64 OXIDATION-REDUCTION POTENTIALS 



form being yellow, and one electron only is concerned in the reaction. The potential 

 of the 50 per cent, oxidised system (Eq) at pH 7-0 is — 0-102 v. 



It is unusual in an organic oxidation-reduction system for one electron only to 

 be concerned. In the organism the pigment appears always to be in the oxidised 

 form and it is considered likely that its oxygen carrying function is of significance in 

 respiration. 



PYOCYANINE AND OTHER PIGMENTS 



Pyocyanine is the blue pigment of B. pyocyaneus and has been studied poten- 

 tiometrically by Friedheim and Michaelis (1931), whose results were confirmed 

 independently by Elema (1931). Pyocyanine is oxymethyl phenazine (Wrede and 

 Strack, 1929). At pH values greater than 6 the pigment undergoes reversible 

 reduction to a colourless leuco-compound, two electrons being concerned as with 

 ordinary quinonoid dyes. The potential of the half-oxidised pigment at pH 7-4 

 is about — 0-06 v. At pH values below 6 {i.e., in acid solutions) the pigment 

 behaves in a different way. In these acid solutions, the pigment is red, and its 

 reduction is effected in two consecutive stages, each one involving one electron — 

 during the first stage of reduction a green compound is formed and in the second 

 stage a colourless leuco-compound. The midway reduction product appears to be a 

 free radical rather than a meriquinone. 



The alternative possibilities for the course of a reduction involving two electrons 

 are as follows : — 



(1) Complete reduction in one stage. 



Oxidant + 2 electrons -> Reductant. 



This applies to the reduction of blue pyocyanine to colourless leuco-compound at pH 

 values over 6. 



(2) Half reduction and formation of mixed or meriquinone : — 



Oxidant + 1 electron -> ^ (Oxidant Reductant) 

 ^ (Oxidant Reductant) + 1 electron -> Reductant. 



As an example, one might consider the half oxidation of hydroquinone to quinone 

 when quinhydrone crystalhses out. 



(3) Reduction in two stages : — 



Oxidant + 1 electron -> Semireductant 

 Semireductant -f 1 electron -^ Reductant. 



This last case is observed with pyocyanine in solutions of pH less than 6. The 

 oxidant is red, the semireductant green and the reductant is the colourless form. 

 Michaelis and Elema independently have developed mathematical treatments of 

 two-step reactions of the last-mentioned type. 



