NaCl ionic conductivity

The parameter c Eqn (2.1), is capable of variation by many orders of magnitude in ionic solids. In good solid electrolytes such as Na "-alumina and RbAg4l5, all of the Na /Ag ions are potentially mobile and hence c is optimised. At the other extreme, in pure, stoichiometric salts such as NaCl, ionic conduction depends on the presence of crystal defects, whether... 

Frenkel defects on the anion sublattice show only anion migration and hence have fa close to 1. The alkali halides NaF, NaCl, NaBr, and KC1 in which Schottky defects prevail and in which the cations and anions are of similar sizes have both cation and anion contributions to ionic conductivity and show intermediate values of both anion and cation transport number. 

Example 3.7 Use molar ionic conductivity data in Table 3-1 to calculate the mobility and diffusivity of Na+, Cl and NaCl at infinite dilution and 298.15 K. 

FIGURE 5.8 The ionic conductivity of NaCl plotted against reciprocal temperature. 

Figure 5.2 The ionic conductivity of pure NaCl as a function of temperature. Intrinsic conduction occurs in stages I and II stage III corresponds to conduction by cation vacancies present as a result of impurities. Vacancies become associated to form neutral pairs in stage IV.

We all know from experience that both sugar (sucrose) and table salt (NaCl) dissolve in water. The solutions that result, though, are quite different. When sucrose, a molecular substance, dissolves in water, the solution that results contains neutral sucrose molecules surrounded by water. When NaCl, an ionic substance, dissolves in water, the solution contains separate Na+ and Cl- ions surrounded by water. Because of the presence of the ions, the NaCl solution conducts electricity, but the sucrose solution does not. 

Figure 3. Ionic conductivity of the Tween microemulsions and PEO solutions, compared with Equations 1 and 2. For these experiments, the aqueous phase was 0.1 N NaCl or 0.1 N KC1, and the measured conductivity values were normalized to that of the suspending electrolyte.

Instead of providing electrons, atoms in defect sites may themselves be mobile and thus provide ionic conduction in a solid. Ionic compounds such as NaCl have high conductivity in their molten form, and such conductivity is important for the manufacture of aluminum by electrolysis of molten cryolite (Na3 A1F6). In most solids however, ionic conduction is much lower and arises largely from... 

For a solution like KCI where the ionic conductances of both ions are the same, t+ = t" and i jm is the same value for both interfaces. However, this is not the case for NaCl where t+ = 0.4 and t = 0.6. The equations above indicate that the value of i jm at the anion exchange membrane would be 1.5 times higher than that at the cation exchange membrane. 

For solutions of simple, pure electrolytes (one positive and one negative ionic species), such as NaCl, equivalent ionic conductivity A°, which is the molar conductivity per unit concentration of charge, is defined as... 

The value of the specific conductivity at room temperature is usually orders of magnitude smaller than in liquid electrolytes. For example, the ionic conductivity of NaCl at room temperature is of the order of 10 S cm". Conductivities with values almost as high as those for liquid electrolytes can nevertheless be achieved by increasing temperature. The transition from the region of low conductance at room temperature to super ionic conductance at higher temperature can occur continuously, as a transition of second-order, or as first-order transition, which is shown in Figure 1.10. 

We have mentioned all along that in addition to all the beautiful molecules we constructed up to now using shared electron pairs (covalent bonds), there are materials wherein the electron pair in the bond is completely owned by one of the bonding partners, and as such the bond involves two oppositely charged ions. This ionic constitution of the bond can be detected in a variety of ways. For example, if you connect a crystal of common salt, NaCl, to a buzzer and wet the crystal a bit, the buzzer will whistle due to the movement of the ions and the creation of an electric current. A crystal of sugar that is made exclusively of covalent bonds will not whistle even if you drown the sugar in water. Similarly, a solution of an ionic material like NaCl will conduct electricity, while a solution of a covalent material like acetone or sugar will not. 

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