ELECTROLYTES IN MIXED SOLVENTS 177 



the values of the two pure media as extremes. If the viscosity of the two 

 solvents differs greatly, then in general the viscosity of a mixture will lie 

 intermediate between that of the two pure components. If the two sol- 

 vents have approximately the same viscosity and particularly if both 

 solvents are associated liquids, the viscosity curve will as a rule exhibit 

 a maximum. Cases in which the viscosity curve passes through a mini- 

 mum are rather exceptional. 



The viscosity of a mixture of two solvents will in all cases be of the 

 same order of magnitude as that of the two components. If the nature 

 of the ions remains fixed, therefore, the speed of the ions may be ex- 

 pected to vary approximately in proportion to the fluidity change. 



In adding a second solvent to a solution of an electrolyte in another 

 solvent, an interaction may take place between the electrolyte and the 

 added solvent. In this case, the nature of the ions will change and with 

 it, in general, their speed. In some instances, the change in the speed 

 of the ions due to this cause is relatively large. 



In general, it may be expected that the ionization of a salt in a mixture 

 of two solvents, particularly in dilute solutions, will have a value inter- 

 mediate between those of the same electrolyte in the pure solvents. For 

 we have seen that the ionization of a salt is a function of the dielectric 

 constant of the medium, and the dielectric constant of a mixture of two 

 solvents is in general intermediate between those of the pure components. 

 Here again, however, we have to take into account the interaction between 

 the electrolyte and the components which form the solvent medium. If 

 interaction takes place between the second solvent and the electrolyte, 

 then a new complex is formed whose ionization may differ greatly from 

 that of the same electrolyte in the first solvent and, in fact, all of whose 

 chemical properties may differ greatly from those of the original electro- 

 lyte in the first solvent. A considerable number of examples of this type 

 are found in aqueous solutions. When, for example, ammonia is added 

 to a solution of a silver salt in water, a complex is formed between the 

 silver ion and ammonia, which apparently has the composition 

 Ag(NH 3 ) 2 + and whose properties are distinct from those of the normal 

 silver ion in water. So, we find that salts of this ion are much more 

 soluble than those of the normal silver ion, particularly in the case of 

 the halides. Similar complexes are formed in the case of many other 

 salts dissolved in water in the presence of ammonia, as, for example, 

 salts of copper, zinc, cobalt, nickel, etc. The distinctive properties of 

 the complex affect all the characteristic properties of the resulting elec- 

 trolytic solution. So the addition of ammonia to a solution of a silver 

 or a copper salt in water decreases the viscosity of the solution, until all 



