Conductivity of electrolyte solution

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. 

We can recognize four main periods in the history of the study of aqueous solutions. Each period starts with one or more basic discoveries or advances in theoretical understanding. The first period, from about 1800 to 1890, was triggered by the discovery of the electrolysis of water followed by the investigation of other electrolysis reactions and electrochemical cells. Developments during this period are associated with names such as Davy, Faraday, Gay-Lussac, Hittorf, Ostwald, and Kohlrausch. The distinction between electrolytes and nonelectrolytes was made, the laws of electrolysis were quantitatively formulated, the electrical conductivity of electrolyte solutions was studied, and the concept of independent ions in solutions was proposed. 

Conductimetry is a method of obtaining analytical and physicochemical information by measuring the conductivities of electrolyte solutions [25]. Conductivity cells have two or four electrodes, but the processes that occur at or near the electrodes are not directly related to the information obtained by conductimetric measurements. 

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. 

Traceability structures for gas analysis, clinical chemistry, pH measurement and electrical conductivity of electrolyte solutions in Germany... 

The conductivity of electrolyte solutions depends on the concentration and the charge number of the ions in the solution. It is expressed as the molar or equivalent conductivity or molar conductivity, which is given by ... 

Electrical conductivity measures a material s ability to conduct an electric current. The high conductivity of metals is due to the presence of metallic bonds. The high conductivity of electrolyte solutions is due to the presence of ions in solution. 

Table 7 Selected Data on the Conductivity of Electrolyte Solutions Based on Polar Aprotic Systems... 

The phenomenon of electrolysis also receives a simple explanation on the basis of the theory of electrolytic dissociation. The conductance of electrolyte solutions is due to the fact that ions (charged particles) are present in the solution, which, when switching on the current, will start to migrate towards the electrode with opposite charge, owing to electrostatic forces. In the case of hydrochloric acid we have hydrogen and chloride ions in the solution ... 

The electrical conductivities of electrolyte solutions and the ion-pair association constant are both very sensitive to ion solvation and permit the calculation of solvation constants. 

This empirical relationship between the equivalent conductivity and the square root of concentration is a law named after Kohlrausch. His extremely careful measurements of the conductance of electrolytic solutions can be considered to have played a leading role in the initiation of ionics, the physical chemistry of ionic solutions. 

It is immediately obvious from Fig. lA, that Ohm s law does not apply, not even as a rough approximation. This observation is not as trivial as it may seem when we recall that in the study of conductivity of electrolytic solutions, Ohm s law is strictly obeyed over a very large range of potentials and frequencies. The difference is that Fig. lA pertains to measurements conducted under dc conditions, whereas ionic conductivity is measured, as a rule, with an alternating current or potential. The implication is that the impedance of the metal-solution interphase is partially capacitive - a subject to be dealt with in considerable detail shortly. 

Nernst, Walther. (1864-1941). A German chemist who won the Nobel Prize in 1920. He was educated atZurich and Berlin and received his Ph.D. at Wurzburg. He wrote many works concerning theory of electric potential and conduction of electrolytic solutions. He developed the third law of thermodynamics, which states that at absolute zero the entropy of every material in perfect equilibrium is zero, and therefore volume, pressure, and surface tension all become independent of temperature. He also invented Nernst s lamp, which required no vacuum and little current. 

Electrolyte solutions contain ions which can move in response to a gradient in electrical potential. The transport properties of these systems are important in devices such as batteries and in living systems. The movement of ions in solution is very different from the movement of electrons in metallic conductors, and it is important to understand the fundamental laws which govern the conductivity of electrolyte solutions. Ions move according to the classical laws of physics, whereas the movement of electrons is quantal. 

The concept of triple-ions arose from the conductance studies of Fuoss and Krauss297) who observed an increase of equivalent conductance of electrolyte solutions at higher salt concentrations. They interpreted these findings as evidence of association of the free ions with their pairs, viz. 

In 1884 Arrhenius obtained his Ph.D. from the University of Uppsala with a thesis on the conductivities of electrolytic solutions. Although poorly rated by his examiners, his thesis attracted the attention of the most distinguished physicists and physical chemists in Europe at the time. Arrhenius collaborated with a number of them from 1886 until 1890. Based on his international reputation, he secured a post at the Technical High School in Stockholm, first as a lecturer, then as a professor, and finally as its rector. He later became director of the new physical chemistry institute of the Nobel Foundation in 1905. By that time, his interests had already shifted toward other fields of science. 

The electrical conductance of electrolyte solutions is measured under isothermal, isobaric conditions with uniform concentration throughout the cell, in which case jik = 0 and Eq. (13.7.8) becomes... 

Classical electrolyte theories were developed to explain equivalent conductance of electrolyte solutions and not mobility of sample ions at infinite dilution. In essence, such theories describe the electrophoretic behavior of the electrolyte and not the sample ions. Theories for the mobility of sample ions are difficult to formulate and are only poorly developed at present. A simple extension of classical electrolyte theories to electrophoretic mobility for ions of finite size, allows the derivation of Eq. (8.4) [32]... 

The conductivity of electrolyte solutions is equal to the sum of the conductivities of each type of ion present. For a single dissolved salt, the equivalent conductance can be expressed as... 

The work of Debye and Erich Hiickel (1896-1880), published in 1923, led to a theory of ionic solutions that explained a number of anomalies concerning conductivities of electrolytic solutions. In 1926, Lars Onsager (1903-76) added the treatment of Brownian motion toward understanding the transport properties of ions in melts, aqueous, and... 

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