The Ion Activity Theory and Its Application to Acid-Base Equilibria

THE ION ACTIVITY THEORY AND ITS APPLICATION TO ACID-BASE EQUILIBRIA 

As early as, 1805 the theory of Gbotthus concerning the electrical conductivity of solutions propwjsed that the positive and negative parts of the electrolyte molecules were transferred from molecule to molecule by a chain mechanism. Consequently these portions must have an independent existence for at least a short time during the electrolysis. 

These dynamic considerations led R. Clausius (1857), fifty years later, to conclude that in an electrolyte solution, even in the absence of a current, there must be present a number of free electrical partial molecules possessing an irregular heat motion. The effect of an electric current is to constrain these particles to move in a definite direction. Simultaneously, W. Hittorf wrote in reports of his classical investigations of the migration of ions, The ions of electrolytes can not be firmly bound to the electrolyte molecule.  

Unfortunately a clear picture of the state of a dissolved electrolyte was not available at the time. Eighty years after Grotthus proposal was made, physical chemistry received its tremendous impetus and not until then was it possible to account for the physical properties of electrolyte solutions. 

At that time van t Hoff made his famous comparison between solutions and gases. He stated that solutes in dilute solution were governed by the same equation of state which applies to gases pv = RT, where p represents the osmotic pressure of the solution instead of the gas pressure. The behavior of nonelectrolytes is well described by this rule. The measurements of Raoult have shown that the lowering in freezing point of solvents is independent of the nature of the solute, and is determined only by the molecular concentration of the solute. Further investigations by Pfeffer, M. Traube, and Hugo de 

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