1X48-20 MANUAL OF METHODS FOR PURE CULTURE STUDY 



sions and bacterial cultures are poorly stabilized with respect to 

 oxidation-reduction potential. Consequently, disturbing polari- 

 zation may occur if even the small amount of current necessary to 

 operate the usual potentiometer and galvanometer circuit is allowed 

 to pass through the half-cell containing the biological system under 

 measurement; and the observed potential may be of uncertain 

 accuracy and reliability. This difficulty can be minimized by the 

 employment of a vacuum tube potentiometer-electrometer of the 

 kind now in common use for glass electrode measurements and pro- 

 vided with a scale graduated in volts. 



The oxidation-reduction cell is set up by joining the saturated 

 calomel half-cell with the half-cell containing the solution or culture 

 to be measured as indicated in the following scheme : 



(Pt) 



Ordinarily, the potential of a culture is negative (reducing) to 

 that of the calomel half-cell, and the metal terminals of the above 

 oxidation-reduction cell are connected accordingly to the terminals of 

 the potentiometer. The reading of potential thus obtained will be 

 that referred to the calomel half -cell; and this observed potential, 

 Eobs, can be converted to Eh, the potential referred to the standard 

 normal hydrogen half-cell, by adding Eobs and Ecai algebraically. 

 That is, Eh = Eobs+Eeai. Thus, if Ecai= +0.250 v. (see p. 3) and 

 Eobs = -0.150 v., then Eh= +0.100 v. 



Significance of E^. measurements. The potentiometric method is 

 direct and relatively simple. The interpretation of the results is, 

 however, another matter. Discounting subsidiary, but sometimes 

 important, instrumental effects such as potentials due to liquid 

 junctions, and temperature differences within the oxidation-reduction 

 cell, all of which can be eliminated or minimized (see Clark, 1928), an 

 observed Eh of a system such as ferric :ferrous iron, under conditions 

 of equilibrium and maximum work, is a measure of the Gibbs free 

 energy change, nFEh = -AG, in the reaction between the components 

 of the two halves of the oxidation-reduction cell. This is the case for 

 a considerable number of oxidation-reductions which, alone or in the 

 presence of catalysts and mediators, can take place more or less 

 rapidly and reversibly as if a transfer of electrons, with or without 

 accompanying protons, were direct and complete. These are re- 

 actions between so-called electromotively active systems, the Eh of 

 which is fixed, at constant pH, by a characteristic constant and by 

 the relative concentrations (more accurately, activities) of the 

 components of each such system. For example, a potential of the 

 ferric rferrous system in acid solution, can be defined by the relation: 



RT [Fe++] 

 Eh = E'o In (8) 



F [Fe+++] 



which implies the limitation that definite and significant potentials 

 are possible only in the presence oi finite ratios of oxidant to reduc- 

 tant. In addition, the total concentration of the reversible system 

 may be decreased to and beyond a level at which traces of electromo- 



