Solutions of Electrolytes

SOLUTIONS OF ELECTROLYTES Properties of Electrolytes.—An electrolyte is a substance which is a conductor of electricity either in the molten state or in solution (see ELECTROLYSIS, and ELECTRICITY, CONDUCTION OF, IN LIQUIDS). In order to account for this electrical conductivity, R. Clausius (1857) surmised that these substances were dis sociated to some minute extent into electrically charged particles or ions which acted as carriers of the electric current. Soon after I88o two lines of evidence converged to show that salt-like sub stances are in fact dissociated in solution to a considerable extent.

In the first place it was found that electrolytes in aqueous solu tion gave abnormally large values for the molecular elevation of the boiling point and the molecular depression of the freezing point. Table VI. shows a few values of the latter.

The molecular depressions produced by non-electrolytes are in fairly good agreement with the value 18.9 calculated by van't Hoff on theoretical grounds. The values for electrolytes are approxi mately twice this figure. Clearly, this could be interpreted to mean that the actual number of molecules in the solutions of the total number of undissociated molecules and ions in the solution is greater than the number of molecules introduced in the ratio (I --y)+k7= 0-(k-1) 7. Arrhenius clinched his theory by showing that the values obtained for this quantity by conduc tivity measurements were in good agreement with the van't Hoff factor i, as shown in Table VII.

latter is considerably greater than the number of molecules of the substance introduced, in fact that the electrolyte has under gone dissociation in solution. Van't Hoff however contented him self with introducing a factor i, the ratio of the observed to the normal calculated effect.

About the same time studies were being made of the electrical conductivities of solutions of electrolytes and their change with concentration. In order to find if there is any change in the state of the electrolyte when the concentration is varied, it is necessary to express the results as the quantity of electricity carried by the same number of molecules in the different solutions in a given time, the measurements being otherwise made under the same conditions. The quantity so obtained is the molecular conduc tivity of the electrolyte in solution. F. Kohlrausch found that the

molecular conductivities of electrolytes increased with their dilu tion (i.e., the amount of solvent containing one molecular weight in grams of the electrolyte), but that two classes could be dis tinguished : (I) Strong Electrolytes, having large molecular conductivities which approach a constant limiting value when the dilution be comes very great. This class includes most salts; mineral acids such as nitric, hydrochloric, sulphuric ; and strong bases such as caustic potash and caustic soda.

(2) Weak Electrolytes, the molecular conductivities of which are smaller and show no signs of reaching a constant value at great dilutions. To this class belong most organic acids and bases, e.g., acetic acid ; also ammonium hydroxide.

Electrolytic Dissociation.

These two groups of facts were correlated in the theory of electrolytic dissociation developed by Svante Arrhenius (1882-87), who conceived the idea that strong electrolytes are largely dissociated in their aqueous solutions into charged parts or ions. Thus a molecule of a binary electrolyte such as sodium chloride yields two ions on dissociation, a positive ly charged sodium ion and a negatively charged chloride ion : NaCI similarly barium chloride yields three ions : In order to account for the change of molec ular conductivity with dilution, Arrhenius assumed that at very great dilutions strong electrolytes were completely dissociated in this way. Now the molecular conductivity is determined by (I) the number of ions formed from one gram-molecule of the salt in solution, (2) their mobility or the rate at which they move in a given electric field. Arrhenius assumed that in fairly dilute solu tions the mobility of the ions was constant, so that he ascribed the changes in the molecular conductivity to changes in the num ber of ions. Thus the decrease in the molecular conductivity as the concentration is increased is due to a decrease in the number of molecules dissociated into ions. The fraction dissociated, or degree of dissociation, in a given solution is the ratio of the molecular conductivity in that solution to its limiting value at infinite dilution (corresponding to complete dissociation).