POLAROGRAPHIC CURVE 



155 



until now is dependent upon the electrolytes present and is known as the Residual 

 Current but when the voltage applied exceeds A (known as the Decomposition 

 Potential) the current flowing increases considerably, until the voltage C is reached 

 when the current settles down to a nearly constant value. The current 

 represented by the vertical distance between the curves at C and A is known as 

 the Limiting Current and gives a measure of the amount of the oxidation-reduction 

 system present. At the voltage corresponding to point B (i.e., the Half Wave 

 Potential) half the limiting current flows. This Half Wave Potential is characteristic 

 of the oxidation-reduction system and is in fact the potential of the half oxidised 

 system. This, as we have already seen, is the characteristic electrode potential or 

 Wq of the system. It will be seen at a glance that the Polarographic curve is of the 

 same S-form as the curves on page 25 relating the electrode potential to the 

 percentage oxidation or reduction of an oxidation-reduction system. 



VoJts 



Fig. 40 



Current-voltage curve obtained with oxidation-reduction system 



A — decomposition potential 

 B — halfwave potential 

 C — limiting current 



Some oxidation-reduction systems which are not truly reversible and do not 

 give electrode potentials at inert electrodes nevertheless are found to be reducible 

 at dropping mercury electrodes and a current flows and hence they can be studied 

 polarographically. The Half- Wave Potentials or " Apparent Reduction Potentials " 

 are not, however, always completely understood and in some cases are due to inter- 

 action of constituents of the system with the mercury of the electrodes. 



Conant (1926) investigated some irreversible oxidation reduction systems by 

 adding reversible oxidation-reduction systems of known potential and studying 

 the rate of reaction— by this means Apparent Keduction Potentials were obtained 

 (cf. also Miiller and Baumberger, 1937). Both Michaelis (1933) and Kolthoff and 

 Lingane (1941), agree that a thermodynamically satisfactory interpretation of 

 Apparent Reduction Potentials is lacking and it therefore remains a convenient 

 but empirical method of characterising some irreversible systems (cf. Shikata and 

 Tachi 1932). 



Before considering biological systems of interest it may be of value to consider 

 the theoretical basis of the polarographic method. It is conditions in the immediate 



