Conductivity of ionic solutions

Q.22.5 What are the primary differences between a true and potential electrolyte Q.22.6 Does temperature alter the conductivity of ionic solutions Why or why not Q.22.7 Using the data from Fig. 23.6 and Table 23.1, estimate the equilibrium... 

Figure 4.3 The electrical conductivity of ionic solutions. A, When electrodes connected to a power source are placed in distilled water, no current flows and the bulb is unlit. B, A solid ionic compound, such as KBr, conducts no current because the ions are bound tightly together. C, When KBr dissolves in H2O, the ions separate and move through the solution toward the oppositely charged electrodes, thereby conducting a current.

Bertrand, G.L. Conductivity of ionic solutions, http //web.mst.edu/ ghert/conductivity/cond. html. Accessed 28 Dec 2010... 

Using basic theory regarding the conductivity of ionic solutions and Faraday s law, the following relationship for the delivered dose of a drug can be derived (Sage, 1995b) ... 

Most of the students were able to distinguish between the eleetrieal eonductivity of metals and the electrical conductivity of ionic solutions and between the characteristics of copper as a metal and copper chloride as an ionie solution. 

The conductivity of ionic solutions is due to movement of both cations and anions. They move in opposite directions (as might be expected), and so we can consider a current due to positive ions, J+, and a current due to negative ions, /. If we consider the current as the change in the amount of ions passing through a cross-sectional area A per unit time, as shown in Figure 8.11, then we can write the current as... 

When combined, equations 8.66 and 8.67 are called the Onsager equations for the conductance of ionic solutions. 

How does the ionic bonding model explain the nonconductivity of ionic solids, and at the same time the conductivity of ionic solutions ... 

A distinguishing property of ionic solutions is electrical conductivity, just as it is a distinguishing property for metals, but the current-carrying mechanism differs. Electric charge moves through a metal wire, we believe, by means of... 

Many recent examples show the importance of ionic radii and solvation in the conductivity of concentrated solutions. Suffice it to refer to three examples from the literature. 

In the classical theory of conductivity of electrolyte solutions, independent ionic migration is assumed. However, in real solutions the mobilities Uj and molar conductivities Xj of the individual ions depend on the total solution concentration, a situation which, for instance, is reflected in Kohhausch s square-root law. The values of said quantities also depend on the identities of the other ions. All these observations point to an influence of ion-ion interaction on the migration of the ions in solution. 

As already mentioned, the criterion of complete ionization is the fulfilment of the Kohlrausch and Onsager equations (2.4.15) and (2.4.26) stating that the molar conductivity of the solution has to decrease linearly with the square root of its concentration. However, these relationships are valid at moderate concentrations only. At high concentrations, distinct deviations are observed which can partly be ascribed to non-bonding electrostatic and other interaction of more complicated nature (cf. p. 38) and partly to ionic bond formation between ions of opposite charge, i.e. to ion association (ion-pair formation). The separation of these two effects is indeed rather difficult. 

This equation is valid for both strong and weak electrolytes, as a = 1 at the limiting dilution. The quantities A = zf- FU have the significance of ionic conductivities at infinite dilution. The Kohlrausch law of independent ionic conductivities holds for a solution containing an arbitrary number of ion species. At limiting dilution, all the ions conduct electric current independently the total conductivity of the solution is the sum of the contributions of the individual ions. 

The presence of ionic species is demonstrated by the conductivity of the solutions. It is a strongly acidic solvent that protonates alcohols, ethers, and acetic acid. These substances are not normally bases, but they have an unshared pair of electrons that can function as a proton acceptors. 

Onset of micellization is detected by sharp changes in such properties as surface tension, refractivity or conductivity (of ionic micelles). To a first approximation the solution is assumed to contain monomeric amphiphiles, whose concentration is given by the cmc, and fully formed micelles, with submicellar aggregates playing a minor role. 

