HYDROGEN TON CONCENTRATION 11 



pliuspliuric acid are — 



For the first hydrogen ion K — 11 1(>~-. />K ^ I -Hoi) 



For the second hj'drogen ion K = 2 < 10~", pK. — 6-699 



For the third hydrogen ion K = 36 x IQ-^^, pK = 12-444 



Obviously the higher the pis. value, the smaller the 

 dissociation constant and the weaker the acid. 



The Measurement of Hydrogen Ion Concentration. — 

 In general there are two main methods of measuring 

 hydrogen ion concentration, electrical and colorimetric. 

 The electrical methods are by far the more accurate, but 

 require the use of costly and delicate apparatus not 

 available in many ordinary laboratories. The colorimetric 

 methods, while not so accurate, are much cheaper, 

 quicker and simpler to perform. 



The electrical methods depend on balancing the 



potential difference set up between the solution of 



unknown hydrogen ion concentration and an electrode 



immersed in it against the potential of a standard cell. 



If a rod of metal is immersed in a solution of one of its 



salts, the metal will tend to dissolve to a degree depending 



on its " electroljrtic solution pressure," producing positively 



charged ions of the metal, and consequently leaving a 



negatively charged mass of undissolved metal. On the 



other hand, the salt in solution ionises, and the metallic 



ions will exert a definite osmotic pressure, depending on 



the concentration of the salt and its degree of dissociation. 



If the ionic osmotic pressure is less than the " solution 



pressure " of the metal, as is the case with zinc, the 



latter will dissolve in order to establish equilibrium. 



If, however, the " solution pressure " is less than the 



ionic osmotic pressure, as is the case with copper or 



mercury, ions will be deposited on the metal and the rod 



will acquire a positive charge relatively to the solution. 



Each metal in contact with a solution of one of its salts 



acquires in this way a characteristic " electrode potential," 



the strength of whicli will depend on the concentration 



