Conductivity of salts

The electrical conductivity.—E. Klein10 showed that if there is a difference between the conductivity of a mixture of salts in soln. and the mean conductivities of the separate constituents, a double salt is probably formed. The molecular conductivity of a salt, and if possible of its components at different dilutions, has been employed to determine the number of component ions in a soln. it was used, for example, by A. Werner (1893-1901) with the cobalt, chromium, platinum, and other ammines.11 In moderately cone. soln. the double salts are but little ionized, and the difference between the conductivities of eq. soln. of potassium zinc chloride, ZnCl2.2KCl, and of the sum of the constituents amounts to nearly 36 per cent., a value which is greatly in excess of that whieh would be due to the mutual influence of salts with a common ion. Tables of the molecular conductivities of salts show that with very few exceptions, at a dilution of 1024 litres and 25°, most salts have conductivities approximating those indicated in Table XIX. 

The conductivity of salts in solvents of low dielectric constant, and of metals in liquid ammonia, exhibit minima which may be explained in terms of an equilibrium between ions and a coulombic compound of two ions, or "ion pairs." This equilibrium conforms to the law of mass action. At limiting conductance in solutions of sodium in liquid ammonia, part of the current is carried by metal ions, but seven-eighths is carried by electrions. Following the BLA model, it is assumed that when two ion pairs, consisting of a sodium ion and an electron, come together, the spins of the two electrons couple to form disodium spinide. Increase in conductivity past the minimum is assumed to be caused by dissociation of disodium spinide into sodium ions and spinions. 

Recently viscosity, density and index of refraction measurements have been recorded for some liquid alkylsydnones (77MI42100). For example, for 3-methylsydnone at 40 °C the viscosity is 5.501 cP, the density is 1.3085 gem-3 and the index of refraction nD is 1.515. Measurements on the conductance of salt solutions in 3-methylsydnone are also available (80MI42111). 

As a result of prolonged and careful study of the conductance of salt solutions down to low concentrations, Kohlrausch found that the difference in molar conductivities of pairs of salts, containing similar anions and always the same two cations, is constant and independent of the nature of the anion. He found for example that the following differences of limiting molar conductivities (measured at 18°C in cm2 Q 1 mol-1 units)... 

E5.10 Electric Conductivity of Salt Crystals, Melts and Solutions... 

Hartley and associates ° 7 have also studied the conductances of salts in ethyl alcohol. With that solvent the equivalent conductances, at least for dilute solutions, follow the relation... 

The Conductance of Salts in Solvents of Low Dielectric Constant. In order to approach a discussion of phenomena that are encountered in solvents of low dielectric constants, Le., less than 25, it will be of service to consider as examples the experimental results of Kraus and Fuoss 20 who have determined the conductance of a single salt (tetraiso-amylammonium nitrate) in mixtures of widely varying composition of dioxane and water. The dielectric constants of these mixtures covered the range of values from the dielectric constant of 2.2 for pure dioxane to 78.6 for water. The experimental results are plotted in Fig. 5, in... 

The equivalent conductance of salts or ions increases as the concentration decreases. This phenomenon is directly related to the interionic forces present in solution a given cation, for example, will have more anions in its vicinity than expected from a purely random distribution. This ionic atmosphere has two effects, electrophoretic and time of relaxation, both of which tend to decrease the ion s mobility. In the former effect, the solvent molecules associated with the ionic atmosphere are moving in a direction opposite to that of the central ion. In the latter, the ionic atmosphere moves slower than the central ion, causing a charge separation (electrostatic retarding force) on the central ion. 

The effect of pressure on the conductance of salts and HBr in MeOH has been studied. The mobility of decreases less with pressure than that of the other ions. For HPic solutions in MeOH, A increases with the pressure because the enhanced dissociation of the acid dominates the variation of mobilities. In MeOH, Bu4NBPh4 has A°rj constant up to 5 x 10 Nm , NaBr shows an increase in the Walden product with pressure, and the i 4N bromides are intermediate cases. Variations of values with pressure were also observed to depend on the particular salt. 

Arrhenius, Svante August won the 1903 Nobel Prize in Chemistry for his work on the electrical conductivity of salt solutions (he was also nominated for the Physics Prize). He is often hailed as a pioneer of modern environmentalism for his work on the greenhouse effect. One of his predictions was that the United States might pump its last barrel of oil in 1935. Fortunately he was proved wrong, but his concern about the world s natural mineral resources and the need for alternative sources of energy was prescient. He died in 1927 at age 68. 

Kohlrausch s principle of the additivity of equivalent conductivities of salts provided Werner and Miolati with a convenient method for determining the number of... 

The earliest application of electrodeless conductivity measurements appears to have been for measurement of salinity at various ocean depths (1, 11, 45, ). Other early uses have also included determination of the equivalent conductances of salts at high concentrations (3, 34, 53) and the monitoring of nitric... 

The British scientist Henry Cavendish (1731-1810) reported that the electric conductivity of water is greatly increased by dissolving salt in it. In 1884 the young Swedish scientist Svante Arrhenius (1859-1927) published his doctor s dissertation, which included measurements of the electric conductivity of salt solutions and his ideas as to their interpretation. These ideas were rather vague, but he later made them more precise and then published a detailed paper on ionic dissociation in 1887. Arrhenius assumed that in a solution of sodium chloride in water there are present sodium ions, Na, and chloride ions, Cl . When electrodes are put into such a solution the sodium ions are attracted toward the cathode and move in that direction, and the chloride ions are attracted toward the anode and move in the direction of the anode. The motion of these ions through the solution, in opposite directions, provides the mechanism of conduction of the current of electricity by the solution. 

Alkali and Alkaline Earth Metal Production by Molten Salt Electrolysis, Fig. 4 Specific electrical conductivity of salt rich mixtures of NaCl-Na and KCl-K from Bredig [5]... 

The equivalent conductance of salts increases on dilution to a limiting value (Ao) called the limiting equivalent conductance (formerly written A, the conductance at infinite dilution ). This value can be tound experimentally and accurateh, but only for salts because the small concentration of hydrogen R)ns. maintained b the ionization of water, interferes with the ionization of weak acids and bases. For these,, o is obtained b summing the ionic conductances (ionic mobilities) of the individual ions, taken from tables such as those of... 

The symbol A (or A°) represents the maximum theoretical value that the molar conductivity of an electrolyte will approach when diluted indefinitely with an inert solvent. At the beginning of this century Kohlrausch found that the molar conductivity of salts in very dilute aqueous solutions showed a linear relation with the square root of the concentration. This, Kohlrausch s square root law , was incompatible with the Arrhenius electrolytic dissociation theory (q.v.), but it has since been justified by the Debye-Hiickel-Onsager theory of interionic attraction effects, which have been shown to have a dependence. 

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