I CYTOPLASM 151 



hydration layer. If a Na ion surrounded by its hydration layer ap- 

 proaches this system, it is held electrostatically, and a hydration 

 equilibrium between the various groups is established. If the Na ion 

 is replaced by a much less hydrated ion like Rb, the latter is able to 

 approach the anionic group more closely because of its smaller 

 hydration layer. This results in a stronger discharge than in the case 

 of the Na ion; the hydration decreases and the neighbouring poly- 

 peptide chains approach each other. 



An explanation along these general lines becomes more difficult if 

 bivalent ions such as Ca take part in these processes. Since bivalent 

 ions carry two elementary charges, they can discharge negative pro- 

 teins more strongly than monovalent ions. For this reason they 

 usually cause shrinkage of protoplasm (decrease in permeability, in- 

 crease in density and viscosity; Cholodny and Sankewitsch, 1935). 

 In the case of the trivalent ions Fe and Al these effects are still more 

 pronounced (tanning). One speaks, therefore, of a valency rule of 

 shrinkaee, which states that the shrinking effect of ions increases with 

 rising valency. 



With increasing charge of the ions, however, the hydration layer 

 also increases. The Ca ion, for instance, is hydrated more strongly 

 than the K ion of the same size. Accordingly, CaClg causes gelatin 

 to swell to a greater extent, and this can even result in the formation 

 of a sol. In the same way the strongly hydrated Zn ion in concentrated 

 ZnClg solutions causes unexpectedly marked swelling of cellulose. 

 The valency rule does not, therefore, apply generally to bivalent 

 ions. 



The valency rule asserts itself more clearly in Hofmeister's series 

 of the anions 



SCN > I > NO3, Br > CI > acetate | > SO4 > tartrate | > citrate. 



The trivalent citrate ion is a weaker swelling agent than the bivalent 

 tartrate and sulphate ions and these last two are weaker agents than 

 the monovalent ions. 



In the case of positively charged proteins with cationic polypeptide 

 chains, Hofmeister's ion series appears to be reversed, because the 

 adsorption now refers to the anions. This inversion is particularly 

 striking if one succeeds in reversing the charge of a negative gel. For 

 instance, with gelatin in a neutral or basic medium, where the gel 



