Electrolytes and Solution Conductivity

1 Distinguish among strong electrolytes, weak electrolytes, and nonelectrolytes. 

Formulas of ionic compounds were introduced in Section 6.8. Chemical compounds are electrically neutral. 

A net zero charge is achieved by combining cations and anions in numbers such that positive and negative charges are balanced. 

A nonelectrolyte does not conduct electricity because no ions are present in solution. 

A strong electrolyte conducts electricity. CUCI2 is completely dissociated into and CT ions. 

---

Figure 4.1 Schematic of the atomic structure of the active three-phase interface between the metal particle that catalyzes the reaction, the carbon support necessary to conduct electrons, and the polymer electrolyte and solution necessary to conduct protons for electrocatalytic systems.

Figure 8.11—Effecl of diffusion on the efficiency obtained in HPLC and CE. Diffusion increases with the square of tube diameter. This is, thus, more important in HPLC. In CE. the electrolyte is repelled by the wall leading to an almost perfect plane-like flow contrary to the usual parabolic profile obtained under hydrodynamic flow. However, other factors that depend on the difference in conductivity between the electrolyte and solutes can lead to peak deformation.

One useful property for characterizing a solution is its electrical conductivity, its ability to conduct an electric current. This characteristic can be checked conveniently by using an apparatus like the one shown in Fig. 4.4. If the solution in the container conducts electricity, the bulb lights. Some solutions conduct current very efficiently, and the bulb shines very brightly these solutions contain strong electrolytes. Other solutions conduct only a small current, and the bulb glows dimly these solutions contain weak electrolytes. Some solutions permit no current to flow, and the bulb remains unlit these solutions contain nonelectrolytes. 

The addition of a suitable amount of PC or a mixture of PC and EC to PEO-(LiX) does increase the room temperature conductivity of the electrolyte to >10" Scm but apparently with a significant reduction in its dimensional stability. This limitation, coming from the fact that PEO is soluble in PC or in a mixture of PC and EC, can be overcome with the use of polymers which are insoluble in the plasticizing solvents. Examples of such materials are the series of dimensionally stable polymer electrolyte films with conductivities of >10" Scm at room temperature we have prepared [33, 34]. A list of these electrolytes and their conductivities at 20°C is given in Table 3.7. They are obtained by immobilizing solutions of Li salts (i.e. Li salt-solvates) formed in a mixture of ethylene carbonate and propylene carbonate, in a polymer matrix such as poly(acrylonitrile), (PAN), poly[(tetraethylene glycol) diacrylate] (PEGDA), and poly(vinyl pyrrolidinone), (PVP). These polymers are insoluble in PC and EC/PC mixtures consequently, dimensionally stable... 

A variety of techniques is available for determining electrolyte and ion conductivity. For a description of the methods and their drawbacks see R. A. Robinson and R. H. Stokes, Electrolyte Solutions (London Butterworths, 1955), Chapter 5. 

In theory, there are no limits to the accessibility of electrokinetic data. However, there are a number of physical situations which limit the range of electrokinetic data which can be obtained in the above described experimental set-ups. The experimental techniques described above require visual determination of particle velocities and are typically limited to the range 3-100 pm/s. Additionally, according to equation (19.24), current and solution conductivity affect the particle mobility. In experimental practice, the limit for current in the cell is around 300 pA with the use of blank platinum electrodes. Under higher currents, electrolytic reactions at the electrodes result in electrode polarization, heating and subsequent formation of gas bubbles and thermal convection in the cell. Furthermore, solution conductivity measurements are not reliable below about 10 pS/cm. The above limitations restrict the typical range of solution ionic strengths at which one can work to O.l-lOOmM. 

In contrast with their oxide counterparts, CaFj and PbFj can form fluorite-type solid solutions with fluorides of higher-valency metals such as YFj, BiFj, UF4, etc. They are expressed, for example, by Cai xYxF2+x, in which the excess F" ions are accommodated in the interstitial sites, forming various kinds of clusters with Vp, depending on the concentration of F . They are usually good F solid electrolytes, and the conductivity of Pbj.xBixFj+x (x = 0.25) is shown in Figure 6.3 as an example. 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

The numerical approaches to the solution of the Laplace equation usually demand access to minicomputers with fast processing capabilities. Numerical methods of this sort are essential when the electrolyte is unconfined, as for an off-shore rig or a submarine hull. However, where the electrolyte is confined, as within essentially cylindrical equipment such as pipework and heat-exchangers, or for restricted electrolyte depths, a simpler modelling procedure may be adopted in the case of electrolytes of good conductivity, such as sea-water . This simpler procedure enables computation to be carried out on small, desk-top microcomputers. 

Immersion in aqueous media open to air Solutions in which tin is cathodic to steel cause corrosion at pores, with the possibility of serious pitting in electrolytes of high conductivity. Porous coatings may give satisfactory service when the corrosive medium deposits protective scale, as in hard waters, or when use is intermittent and is followed by cleaning, as for kitchen equipment, but otherwise coatings electrodeposited or sprayed to a sufficient thickness to be pore-free are usually required. 

When samples of about 1 cm were taken from a single cast film of 100 X 200 mm of a number of paint and varnish films, their resistances varied with the concentration of potassium chloride solution in one of two ways (Fig. 14.2). Either the resistance increased with increasing concentration of the electrolyte (inverse or / conduction) or the resistance of the film followed that of the solution in which it was immersed (direct or D conduction). The percentage of / and D samples taken from different castings varied, but average values for a number of castings were 50% D for the pentaerythritol alkyd and the tung oil phenol formaldehyde varnishes, 57% for urethane alkyd, 76% for epoxypolyamide and 78% for polyurethane varnishes... 

An investigation has been made of the factors which control / and D conduction and it has been found that the difference is only one of degree and not of kind . Thus, if the varnish films are exposed to solutions of decreasing water activity, then the resistance falls with increasing concentration of electrolyte, but a point is eventually reached when the type of conduction changes and the films exhibit /-type behaviour. It appears that D films can be converted into / films, the controlling factor being the uptake of water. 

When an ionic solid such as NaCl dissolves in water the solution formed contains Na+ and Cl- ions. Since ions are charged particles, the solution conducts an electric current (Figure 2.12) and we say that NaCl is a strong electrolyte. In contrast, a water solution of sugar, which is a molecular solid, does not conduct electricity. Sugar and other molecular solutes are nonelectrolytes. 

Aqueous solutions of many salts, of the common strong acids (hydrochloric, nitric and sulphuric), and of bases such as sodium hydroxide and potassium hydroxide are good conductors of electricity, whereas pure water shows only a very poor conducting capability. The above solutes are therefore termed electrolytes. On the other hand, certain solutes, for example ethane-1,2-diol (ethylene glycol) which is used as antifreeze , produce solutions which show a conducting capability only little different from that of water such solutes are referred to as non-electrolytes. Most reactions of analytical importance occurring in aqueous solution involve electrolytes, and it is necessary to consider the nature of such solutions. 

For strong electrolytes the molar conductivity increases as the dilution is increased, but it appears to approach a limiting value known as the molar conductivity at infinite dilution. The quantity A00 can be determined by graphical extrapolation for dilute solutions of strong electrolytes. For weak electrolytes the extrapolation method cannot be used for the determination of Ax but it may be calculated from the molar conductivities at infinite dilution of the respective ions, use being made of the Law of Independent Migration of Ions . At infinite dilution the ions are independent of each other, and each contributes its part of the total conductivity, thus ... 

---