OXYGEN WAVES 



159 



OXYGEN AND HYDROGEN PEROXIDE 



Dissolved oxygen in solution gives a typical polarogram and this may be of 

 importance in determining traces of oxygen in biological systems such as bacterial 

 cultures. There are two waves corresponding to the two stages of reduction of 

 oxygen, first to hydrogen peroxide and then to water : — 



02 + 2H+ + 2e->H202 

 H202 + 2H++2e-^2H20 



The second wave, which is less sharp than the first is identical with that observed 

 in an air-free solution of hydrogen peroxide. 



volts 



2-0 



Fig. 43 

 Polarographic oxygen waves in presence of maximum suppressor 



The reactions are irreversible at the electrode and are subject to over- voltage 

 effects. The fact that dissolved oxygen gives waves on the polarographic curves 

 accounts for the necessity of removing the oxygen by passing inert gases or by 

 sulphite reduction when investigating other systems. 



The polarographic method has been used for the study of respiration of green 

 algae and in studying soils. 



POLAROGRAPHY OF THE CYSTINE-CYSTEINE SYSTEM 



In Chapter III it was pointed out that the electrode potentials of the cystine- 

 cysteine system could not be explained on the basis of the equation : — 



2 RSH ^ RSSR+2H++2e 



since the potential depended only upon the concentration of cysteine (RSH) and 

 hydrogen ions and not upon the concentration of the oxidised form, cystine 

 (RSSR). When the dropping mercury electrode is used the mercury reacts with 

 cysteine and does not behave as an indifferent electrode. 



Studying the anodic waves of cysteine Kolthoff and Barnum (1940) found 

 that the results indicated not oxidation to cystine but to the reversible reaction : — 



RSH+Hg ^ Hg RS+H++e 

 a compound of cysteine with mercury being formed. 



