192 OXIDATION-REDUCTION POTENTIALS 



blue-methylene white; but if too little ferric iron is used to oxidize completely methy- 

 lene white, there will be left ferrous iron as the only form of iron and a mixture of 

 methylene white and methylene blue. If excess ferric is used, all the methylene white 

 will be oxidized to methylene blue, and there will be left a mixture of ferrous and 

 ferric ions. In the first case the potential will lie on the methylene blue — methylene 

 white curve; and in the second case it will lie in the ferrous-ferric curve. 



Figure 3 represents a series of actual measurements with sulphonates of indigo. 

 The experimental data are shown by the centers of circles, and the theoretical form- 

 curve is shown in each case by the line to which the loci of the experimental data evi- 

 dentally conform. This theoretical curve is determined by 



„ _,, - Total reductant 



£ = £0-0.03 log Total oxidant 



Since all the systems to be described conform to this equation when the hydrion con- 

 centration of the solution is fixed and known, and differ only in the characteristic 



T^, rr^ 1 1 -r • • 1 ,-1 1 Total rcductaut . 

 constant L'o, 1 able I, givmg values for the term 0.03 log -. —, — — for various 



percentages of reduction, will be found useful. 



Now it is found that a system which has been studied in a solution of constant 

 hydrogen-ion concentration and which has been found to give a curve such as that of 



Figure 3 will give the same form of curve 



77 77 at another value of pH, but the value of 



. C. .. . C . E'o will depend on the value of pH. Thus 



H:C •C:N::C •C:H ^t^q whole curve will be displaced in the 



\ ':'' C : S : C C :: N : R Potential scale. 



R ! N . C ^ . " •*• : + Why is this displacement? 



C *•* C Consider the case of methylene blue- 



g g methylene white and the conventional 



^ ^ . , , . electronic structure of a thiazine shown 



riG. 4. — Conventional electronic structure . , , . , 



of a thiazine. by Figure 4. The terminal substituted 



amino group at the right is polar and in 

 methylene blue is a very strong base. The opposite dimethyl amino group in methy- 

 lene blue is very weak. When the compound is reduced, the polarity of the one 

 group is destroyed and this group becomes weakly basic. The other group in the 

 now symmetrical structure is enhanced in strength. At the same time the bridging 

 nitrogen fixes a hydrion. These changes make themselves felt in the thermodynamic 

 measurements of free energy-change because as the several groups ionize, a virtually 

 new oxidant or a new reductant is formed, each differing from the parent by the 

 energy of ionization. Putting the matter another way, we may say that the covering 

 or uncovering of points by hydrogen ions or hydroxyl ions alters the ease of escape- 

 ment and acquirement of electrons. It is for this reason that the hydrion concentra- 

 tion must be kept rigidly constant if we are to obtain titration curves such as those 

 of Figure 3. 



Remembering that if the hydrion concentration is constant we shall always ob- 

 tain a curve of the form shown in Figure 3, our interest centers upon the displacement 



