DIELECTRIC CONSTANT 



247 



constant with distance from a univalent ion as calculated by different 

 methods. Carve (1) shows the most reasonable values as determined by 

 Conway et al. (1951) from several previous estimations. This curve can be 

 quite accurately expressed by the equation: 



D = U - 7 



(6-74) 



for distances between 3-10 A and this relationship will be adopted as a 

 basis for calculation of interaction energies between ions. Curve (2) shows 

 the familiar Schwarzenbach (1936) variation as used by Pressman et al. 

 (1946) and others; this is a plot of Z) = 6fZ — 11 corrected to 37.5o. Curve 

 (3) shows typical results of the theoretical procedure (Ritson and Hasted, 

 1948; Grahame, 1950) which minimizes the effects of the ion field on the 

 dielectric constant beyond the first bound layer of water. 



Several points in connection with the evaluation of D may be brought 

 up. (rt) For interactions involving di- and trivalent ions, lower values of 

 D should be used inasmuch as the field strength of such ions is greater. 

 Figure 6-12 shows the variation of D with field strength, curve (1) giving 

 values in a vacuum and curve (2) the actual values in the dielectric. As an 

 approximation, D should be reduced by 10-15 for divalent ions and by 



Fig. 6-12. Variation of the dielectric constant with the electrical field strength, 

 curve (2) giving the actual values in the dielectric water. 



