THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 93 



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 provided 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 meas- 

 ured 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 potentiom- 

 eter. The reading of potential thus obtained mil be that referred to the 

 calomel half-cell, and this observed potential, Fobs, 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 + Ecai- Thus, 

 if Ecai = +0.250 volt (see page 75) and Eobs = -0.150 volt, then 

 Eh = +0.100 volt. 



Significance of Eh measurements. The potentiometric method is 

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

 ever, another matter. Discounting subsidiary, but sometimes impor- 

 tant, 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 sys- 

 tem such as ferric : ferrous iron, under conditions of equilibrium and maxi- 

 mum work, is a measure of the Gibbs free energy change, nFEh = —AG, 

 in the reaction between the components of the two halves of the oxida- 

 tion-reduction cell. This is the case for a considerable number of oxida- 

 tion 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 elec- 

 trons, with or without accompanying protons, were direct and complete. 

 These are reactions 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 compo- 

 nents of each such system. For example, a potential of the ferric : ferrous 

 system in acid solution can be defined by the relation which implies the 



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