242 HOWARD J. CURTIS 



of the metal. Thus if copper is placed in water some Cu++ will mi- 

 grate from the metal into the water, leaving the metal negatively 

 charged with respect to the water. Ions will continue to leave the 

 metal until the negative potential developed is strong enough to pre- 

 vent further escape. This potential is very unstable, and any move- 

 ment of the water, addition of foreign ions, etc. would upset the 

 equilibrium. However, if the metal is in a solution of one of its 

 salts, for example, copper in copper sulfate, the Cu++ will tend to de- 

 posit on the metal, tending to increase the positive potential of the 

 metal. Thus there would be a tendency for the Cu++ to leave the 

 metal and go into solution and also a tendency to be deposited on 

 the metal from the solution, and so an equilibrium would be estab- 

 lished. This causes a potential between the metal and the solution 

 that is quite stable and depends only upon the metal, the salt con- 

 centration, and the temperature. It is known as a reversible elec- 

 trode since, if current flows from the copper to the solution, copper 

 goes into solution and if the current is reversed the reverse is true 

 quantitatively. 



There have been many theories and a good deal of experimental 

 work attempting to determine the absolute potential of an electrode. 

 Whereas some of the theories have been quite successful, the problem 

 is still not in a completely satisfactory state, and the reader is referred 

 to a text on physical chemistry for a complete discussion of the basic 

 theory of electrode potentials. 



In any real system it is of course necessary to have two electrodes, 

 and it is never possible to do more than measure the potential dif- 

 ference between two electrodes. For practical reasons the standard 

 hydrogen electrode has been taken as a zero of potential, and all un- 

 known electrode potentials referred to it. It is not customary to 

 think of a gas as an electrode, but it can function very satisfactorily as 

 such under proper conditions. The standard hydrogen electrode 

 (see Glasstone, 15) is made by placing an electrode of platinized 

 platinum half in an atmosphere of hydrogen gas and half in a solution 

 of sulfuric acid of such a concentration that it contains 1 g. of H+ per 

 liter. Hydrogen gas is bubbled through the solution until equilibrium 

 is established. There are a number of precautions that must be 

 taken in using this electrode, which will not be treated here, and when 

 these are taken it makes a very good standard electrode. However, 

 for practical reasons it is usually more convenient to use a calomel 

 electrode for everything but very precise chemical work. 



