ELECTROLYTES AND THEIR ACTION 223 



SUMMARY 



There is a group of substances, which, investigated in various methods, are 

 found to show, in solution in water, a higher osmotic pressure than that 

 corresponding to their molar concentration. All these substances are found 

 to be conductors of electrical currents, that is, they are electrolytes, to use the 

 name introduced by Faraday. 



It is clear, therefore, that the molecules of electrolytes are split up, dissociated, 

 in solution in water, so that there are more osmotically-active elements in their 

 solutions than in those of non-electrolytes in the same molar concentration. 



Since electrolytes conduct electricity by means of their " ions," which appear 

 at the two electrodes (Faraday), the view was put forward by Arrhenius that 

 these ions exist in solutions of electrolytes in ordinary conditions, independently 

 of the passage of electrical currents. 



Evidence of various kinds has been brought to show that this is the case. 

 Hydrochloric acid, for example, is more or less completely split up into hydrogen 

 ions, each carrying a unit positive charge, and chlorine ions, each carrying a 

 unit negative charge. This is known as " electrolytic dissociation." 



The more dilute the solution, the more complete is the dissociation. 



The power of conducting a current depends both on the actual number of 

 ions engaged in the carriage of the charges and also on the rate at which 

 they move. The rate has considerably different values for different ions and is in 

 relation not only to the atomic or molecular weight of the ion, but to the 

 number of molecules of water which are attached to it (Hydration of Ions). The 

 value is constant for each ion under similar conditions. The absolute rate of 

 movement is slow. Hydrogen ions, the most rapid, have a velocity, under a 

 potential fall of one volt per centimetre, of only - 0033 cm. per second ; but 

 the rate is, of course, dependent on the force producing the motion. 



The reason why it is impossible to separate the oppositely charged ions by 

 diffusion, or other means except an electrical one, is the enormous electrosfcft&e- 

 attraction between them, which prevents a positive ion from being separated 

 ^ from its fellow negative one beyond infinitesimal distances. 



When, however, one of the ions moves faster than the oppositely charged one, 

 it does actually form a layer in front of that of the more slowly moving ions, at 

 a very minute distance. This phenomenon is known as the " Helmholtz double- 

 layer" and is the cause of the appearance of an electromotive force at the 

 boundary surface between solutions containing ions of differing mobility. 



The source of the energy required to dissociate the molecules of electrolytes 

 when dissolved in water is discussed in the text, as also the relation of the 

 process to the dielectric constant of the solvent. 



While the equilibrium between non-dissociated molecules and ions in the cases 

 of weak acids and bases obeys the law of mass action, as shown by their behaviour 

 on dilution (Ostwald's Dilution Law), that of strong acids, strong bases and salts 

 obeys a different law. The explanation of this fact has not yet been given. It 

 has been suggested by Noyes and his co-workers that there may be two different 

 kinds of combination between ion,s to form molecules, one rather of an electrical 

 nature and somewhat loose, the other more strictly chemical and more stable. 

 The former would be the case with the strong electrolytes. 



^JVi f.Vip.ir intervention in physio^og^a! processes, electrolytes may be said to act 

 "mainly in three ways. By the electrical charges on their ions, as in colloidal 

 phenomena: by their effect on the properties of the solvent, " lyotropic " action ; 

 and by the purely chemical properties of their ions or molecules. 



The important part played by acidity and alkalinity shows the value of the 

 electrolytic dissociation theory in an especially striking way. These properties of 

 solutions can be expressed by the numerical values of their concentration in 