The electrical conductance of a solution is a measure of its current-carrying capacity and is therefore determined by the total ionic strength. It is a nonspecific property and for this reason direct conductance measurements are of little use unless the solution contains only the electrolyte to be determined or the concentrations of other ionic species in the solution are known. Conductometric titrations, in which the species of interest are converted to non-ionic forms by neutralization, precipitation, etc. are of more value. The equivalence point may be located graphically by plotting the change in conductance as a function of the volume of titrant added. 

In some experiments, we need to enhance the ionic conductivity of a solution, so we add an additional ionic compound to it. Rather confusingly, we call both the compound and the resultant solution an electrolyte . 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

In ionic solids, electrons are held in place around the ions so they don t conduct electricity. However, in aqueous solution and molten state, they do conduct electricity. Electrical conductance of ionic compounds is not due to movement of electrons but to the movement of ions. 

If the reaction system has a ionic species and if there is a change in number and/or nature of the ionic species during the reaction, the electrical conductance of the solution can be measured as a function of time. Let us consider the hydrolysis of an ester in presence of NaOH... 

Figure 4.18 Conductimetric titration curves. As an acid is titrated with an alkali, so the ionic composition of the mixture changes and is reflected in the conductivity of the solution, (a) A strong acid and a strong base, (b) A strong acid and a weak base, (c) A weak acid and a weak base, (d) A weak acid and a strong base.

The increase in conductivity of a solution is directly proportional to an increase in ionic concentration... 

The safety sensor, however, gives only qualitative information. For a quantitative determination of the concentration of HF in a solution, it is necessary to determine JpS, which can be done by scanning the anodic potential from about 3 V to 0 V and measuring the relative current maximum in a unstirred solution. If JPS and the temperature T are determined, the electrolyte concentration c can be calculated using Eq. (4.9). This method of determining the concentration of HF is superior to simple measurements of the conductivity of the solution, because it is insensitive to dissolution products of Si or Si02, or to other ionic species in the analyte. 

The ionic conductivity of a solution depends on the viscosity, diffusivity, and dielectric constant of the solvent, and the dissociation constant of the molecule. EFL mixtures can carry charge. The conductivity of perfluoroacetate salts in EFL mixtures of carbon dioxide and methanol is large (10 to 10 " S/cm for salt concentrations of 0.05-5 mM) and increases with salt concentration. The ionic conductivity of tetra-methylammonium bicarbonate (TMAHCO3) in methanol/C02 mixtures has specific conductivities in the range of 9-14 mS/cm for pure methanol at pressures varying from 5.8 to 14.1 MPa, which decreases with added CO2 to a value of 1-2 mS/cm for 0.50 mole fraction CO2 for all pressures studied. When as much as 0.70 mole fraction... 

When the relative permittivity of the organic solvent or solvent mixture is e < 10, then ionic dissociation can generally be entirely neglected, and potential electrolytes behave as if they were nonelectrolytes. This is most clearly demonstrated experimentally by the negligible electrical conductivity of the solution, which is about as small as that of the pure organic solvent. The interactions between solute and solvent in such solutions have been discussed in section 2.3, and the concern here is with solute-solute interactions only. These take place mainly by dipole-dipole interactions, hydrogen bonding, or adduct formation. 

The latter indicates that some ions (in the present case, cations, Na+) are partially bound to the SDS-micelles, which results in a change in the slope of the conductivity of the solution. Similar behavior is observed for other ionic detergents such as cationic (CTAB) surfactants. 

As described in Section 5.8, the conductivity of electrolyte solutions is a result of the transport of ions. Thus, conductimetry is the most straightforward method for studying the behavior of ions and electrolytes in solutions. The problems of electrolytic conductivity and ionic transport number in non-aqueous solutions have been dealt with in several books [1-7]. However, even now, our knowledge of ionic conductivity is increasing, especially in relation to the role of dynamical solvent properties. In this chapter, fundamental aspects of conductimetry in non-aqueous solutions are outlined. 

---