OXIDATION-REDUCTION 

 POTENTIAL 



250 



system is determined by its oxidation 

 potential. The potential produced is 

 determined by the ratio of ferrous to 

 ferric ions, and is given by the relation : 



RT (Fe^) . 

 £a - E. - — In ^-p^^j . 



Fe-^^ ^ (Reductant) 

 Fe*++ (Oxidant) 



Eh is the observed difference in electro- 

 motive force between the electrode and 

 the normal hydrogen electrode; Eo is a 

 constant characteristic for the ferrous- 

 ferric system (the so-called normal po- 

 tential); R, T, and F have their cus- 

 tomary significances. The parentheses 

 represent concentrations of the two com- 

 ponents. 



Certain groups of organic dyes are 

 likewise able to induce upon electrodes 

 reversible potentials. These organic 

 dyes can be used as indicators of oxida- 

 tion-reduction, and the following rela- 

 tion holds : 



RT (Red) 



^^ - ^o - -^ '° loir 



If the reductant is identified as an ion, 

 or the oxidant as a cation, for two simple 

 cases there would be 



Ox + electron ^::± Red" (1) 



Ox+ + electron ;=± Red (2) 



For equation (1), the relation would be 



The active reductant of equation (1) 

 is the anion of an acid, and its concen- 

 tration depends not only upon the 

 amount of reductant, but also upon the 

 hydrogen ion concentration. The rela- 

 tion then becomes 



at any constant pH (For development 

 see Cohen, B., Symposia Quant. Biol., 

 1933, 1, 195-204). 



The use and interpretation of indica- 

 tor dyes in biological systems is given 

 by Cohen, B., ibid, 214-223, and Cham- 

 bers, R., ibid, 205-213. Sources of error 

 are also indicated by Cohen, B., Cham- 

 bers, R. and Reznikoff, P., J. Gen. 

 Physiol., 1928, 11, 585-612. Most of the 

 following material is taken from the 

 above papers. 



On a microscopic basis, the measure- 

 ments, like those of pH,are madejwith 

 indicators in which the cells are bathed 

 or which are injected with them. They 

 are applied in sequence and their reac- 

 tions observed. Methylene blue, for 

 instance, will be oxidized (retain color) 

 or be reduced (lose color) depending 



OXIDATION-REDUCTION 

 POTENTIAL 



upon the relative activity of the proces- 

 ses of oxidation and reduction. 



Although it is difficult to accurately 

 measure the amount of indicator in- 

 jected into cells, it is imperative that 

 the quantity be small. Otherwise too 

 much indicator may be more than the 

 cell can reduce, or be greater than the 

 reducing intensity which the cell can 

 generate. The following indicators from 

 Cohen provide a useful range in potential 

 values : 



Name of Oxidant E at pH 7.0 



Phenol m-sulfonate indo-2,6 dibromo- 



phenol 0.273 



m-Bromophenol indophenol 0.248 



o-Chlorophenol indophenol 0.233 



Phenol blue chloride 0.227 



Phenol indo-2, 6 dichlorophenol 0.217 



o Cresol indophenol 0. 195 



o Cresol indo-2, 6 dichlorophenol 0. 181 



l-Naphthol-2-sulfonate indophenol o-sul- 



fonate 0.135 



l-Naphthol-2-8ulfonate indophenol 0.123 



Toluylene blue chloride 0.115 



Brilliant creayl blue chloride 0.047 



Methylene blue chloride H-0.011 



Ki indigo tetrasulfonate —0.046 



Ethyl capri blue nitrate —0.072 



Ki indigo trisulphonate —0.081 



Ki indigo disulphonate ...—0.125 



Cresyl violet —0.167 



E'o represents the potential at any given 

 pH of a system in which the ratio of oxi- 

 dant to reductant is unity. 



In order to get the indicator dyes into 

 single cells the microinjection technique 

 of Chambers is used. Chambers recom- 

 mends dilute aqueous solutions of the 

 basic dyes, i.e., 0.05% to 0.1%, and in- 

 jects successive small doses. Needhara, 

 J. and D. M., Proc. Roy. Soc. B, 1926, 

 99, 173-199; 383-397 used 1% solution 

 since weaker solutions of particular 

 dyes could not be seen under the micro- 

 scope when injected into cells. 



The determinations are carried out 

 aerobically (cells maintained in a micro 

 drop in water-saturated air at atmos- 

 pheric pressure) andanaerobically (cells 

 held in an atmosphere of purified process 

 nitrogen saturated with water). 



For example, under aerobiosis, if all 

 the indicators down to and including 

 methylene blue are reduced at pH 7.0 

 by cells of a particular type; and if 

 ethyl capri blue is only partially re- 

 duced (and the rest of the indicators not 

 reduced), the reducing intensity of the 

 aerobic cell is approximately —0.072 

 volts at pH 7.0. The same procedure is 

 followed for cells anaerobically. 



To detect the presence of the indicator 

 after decolorization by the cell proto- 



