**5.2 Exchange reactions**

86 Electrochemical Cells – New Advances in Fundamental Researches and Applications

(-) Cu(sol.) KI + NaI + CuI CuxNil-x (sol.) (+) This experiment have made possible to

By working with electrochemical cells without separation of electrodes, enthalpy of

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).

® = (nB/nA) (Y°/YA) (nA/nB) (1/x) (I + (nA/nB)) (D°/D)1/2. (V'm/V"m)exp (-(nBE°F)/RT) (12) where YA is the coefficient activity of component A, and Y° is the molar salt fraction of

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

2. that the higher is potential difference of the electrodes the smaller is the relative

3. that increasing of the salt concentration AXn causes the increase of error ®, nevertheless, too small concentration may give a relative error greater than the reaction (II). The optimal concentration of salt in the molten electrolyte AXn, according to Wagner, is

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

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

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.

Simplified equation, admitting a relative error, expressed by the expression:

1. that the error is maximum for the minimum concentration of component A,

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

chloride absorbed by the electrolyte, with metallic indium:

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

*fH)* salt of the metal B must not exceed 75% of enthalpy of formation of metal

formulate an empiric rule:

component A in the melt.

In the formula (12) we can see:

the alloy respectively,

error,

1 - 3%

+ …,

soluble in the electrolyte.

salt A (Geyderih et al. 1969).

formation *(*

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.

If we study the cells of the type:

(-) 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 systems. The speed of the exchange reaction depends on:


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

Electrochemical Cells with the Liquid Electrolyte

calculated from thermodynamic data.

700°C compared with reference electrode H2.

chloride.

**Electrochemical system** 

in the Study of Semiconductor, Metallic and Oxide Systems 89

Table 2. Calculated electrochemical potential ( Cl2/Cl-) in individual molten chlorides,

**F-**

Ba 2+Ba - 1.31 - 2.59 - 2.62

Table 3. Electrochemical potential of metals in molten liquid halides at the temperature

**Elecrtocemical system / V** 

Table 4. Comparison of certain electrochemical potentials of metals in molten liquid

Zn2+Zn 1.706 1.603 In+ In 1.520 1.414 Sn2+Sn 1.428 1.320 Pb2+Pb 1.420 1.271 Sb3+Sb 1.019 -

 **Cl-**

K+K - 0.62 -2.50 - 2.53 - 1.98 Sr2+Sr - 1.16 - 2.51 - 2.41 - 1.94 Li+Li - 0.08 - 2.39 - 2.40 - 1.95 Na+Na - 1.84 - 2.36 - 2.35 - 1.81 Ca2+Ca - 1.13 - 2.35 - 2.25 - 1.53 Mg2+Mg - 0.28 - 1.58 - 1.58 - 1.01 Mn2+Mn - 0.17 - 0.85 - 0.83 - 0.44 Zn2+Zn - 1. 09 - 0.40 - 0.50 - 0.27 Cd2+Cd - 0.91 - 0.25 - 0.46 - 0.19 Tl+Tl - 0.44 - 0.29 - 0.41 **Sn2+Sn** - **- 1.270** - 0.981 **- 0.462 Pb2+ Pb** - **- 1.215** - 0.976 **-0.620**  Cu+Cu + 1.37 + 0.29 - 0.06 - 0.17 Co2+Co + 0.52 + 0.06 - 0.05 + 0.43 2H+H2 0 0 0 0 Bi3+Bi - 0.14 + 0.39 + 1.19 + 0.33

Pb2+Pb - 1.30 - 1.163 - 1.112 - 1.039 Fe2+Fe - 1.297 - - 1.118 - 1.050 Co2+Co - 1.171 - 1.028 - 0.977 - 0.900 Ni2+Ni - 1.104 - 0.939 - 0.875 - 0.763 Sb3+Sb - 1.019 (300°C) - - - Cu+ Cu - 1.035 - 0.987 - 0.970 - 0.943 Ag+ Ag - 0.911 - 0.848 - 0.826 - 0.784 Bi3+Bi - 0.844 - 0.817 - - Pd2+Pd - 0.487 - 0.487 - 0.340 - 0.285 Pt2+Pt - 0.299 - 0.180 - - Au+Au + 0.223 - - -

