164 HYDROGEN ION CONCENTRATION 



trode, since, depending upon the amounts of impurities in the water, 

 especially of O2 and CO2, varying traces of Hg will go into solution. 

 On the other hand, Hg in a saturated HgCl solution is a reversible 

 electrode, in spite of the fact that the solubility of calomel depends 

 on the total Cl~ content of the solution. For its dissociation occurs 

 in the following manner: 



Hg CI ^ Hg+ + Cl- 

 hence 



[Hg CI] 

 [Hg+] [C1-] 



In a saturated calomel solution the concentration of HgCl mole- 

 cules is constant, hence, 



[Hg+] X [Cli = a constant 



Therefore the concentration of the Hg-ions in solution is inversely 

 proportional to that of the Cl-ions. Therefore, two Hg-electrodes 

 immersed in two solutions of different concentrations of KCl, both 

 saturated with calomel, form a reversible concentration chain for 

 Hg-ions. Since the ratio of the Hg-ions is reciprocally equal to that 

 of the Cl-ions, such a chain may also be used as a reversible chain for 

 Cl-ions. These are designated as electrodes of the second order, or, 

 reversible electrodes for anions. These electrodes may therefore be 

 utilized for the determination of Cl-ion concentrations. 



An alloy of a noble with a common metal behaves, in the electro- 

 motive sense, as if it consisted of the common metal only. The same 

 is true of gas-electrodes. Platinum (as well as gold, palladium, 

 iridium) absorbs H2-gas from an atmosphere of hydrogen, the 

 absorption equilibrium being determined by the partial pressure of 

 the hydrogen. Such an electrode behaves as if it consisted only of a 

 metal-like, conductive hydrogen, hence it is a reversible electrode 

 for H+-ions. 



49. The theory of gas chains 



We saw above that when a concentration chain is made up of 

 Pt — H2 electrodes and of two solutions of a different [H+], (hi and 

 h2) then its E.M.F. is: 



lii = — — In — 



