Organic ions, limiting equivalent conductances

Lithium, sodium, potassium, or other salts of benzoic acid, phthalic acid, sulfoben-zoic acid, citric acid, and others are useful eluents for anions. These are rather large organic anions that are less mobile than most inorganic anions and therefore have lower equivalent conductances. For example. Table 4.1 shows that the benzoate anion has a limiting equivalent conductance of 32 S cm equiv , while chloride, nitrate, sulfate, and other typical sample anions have higher equivalent conductances (approximately 70 S cm equiv. If a sodium benzoate eluent is used, the equivalent conductance is the sum of sodium ion (50) and benzoate (32), or 85 Scm equiv . The equivalent conductance of an anion is the sum of equivalent conductances of the sodium ion (50) and the anion (70), or 120 S cm equiv . On an equivalent basis, this amounts to almost a 50% increase in conductance. 

Electrolyte conductivity depends on three factors the ion charges, mobilities, and concentrations of ionic species present. First, the number of electrons each ion carries is important, because A, for example, carries twice as much charge as A . Second, the speed with which each ion can travel is termed its mobility. The mobility of an ion is the limiting velocity of the ion in an electric field of unit strength. Factors that affect the mobility of the ion include (1) the solvent (e.g., water or organic), (2) the applied voltage, (3) the size of the ion (the larger it is, the less mobile it will be), and (4) the nature of the ion (if it becomes hydrated, its effective size is increased). The mobility is also affected by the viscosity and temperature of the solvent. Under standard conditions the mobility is a reproducible physical property of the ion. Because in electrolytes the ion concentration is an important variable, it is usual to relate the electrolytic conductivity to equivalent conductivity. This is defined by... 

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