RESPIRATION OF PLANTS 107 



degree of oxidation, e.g., FeCl2 and FeCU, the electrode will 

 assume a potential dependent upon the proportional concentra- 

 tions of the two ions Fe++ and Fe+++. If Fe++ ions are in excess 

 of Fe+++ ions, the electrode will be negatively charged. On 

 the other hand, with prevalence of Fe+++ over Fe++ the electrode 

 will be charged positively. If the two solutions are placed in 

 separate containers, divided by a porous membrane and with 

 platinum electrodes in each, and the electrodes are connected co 

 a galvanometer, a current will flow owing to the different poten- 

 tials assumed by the two electrodes. In the containers, there 

 will take place, in one Fe+++, reduction to Fe++ ions; and, in 

 the other Fe++, oxidation to Fe+++: and these processes will 

 stop only when there is an equal proportion between the ions 

 Fe+++ and Fe++ in both solutions. Thus under conditions 

 permitting the exchange of charges, there is oxidation or reduction 

 of ions carrying different charges, and there is obtained an 

 oxidation-reduction system, which may produce energy, depend- 

 ing upon the difference in the potentials of the two immersed 

 electrodes. 



The electrical potential that is assumed by a platinum electrode 

 when immersed in an oxidation-reduction system represents a 

 definite magnitude for each solution. The magnitude of this 

 potential, called the '^ reduction oxidation" or ''redox" potential, 

 indicates the direction in which the reaction will proceed. Sub- 

 stances having a higher potential will oxidize substances having 

 a lower redox potential, and conversely the latter will reduce 

 the former. The highest potential found in plants may be taken 

 as that of an electrode in pure oxygen; the lowest, that of an 

 electrode in pure hydrogen. Most of tl\e oxidation-reduction 

 systems in plants possess an intermediate potential; and accord- 

 ing to their redox potentials, plant substances may be placed in 

 a series in which each preceding membei may oxidize each 

 following member, and conversely the latter may reduce the 

 former. 



The determination of the redox potential may be accomplished 

 by means of the electrometric method, just Uke th^ determination 

 of pH. Various organic dyes also are used as indicators for this 

 purpose. The use of redox indicators is based on the fact that 

 most of them are transformed with reduction into colorless leuco 

 compounds or have different colors at different redox potentials. 



