HYDRATION OF IONS 



251 



values are seen to correlate with ionic charge and radius; thus they may 

 be of use in estimating hydration magnitudes for ionic groups of biological 

 importance. Unfortunately, little is known experimentally of the hydra- 

 tion characteristics of groups such as R— NHg"^, E, = NHo^, <p — 0~, and 

 R— C00~ but there is no reason to expect marked deviation from the inor- 

 ganic ions, except for the steric factors introduced by the part of the mol- 

 ecule to which the ionic group is bonded. Taking into account ionic size 

 and steric factor, the primary hydration numbers for R— NH./ and R— COO" 

 may be assumed to be approximately 3. 



Table 6-17 

 Ionic RAon, Primary Hydration Numbers, and Hydration Energies for 



Some Common Ions " 



" The experimentally estimated values for AH and J*S' were selected from Latimer 

 et al. (1939), Bockris and Conway (1954), and Moelwyn- Hughes (1957) mainly; 

 AF was calculated from these. The ionic radii were taken from Moelwyn-Hughes 

 (1957) and the primary hydration number estimated from values obtained by different 

 methods in Bockris and Conway (1954). There is a good deal of variation in all these 

 values for individual ions and the figures in the table must be taken as only approxi- 

 mate; the attempt has been made to present the most reasonable values on the basis 

 of recent data. 



