**1. Introduction**

70 Electrochemical Cells – New Advances in Fundamental Researches and Applications

External-loop airlift reactors have been shown to be versatile tools to carry out EC with complete flotation, using only electrochemically generated H2 bubbles to achieve an overall liquid circulation and good mixing conditions. Consequently, the use of mechanical agitation, pumping or compressed air was not necessary. This could not be achieved in other kinds of conventional gas-liquid contacting devices than airlift reactors. External-loop devices are particularly adapted because they offer specific designs for the disengagement section that allow large distance between riser and downcomer. This improves flotation by minimizing the recirculation of aluminum or iron particles in the downcomer. These results were obtained by the adequate selection of the axial position of the electrodes (*H1*) and the liquid height in the separator section (*h*) in order to avoid floc break-up in the riser and floc erosion at the free surface. A limiting value of the liquid velocity in the downcomer was defined, while *ULd* was correlated to dispersion height *hD* and current density *j* (Equation 17). These can be used at

To increase the efficiency of EC in a continuous reactor, the mean residence time should be increased. The experiments showed that this effect is reached in the case of a relatively high value of current density and weak value of the inlet flow-rate. This study highlighted the hydrodynamic aspect of the flow in the external airlift reactor functioning as a batch and continuous reactor. The design of this kind of reactor should be improved to allow the

Essadki H., Nikov I., Delmas H., (1997), *Electrochemical Probe for Bubble Size Prediction in a* 

Essadki A.H., Gourich B., Vial Ch., Delmas H., Bennajah M., (2009), *Defluoridation of drinking* 

Levart E., and Schumann D., (1974), *Analyse du Transport Transitoire sur un Disque Tournant* 

L.P.Reiss and T.I.Hanratty , *An experimental study of the unsteady nature of the viscous sublayer*,

*water by electrocoagulation/electroflotation in a stirred reactor with a comparative performance to an external-loop airlift reactor,* Journal of Hazardous Materials 168,

*en Régime Hydrodynamique Laminaire et Permanent*, Int. J. Heat Mass Transfer 17,

*Bubble Column*, Experimental Thermal and Fluid Science, 243-250.

reactant to follow the compartment in which the reaction takes place (riser).

**4. Conclusion** 

**5. References** 

1325-133.

555–566.

constant *j* and *Ar*/*Ad* ratio for scale-up purpose.

Chen G., (2004), Sep. Purif. Technol. 38, 11–41. Chisti Y., (1989), *Airlift Bioreactors*, Elsevier, London.

AIChE J, 9, (1963), 154-160.

The first publications devoted to a study of the thermodynamic properties of metallic alloys, using electrochemical cells (EMF method) was known since 1936 year (Strikler & Seltz, 1936). This was the groundwork for all the next studies.

Co-workers from Moscow State University (Geyderih et al., 1969) have considered some questions about this experimental method.

A new attempt to generalize the electrochemical methods on thermodynamic studies of metallic systems was made again in the book (Moratchevsky, 1987). The general aspects of the thermodynamics of nonstoichiometric compounds were presented there and the methods for experimental studies of the thermodynamic properties of molten metal and salt systems were discussed.

The different types of electrochemical cells with solid and liquid electrolytes and dynamic EMF methods were examined in the recent book (Moratchevsky et al., 2003). A separate chapter of this book deals with methods of treatment and presentation of experimental data. In recent decades the important step of qualitative development of EMF method had been made and it was not considered in this book.

In the present chapter we focus on those experimental techniques that help to increase significantly the experimental result precision.

The knowledge of thermodynamic properties and phase diagrams of binary, ternary and multi component systems is necessary for solving materials science problems and for designing new products and technologies fitted to actual needs. A rational study of equilibria among phases and of the given system thermodynamic properties not only leads to the discovery of unknown phases but also to the determination of phase thermodynamic stability, to homogeneity domain boundaries, and finally to the elaboration of analytical description of the system by using thermodynamic models which are based on the dependence of phase Gibbs energies on such parameters as temperature, concentration and pressure.

Electrochemical Cells with the Liquid Electrolyte

where *b* is *tg*( ) (see Fig.1)

= - *HA* and b = *SA* .

respectively.

Gibbs-Duhem- Margules equations:

Gibbs-Duhem equation for the two-component system AB is:

The Margules equation is generalized by Gibbs-Duhem equation:

in the Study of Semiconductor, Metallic and Oxide Systems 73

Fig. 1. Graphical relation of measured values E(T) with partials thermodynamic functions a

The integral thermodynamic functions can be calculated with help of Gibbs-Duhem or

1 1 /1

(1 ) <sup>1</sup> *x x i A <sup>x</sup> x d*

The partial and integral thermodynamic values are presented by the terms *Фi* and *Ф* ,

*<sup>A</sup> H TS A A* (3)

*<sup>A</sup> nFE* (4)

*H nFa <sup>A</sup>* (5)

*S nFb <sup>A</sup>* (6)

*A A* / / *P P S TnF E T* (7)

*AA A* / *<sup>P</sup> H T S nF T E T E* (8)

11 1 2 *x x* (1 ) (9)

1

*x* (10)

0 1

Experimental studies are the primary information sources for thermodynamic properties and phase diagrams of all systems. The method of electromotive force (EMF) is one of the most important methods of the physicochemical analysis. One peculiarity of the EMF is its proportionality to the chemical potential:

*i = nFE* of one of the system components,

where *n* is a charge of the ion responsible for the potential,

*F*=96485.34 C/mol is the constant of Faraday,

*E* is electromotive force.

Improving the accuracy and reproducibility of measurements leads to the increase of the quality and quantity of information about the system. Values of *i (T, xi)* versus temperature *(T)* and atomic fraction *(xi)* obtained with uncertainties of 500 J/mol (especially in a narrow temperature range) give only rough estimates of partial entropy and enthalpy of the components.

An accuracy improvement in determining of the chemical potential *(i(T, xi)* versus temperature *(T)* and atomic fraction *(xi)* from 10 to 50 J/mol not only leads to the various thermodynamic properties of the system (partial entropies ( *<sup>f</sup> Si* ) and enthalpies ( f i H ) of components, phase enthalpies of transformation ( *<sup>f</sup> Hitr*, ), partial enthalpies at infinite dilution ( *<sup>f</sup> Hi* ), thermal capacities (Сp), but also gives a possibility to study the phase diagram in detail (liquidus and solidus, miscibility gaps, invariant points, stoichiometry deviations, ordering, etc...)
