Theory of Strong Electrolyte 

            The conductance of the solution of a given electrolyte depends on two factors, namely the number of ions and their speed. If the electrolyte is completely dissociated, it is called a strong electrolyte and if it is dissociated partially, it is called the weak electrolyte. Concerning the conductance of both weak and strong electrolytes, it is observed that the equivalent\molar conductance decreases with an increase in concentration. Why?   

            The decrease in equivalent conductance with an increase in concentration for weak electrolytes can be explained as due essentially to a decrease in the degree of ionization. Degree of ionization decrease with an increase in concentration. With the decrease in the degree of ionization, the number of ions per unit volume decreases and this results in the decrease in the equivalent conductance. However, such an explanation cannot apply to strong electrolytes, since these are almost completely dissociated in all dilute solutions. Consequently, to account for the verification of equivalent conductance with a concentration in strong electrolytes, some other explanation must be sought, and this is found in the Debye-Huckel-Onsager Theory of conductance.

            In 1923 P. Debye and E. Huckel put forward a theory of interionic attraction for a dilute solution of strong electrolytes. This theory occupies a dominant place in all considerations involving electrolytes and their kinetic and thermodynamics behavior. Debye and Huckel suggested that strong electrolytes exist in solution as ions. They believe that strong electrolytes at least in dilute solution are completely ionized and the effect observed is due to unequal distribution of ions resulting from the interior attraction. Debye Huckel showed that because of the electrostatic attraction between charged ions, each positive ion in the solution must be surrounded on an average with more negative ions than ions of like charge; and conversely, each negative ion must be surrounded on an average with more positive than negative ions. In other words, each ion in the solution is surrounded by an ionic atmosphere whose net charge is opposite to that of the central ion. They showed, further that the properties of an electrolyte are determined by the interaction of the central ion with its atmosphere. Since the nature of the atmosphere is determined by the valences of the ions in the solution, their concentration, the temperature, and the dielectric constant of the medium, it must follow that there are also factors controlling the thermodynamic property of the electrolyte. At any given temperature and in a given solvent, the temperature and dielectric constant are fixed, hence the properties of an electrolyte depend only on the charges of the ions and their concentration not at all on the nature of each electrolyte. These conclusions are only valid for dilute solutions.

            According to Debye-Huckel's theory of interionic attraction, each ion in solution is surrounded by an ionic atmosphere of opposite ions whose net charge is on average opposite to that of the central ion. This ionic atmosphere is spherical and symmetrical in nature in the absence of any disturbing factors such as the application of a potential across the two electrodes immersed in the solution during the conductance measurement. However, when potential is applied, the ions start moving towards the respective electrodes and as a result, the ionic atmosphere is distorted which results in a decrease in the speed of the ions. Debye and Huckel showed that these effects are due to two factors, namely, (1) Asymmetric or Relaxation effect and (2) Electrophoretic effect.

1 Asymmetric or Relaxation Effect           

            According to Debye and Huckel's theory, each ion in the solution of an electrolyte is surrounded by an ionic atmosphere of oppositely charged ions. For example, a positively charged ion is surrounded by the atmosphere of negatively charged ions and vice versa. When the ions have no external force applied to them, this atmosphere is spherically and symmetrically distributed about the ions. However, when an external force is imposed, as when a potential is applied across the two electrodes the ions present in the solution move toward their respective electrodes. The central positive ion will tend to move towards the cathode, while its negatively charged ion atmosphere will move towards the anode. As a result, the symmetry of the ionic atmosphere about the central ion is destroyed. As long as the ionic atmosphere is symmetrical: it will exert the uniform force of attraction on the central ion from all possible directions and the net effect will be nil.

            But in the case of a distorted ionic atmosphere, the force of attraction from the rear becomes prominent on the moving central ion. Therefore, the central moving ion experiences a backward pull or a retarding force. Its mobility is slowed down and hence, the effective conductance of the solution decreases. This backward pull on the central ion is known as the asymmetric effect as it is due to the lack of symmetry in the ionic atmosphere. Once the ionic atmosphere is destroyed, a finite time is required for rebuilding another symmetric ion atmosphere. This time is called the relaxation time. This time of relaxation is considered to explain the retardation in the motion of the central moving ion by the ionic atmosphere, therefore the asymmetric effect is also known as the relaxation effect.

2 Electrophoretic Effect

            In addition to the asymmetric effect, there is another factor with retards the mobility of ions, the electrophoretic effect. This effect arises from the fact that an ion, in moving through the solution, does not travel through a stationary medium but through one that moves in a direction opposite to that of the ion. Ions are generally solvated, and when this moves, they carry with them solvent. Any positive ion migrating towards the cathode has then to the thread (pass into) its way through the medium moving with negative ions toward the anode. Similarly, negative ions have to migrate through the molecules of solvent carried by negative ions in the opposite direction. These counter-movements have a retarding influence on the mobility of the central ion. This effect is known as the electrophoretic effect. This causes an additional viscous force which further slows down the speed of the ions and hence the conductance.

            It is called the electrophoretic effect because it is somewhat analogous to opposing the movement of colloidal particles in the electric field. It slows down the speed of the ions in the same way as swimming against the current in a river would slow down swimming. For any dilute solution, the ionic atmosphere is almost nonexistent, so the asymmetric effect is zero. Similarly, at infinite dilution, the electrophoretic effect is also negligible but both effects have definite values in concentrated solution. Debye and Huckel showed that both these retarding effects on an ion produce a decrease in the equivalent conductance. These are the theoretical explanation for the decrease of equivalent conductance in the case of solutions of strong electrolytes.       



By: Muzammil  Khalid 

The writer is a post-graduate student from the Chemistry Department University of Turbat   

Turbat Kech Balochistan