COMPOUNDS OF OXIDANT AND REDUCTANT 



43 



Consequently if Eh is plotted against log {Sb — S) the curve so obtained 

 has a slope of O.l'^ at its central point. 



Figure 3, and the description of the method employed, are taken from 

 the paper of Clark and collaborators, with appropriate modifications, and 

 should be sufficiently explanatory. 



LOG {Sb-S) 



Fig. 3. The solid lines represent the theoretical relation of En — E'q to log (Sb — S) 

 when g = r = 2, 0.5.S = Sr = So = 1 X 10"^ .1/. The broken lines represent the 

 same theoretical relation with log (.Sb — ().5.S) as abscissa. Curve A, K« = l X 10'*, 

 0.5 log Kr= - 4.00 (point 1); Ko = 1 X 10-^ 0.5 log K,, = - 2.00 (point 2); 

 Ei- Eo = 0.2400 (0.06 coefficient). Curve B, K/J = 1 X 10■^ 0.5 log Kr=- 3.00, 

 (point 3); Ko = 5 X lO'', 0.5 log Ko = - 1.15 (point 4); E^ - Eo = 0.2219 (0.06 

 coefficient). Curve C, Kfl= 1 X \0'\ 0.5 log Kft = - 2.00 (point 5); Ko = 1 X lO"' 

 0.5 log Ko = -0.50 (point 6); £2 - ^o = 0.1800 (0.06 coefficient). After Clark et al. 

 {1^5S). 



"Figure 3 illustrates a method of graphical analysis based upon the use 

 of these approximations and unique points. Since Z is unity at the center 

 point, log {Sb — S) is made the abscissa in order that the important orient- 

 ing diagonal, of slope {).\i, mav coincide with the center point. When the 

 ratio of Kq to K^ is as large as any of the values indicated, the diagonal is 

 very close to the tangent through the mid-point and accordingly can be 

 well placed. This diagonal will intersect the line of E'^ where: 



0.5 log Kr = log [B] 



Now if [B] = {Sb - S), as it does in Curve C, this point of intersection will 

 be 0.0181 volt below the corresponding point of the association curve, 

 drawn with log {Sb - S) as abscissa. Curves B and A show progressively 

 greater departure from this relation, yet at the unique point in question 

 Z = 0.5, .so that, if a supplementary curve be plotted with log (iS^ — 0.5 S) 



