194 



OXIDATION-REDUCTION POTENTIALS 



of the curve as a whole when pH is changed. In Figure 5 is added the new dimension 

 pH, the case illustrated being that of 2-6 dibromo indophenol (or 2-6 dibromo ben- 

 zenone indophenol). At a given level of pH the familiar curve relating percentage 

 oxidation (or reduction) to potential is similar to one in Figure 3. If any fixed per- 

 centage oxidation is carried through changes of pH, the curve is one of those shown 

 running from the upper right-hand to the lower left-hand part of the figure. For con- 

 venience the curve of 50 per cent oxidation is chosen for use in two dimensional charts. 



Fig. 5. — Isometric drawing showing the surface descriptive of the system of which 2-6 dibromo 

 indophenol is the oxidant. The co-ordinates are percentage oxidation, pH, and electrode potential. 



In Figure 6 are several such curves showing the potentials of half-reduced systems 

 at different levels of pH. Curves 1-4 are respectively those of the mono-, di-, tri-, 

 and tetra-sulphonates of indigo; 5 that of methylene blue, 6 that of Lauth's violet, 

 the remainder being curves of indophenols. 



In Table II are given the potentials of half-reduced solutions of several indicators 

 at intervals of 0.2 pH unit. This table may be used in conjunction with Table I as 

 follows. 



Suppose a culture known to be at pH = 7.2 has reduced methylene blue to the ex- 

 tent of 90 per cent. Table II shows that the potential of a 50 per cent reduced methy- 

 lene-blue solution is + .004 at pH 7.2. Table I shows that the 90 per cent reduced solu- 

 tion is .029 volts more negative. Hence the culture has a reduction intensity wliich 



