**5.4 Selection of different types of halides such as electrolytes**

It is possible to use different halides such as electrolytes: chlorides, bromides and iodides. All these salts are hygroscopic and their treatment by the corresponding hydrogen halide (HCl, HBr, HI) is needed to avoid water marks. If some halogenade is not appropriate due to an exchange reaction it is replaced by another.

The choice of the electrolyte is determined by its melting temperature and by the need to minimize the exchange reaction. The study of Sn-Pb system is not possible in molten chlorides (the difference of potential electrodes is 0. 05 V at 500°C ; Table 4).The substitution of chloride by iodide significantly increases this difference to 0.168 V (see Tabl.3) and decreases the exchange reactions:

$$\begin{aligned} \mathrm{SnI\_2 + Pb} &\rightarrow \mathrm{PbI\_2 + Sn} \\\\ \mathrm{PbI\_2 + Sn} &\rightarrow \mathrm{SnI\_2 + Pb\_2} \end{aligned}$$

Although it is impossible to eliminate their influence completely, especially for tin-rich alloys, the electrochemical chain:

(-) Pb KI + LiI + PbI2 PbxSn1-x (+)

can be studied.

### **5.5 Effect of exchange reaction on the EMF measurements in ternary system In-Sn-Sb**

Let us consider the example of spontaneous exchange reaction in the cell that contains the series of four alloys with low indium content, number 1-4, ( xIn from 0.05 to 0.11), and one indium-rich alloy № 5 (xIn=0.5) at presence of two electrodes of pure indium. (See Table 5)

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

(-) In In+ in electrolyte InxSn(1-x) (+) , the exchange reactions occur readily in these systems if the concentration of the second element has reached 90% (Fig.14). We observed a continuous fall of the EMF for alloys with x 0.1 when the duration of the experiment was a few weeks (Vassiliev et al., 1998b; Mozer, 1972). The rate of the exchange reaction was augmented with increasing temperature,

Exchange reactions occur not only between a pure A metal and alloy AxB(1-x) but also between alloys of different compositions, if the activities *aA* of them are very different. This phenomenon is very pronounced in the liquid ternary system In-Sn-Sb (Vassiliev, 1998). Table 4 shows that the attraction between atoms In and Sb is greater than between In and Sn or between Sn and Sb. Accordingly, the tin atoms of the ternary alloy are very free, and exchange reactions between electrodes of different compositions of alloys, with indium, occur easily.

It is possible to use different halides such as electrolytes: chlorides, bromides and iodides. All these salts are hygroscopic and their treatment by the corresponding hydrogen halide (HCl, HBr, HI) is needed to avoid water marks. If some halogenade is not appropriate due

The choice of the electrolyte is determined by its melting temperature and by the need to minimize the exchange reaction. The study of Sn-Pb system is not possible in molten chlorides (the difference of potential electrodes is 0. 05 V at 500°C ; Table 4).The substitution of chloride by iodide significantly increases this difference to 0.168 V (see Tabl.3) and

SnI2 + Pb PbI2 + Sn

PbI2 + Sn SnI2 + Pb, Although it is impossible to eliminate their influence completely, especially for tin-rich

(-) Pb KI + LiI + PbI2 PbxSn1-x (+)

**5.5 Effect of exchange reaction on the EMF measurements in ternary system In-Sn-Sb**  Let us consider the example of spontaneous exchange reaction in the cell that contains the series of four alloys with low indium content, number 1-4, ( xIn from 0.05 to 0.11), and one indium-rich alloy № 5 (xIn=0.5) at presence of two electrodes of pure indium. (See Table 5)

So, if we study the electrochemical cell of the type:

**5.3 Influence of a third component on the exchange reaction** 

**5.4 Selection of different types of halides such as electrolytes** 

to an exchange reaction it is replaced by another.

decreases the exchange reactions:

alloys, the electrochemical chain:

can be studied.

especially in the case of liquid systems.


Table 5. Composition of In-Sn-Sb alloys used for detection of kinetic of spontaneous exchange reaction in the electrochemical cell.

We can state that the measured values E(T, хIn) for alloys with number 1-4, and slightly for number 5, are exposed to such reactions. So, we used only the first points of the measurements E(T, хIn), which were less susceptible to this influence. Exchange reaction is more pronounced for alloy 1 and 2. Dynamics of a regular drift of EMF values for alloys № 2 and 5 versus time and temperature were shown in Fig. 15 and Fig. 16. Experimental points are divided into two series. The gap of EMF values between two series is connected with the study of other phases at lower temperatures are not indicated in Fig. 15 and 16. We took in consideration only the black dots. The different stages of the experiment are marked in time. Fig. 15 and 16 show that the exchange reaction depends on the time and temperature. Two main types of exchange reactions (*a* and *b*) take place in cell:

$$\begin{array}{ll} \text{(-) In} \text{ In} \text{2+} \text{ in} \text{ electrolyte} \text{ In} \text{-Sn-Sb} \text{ +} \\\\ \text{a) In} \text{ (pure)} \text{ } \text{n}^{\text{-}} \text{ } \text{-Sn} \text{ SnSnSb} \text{ (Ne 1-4)} \\\\ \text{a} \text{In} = \text{1} & \text{a}^{\text{-}} \text{In} \text{-1} \\\\ \text{b) In} \text{SnSn} \text{ (N} \text{55} \text{ )} \text{n}^{\text{-}} \text{ } \text{-SnSnSb} \text{ (Ne 1-4)} \\\\ \text{a}^{\text{n}} \text{n} & \text{ >} & \text{a} \text{n}^{\text{-}} \end{array} \tag{14}$$

Reactions *a* and *b* lead to a decrease of the EMF values for alloys (№№ 1-4) and the reaction of *b* increases the EMF values of the alloy number 5 in relation to the reference electrode made of pure indium. The rate of exchange reaction prevails over the reaction (13) and (14). Kinetics of exchange reactions is shown in Fig. 17 and Fig.18 in accordance with 4 passes at the same temperature 755K versus the time. We did not observe exchange reactions for alloys with хIn> 0.1, although the duration of the experiment exceeded more than two months, the maximum temperature reached 822K.
