28 OXIDATION-REDUCTION POTENTIALS 



of electrode potentials in biological systems, which behave, not as systems of fixed 

 capacity or poising effect, but rather like a fountain-pen from which a small quantity 

 of ink may be withdrawn for long periods at a slow rate. So that although the 

 effective capacity of the biological system at any time may be very low, an almost 

 inexhaustible process of reduction may proceed. If therefore a series of dyes be 

 added to a biological system, they may all at first remain in a highly oxidised 

 condition, but one after another may be reduced as the system effects its gradual 

 reducing processes. Very long periods may be necessary therefore to determine 

 the oxidation-reduction condition of a biological system, and as such a system, 

 particularly in bacteriology, is itself a constantly changing one, it may be impossible 

 to determine the electrode potential at different periods by the use of dyes. 



Catalytic Effect of Dye 



To be useful as an oxidation-reduction potential indicator it is evident that 

 a dye must behave as an inert indicator of the potential and must not participate 

 directly or indirectly in the system studied by exerting any catalytic effect. If the 

 dye has such a catalytic effect the equilibrium of the system may be permanently 

 disturbed, and no useful results can be expected with such a dye. 



Barron and Hoffman (1930) observed the catalytic effect of different dyes on 

 the oxidation processes taking place in living cells. 



As a result of their work on adrenaline, Ball and Chen (1933) find it necessary 

 to give the following warning : " It is emphasised that results obtained by means of 

 oxidation-reduction indicators on unstable systems or on biological material where 

 such systems are present must be interpreted with great care." Certain oxidation- 

 reduction potential indicator dyes have been found to promote reactions which 

 will not proceed in their absence. Schott and Borsook (1933) describe such an effect 

 in the case of toluene treated B. coli and call the results " coupled reactions." Green, 

 Stickland and Tarr (1934), however, prefer the term " carrier linked reactions," 

 which they apply to the reactions between dehydrogenase systems isolated from the 

 cell structure. These do not proceed in the absence of appropriate artificial hydrogen 

 carriers, such as certain oxidation-reduction potential indicator dyes. These dyes 

 function by being continually reduced by the negative enzyme system and oxidised 

 by the more positive system. It is very significant that the effect appears to possess 

 considerable specificity. Of the naturally occurring substances examined only 

 pyocyanine functioned in this way, and dyes even of similar oxidation-reduction 

 potential differed greatly in their activity. It is evident that the greatest caution 

 must be exercised before dyes or even natural pigments are treated as inert indicators 

 of the potential (see also Meio, Kissin and Barron, 1934 ; Greville and Stern, 1935 ; 

 Lennerstrand and Kunnstrom, 1935 ; Young, 1937). 



Toxic Effect of Dye 



The dye may damage or even kill the cells in biological studies, and the damaged 

 or dead cells may have entirely different oxidation-reduction properties, so that no 

 trustworthy results can be obtained. It must be remembered that either the 

 oxidised or reduced form alone, or both, may have toxic effects. Chambers, Cohen 

 and Pollack (1931) have shown that indicator dyes which penetrated echinoderm 

 ova were toxic and killed the cells. Dyes, particularly basic dyes, have been found 