450°C 700°C 800°C 1000°C

 **/ V for the anions in Volt** 

 **Br-**

 **I-**

T=300°C T=500C°

**Electrochemical system φ / V** 

Fig. 14. Effect of exchange reaction between pure indium and its alloy In0/05Sn0.95 on the EMF mesures as a function of temperature and time experience in the cell (-) In | In+ in electrolyte | InxSn1-x (+) (- ○ -) - used points (-+- and -△-) - unused points.


Fig. 14. Effect of exchange reaction between pure indium and its alloy In0/05Sn0.95 on the EMF mesures as a function of temperature and time experience in the cell (-) In | In+ in

> Zn 2+Zn - 1.629 - 1.512 - 1.476 Tl+Tl - 1.629 - 1.512 - 1.473

> Sn2+Sn - 1.34 - 1.264 - 1.259

Cd 2+Cd - 1.442 -1.262 - 1.193 - 1.002 In+In - 1.43 - - - Cr3+Cr - 1374 - - 1.113 - 1.006

Li+ Li - 3.684 - 3.514 - 3.457 - 3.352 Na+ Na - 3.566 - 3.332 - 3.250 - 3.019 La3+La - 3 .241 - 3.016 - 2.997 - 2.876 Ce3+Ce - 3.193 - 3.014 - 2.945 - 2821 Nd3+Nd - 3.103 - - 2.856 - 2.736 Gd3+Gd - 3.013 - - 2.807 - 2.709 Mg2+Mg - 2.720 - 2.536 - 2.460 - 2.346 Sc3+Sc - 2.621 - 2.455 - 2.375 - 2.264 U3+U - 2.530 - 2.350 - 2.280 - 2.162 Be2+Be - 2.167 - - - Al3+Al - 2.018 - - - Mn2+Mn - 1.999 - 1.854 - 1.807 - 1.725 V2+V - 1.794 - 1.794 - 1.636 - 1.441

450°C 700°C 800°C 1000°C

electrolyte | InxSn1-x (+) (- ○ -) - used points (-+- and -△-) - unused points.

**Electrochemical system φ / V** 


Table 2. Calculated electrochemical potential ( Cl2/Cl-) in individual molten chlorides, calculated from thermodynamic data.


Table 3. Electrochemical potential of metals in molten liquid halides at the temperature 700°C compared with reference electrode H2.


Table 4. Comparison of certain electrochemical potentials of metals in molten liquid chloride.

Electrochemical Cells with the Liquid Electrolyte

exchange reaction in the electrochemical cell.

main types of exchange reactions (*a* and *b*) take place in cell:

aIn=1 a' In<1

a''In > aIn'

months, the maximum temperature reached 822K.

**5.6 Issues are related to the valence of An+ ion** 

in the Study of Semiconductor, Metallic and Oxide Systems 91

**№ xIn xSn xSb**

1 0.0500 0.4751 0.4749 2 0.0503 0.4497 0.5000 3 0.1002 0.3996 0.5003 4 0.1112 0.4444 0.4444 5 0.5000 0.2499 0.2501

Table 5. Composition of In-Sn-Sb alloys used for detection of kinetic of spontaneous

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

(-) InIn2+ in electrolyteIn-Sn-Sb (+)

b) InSnSb (№5) In Sn InSnSb (№ 1-4) (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

To obtain good experimental results, it is necessary to know rigorously the ion charge that is responsible for the EMF in cell of type (I). However, the difference in activities of pure metal A and alloy AxB(1-x) can lead to different charges of the An+ ion in the vicinity of the electrodes A and AxB(1-x). In this case, even in open circuit, a spontaneous transfer of component A to alloys AxB(1-x) is possible and a constant drift of the EMF occurs over time.

a) In (pure) In Sn InSnSb (№ 1-4) (13)

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

(-) 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, especially in the case of liquid systems.
