EFFECT OP pH 



II 



The hydrogen electrode is therefore in equilibrium with a partial pressure of 

 1 atmosphere of hydrogen ; rH = 0, and at pH the electrode potential is 0. A 

 reversible oxidation-reduction system in equilibrium with a partial pressure of 

 10~^^ atmospheres of hydrogen has : — 



(33) . rH = log vttV. = 15 



10 



-15 



and at pH 7 the electrode potential will be [equation (31)] at 30°C. 

 (34) Eh = 0-03 (15 - 2pH) = + 0-03 volt. 



That is assuming that one electron only is concerned in the reversible oxidation- 

 reduction process and that only one dissociation constant is concerned and that 



■H41 



rH30 



H20 



rHIO 



HO 



pH 



Fig. 1 

 Electrode potential (Eh) : pH curves 

 Theoretical curves — continuous lines. 

 Actual systems (Eq : pH curves) — broken lines 



this constant is effective at pH 7. With other systems with more complex relation- 

 ships this simple expression has no significance. It is essential, therefore, especially 



