972). The rate of the exchange reaction increases with increasing temperature especially in liquid

3. the presence of metal ions of different charges (A(n+) et A(m+) ; m+ > n+) in the

This set of electrochemical potentials characterizes the chemical activity of metals against each other under consideration system and a given electrolyte. The more metal is electronegative the more this metal is chemically active. Especially, in the set of the electrochemical potentials each metal replaces in the electrolytes of all metals with inferior potential. In turn, it was replaced in the same electrolyte by metals with superior potential. If the data for the electrode potentials are incomplete, it is possible to judge about the relative chemical activity of two elements by comparing the Gibbs energies or enthalpy of formation of salts AXn and BXn (X being the corresponding a salt anion). See Table 4 bottom.

(-) Cu(sol.) KI + NaI + CuI CuxNil-x (sol.) (+) This experiment have made possible to formulate an empiric rule:

By working with electrochemical cells without separation of electrodes, enthalpy of formation *(fH)* salt of the metal B must not exceed 75% of enthalpy of formation of metal salt A (Geyderih et al. 1969).

As the equilibrium constant of the reaction Kp (II) has a finite value, there will be always an exchange reaction, even partial, between the salt ions An+ and alloy AxB(1-x). The authors have attempted to assess the relative error in determining the activity of the less noble metal by the EMF method based on constant exchange reaction (II) (Wagner& Werner, 1963). Simplified equation, admitting a relative error, expressed by the expression:

$$\mathbb{T} \oplus = \left(\mathbf{n}\_{\mathbb{B}}/\mathbf{n}\_{\mathbb{A}}\right) \left(\mathbf{Y}^{\diamond}/\mathbf{Y}\_{\mathbb{A}}\right) \left(\mathbf{n}\_{\mathbb{A}}/\mathbf{n}\_{\mathbb{B}}\right) \left(\mathbf{1}/\mathbf{x}\right) \left(\mathbf{I} + \left(\mathbf{n}\_{\mathbb{A}}/\mathbf{n}\_{\mathbb{B}}\right)\right) \left(\mathbf{D}^{\diamond}/\mathbf{D}\right) \mathbb{I} \left(\mathbf{2}^{\diamond}/\mathbf{D}^{\diamond}\right) \exp\left(\cdot \left(\mathbf{n}\_{\mathbb{B}} \mathbf{E}^{\diamond} \mathbf{F}\right) / \mathbf{RT}\right) \left(\mathbf{1}\mathbf{2}\right)$$

where YA is the coefficient activity of component A, and Y° is the molar salt fraction of component A in the melt.

D ° and D are the diffusion coefficients of salt BXn in the electrolyte and the component A in the alloy respectively,

E ° is difference of the reference potentials of the components,

V'm and V''m molar volumes of alloy and electrolyte.

In the formula (12) we can see:


In concordance to our experience, the concentration of AXn salt in liquid electrolytes must not exceed 0.1%. In some cases, this concentration can be reduced if the salt AXn is slightly soluble in the electrolyte.

If there is a problem of chemical stability or hygroscopicity AXn of certain salts (e.g. as the indium chloride or zinc chloride) we can do forming potential without salt. Synthesis of indium monochloride (InCl) is carried out inside the cell by the interaction of hydrogen chloride absorbed by the electrolyte, with metallic indium:

#### 2In + 2HCl (gas) = 2InCl + H2.

Contact of indium monochloride (InCl) with moist air provokes the formation of indium ions with different valence states: InCl+O2+H2OIn(OH)Cl2 +In(OH)2Cl +In(OH)Cl + InOCl + …,

that leads to the exchange reaction between the electrodes of electrochemical cell.
