OXIDASE 



182 



OXIDATION-REDUCTION 



Frozen sections of formalin fixed tissues 

 treated with this reagent show violet or 

 black granules, which quickly disappear. 

 Oxidation-Reduction Potential. Details 

 supplied by Dr. Christopher Carruthers 

 of The Barnard Free Skin and Cancer 

 Hospital. 



This very important measurement is 

 particularly well explained by Seifriz, 

 W., Protoplasm, New York: McGraw- 

 Hill Book Co., 1936, 584 pp. For a 

 comprehensive developmenta.1 treat- 

 ment of the subject see Clark, W. M. 

 and coworkers. Hygienic Laboratory 

 Bull., 1928, 151, 1-352. 



O.xidation is the process in which a 

 substance loses electrons, and reduction 

 is the process in which a substance takes 

 on electrons. For example when ferric 

 chloride FeCla gains an electron it is 

 reduced to FeCU, or 



Fe'"'*^ -h electron — ► Fe''^ 



Because the ion, Fe"*^, can lose an elec- 

 tron it is a reducing agent or reductant, 

 and since Fe+++ can gain an electron it 

 is an oxidizing agent or oxidant. The 

 change is reversible 



Fe''"'^ -|- electron ;;zi Fe"*^. 



When an acid mixture of ferrous and 

 ferric chloride is placed in an electrode 

 vessel it will j'ield a potential — the oxi- 

 dation potential. This potential can 

 be measured by placing a noble metal, 

 such as a bright platinum wire in the 

 solution, and measuring the potential 

 against the normal calomel electrode 

 with a potentiometer. The intensity 

 of the oxidizing or reducing action of a 

 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++) 

 (Fe+++) • 



Fe+-^ ^ (Reductant) 

 (Oxidant) 



Eh^Eo - 



Fe+ 



Eh is the observed difference in electro- 

 motive force between the electrode and 

 the normal hydrogen electrode; £'<, is a 

 constant characteristic for the ferrous- 

 ferric system (the so-called normal po- 

 tential); R, T, and F liave 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 : 



R., _ p liT , (Rod) 



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 ;z± Red (2) 



For equation (1), the relation would be 

 p, _ r. ff^ ,„ (Red-) 



E, - So - — In -^Q^ 



The active reductant of equation (1) 

 is the anion of au 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 



, _ RT (Red-) 



nF (Ox) 



at any constant pH (For development 

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

 1933, 1, 195-20-1). 



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 ReznikofT, 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 made with 

 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 

 upon the relative activity of the proces- 

 ses of o.xidation and reduction. 



Although it is difficult accurately to 

 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 

 genei"ate. 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 



ni-Bromophenol indophenol 0.24S 



o-Chlorophenol indophenol 0.233 



Phenol blue chloride 0.227 



Phenol indo-2,6 dichlorophenol 0.217 



o Cre<5ol indophenol 0.195 



o Cresol indo-2,6 dichlorophenol 0.1 SI 



i-Naplithol-2-sulfonate indophenol ©-sul- 

 fonate 0.135 



l-Naphthol-2-gulfonate indophenol 0.123 



Toluylene blue chloride 0.115 



Brilliant cresyl blue chloride 0.047 



Methylene blue chloride -t-0.011 



K4 indigo tetrasulfonate —0.046 



Ethyl capri blue nitrate —0.072 



K« indigo trisulphonate —0.081 



Kj indigo disulphonate —0.125 



Cresyl violet -0.167 



