166 



HYDROGEN ION CONCENTRATION 



amounts to 2 gram-ions of H+, the number of coulombs transported 

 in this case will amount to 2 F. Therefore, 



Pi 



P2 



and 



or 



2 F . E = RT In 



2F p2 



0.0001983 , pi 



E = — — log — volts 



2 F ^ p2 



Table 23 gives the values of E of a chain in which the ratio of the 

 two partial pressures is 1 :n. 



By "pressure of H2-gas" we always mean the partial pressure. 

 Thus, for instance, in an atmosphere of hydrogen saturated with 

 water vapor by H2-pressure we mean only the partial pressure of 

 hydrogen and not that of the water vapor. 



These "pressure-chains" are of interest in the practice of pH 

 determinations, for the pressure of the atmosphere of hydrogen in 

 the electrode vessels depends in the first place upon the barometric 

 pressure, and, secondly, because of its continuous saturation with 

 water vapor, also upon the temperature. 



Gases which form reversible electrodes on platinum surfaces for 

 their own ionic species are primarily hydrogen and chlorine. Oxygen 



