An Introductory Course of Quantitative Chemical Analysis

ELECTROLYTIC DISSOCIATION THEORY

The following brief statements concerning the ionic theory and a few of its applications are intended for reference in connection with the explanations which are given in the Notes accompanying the various procedures. The reader who desires a more extended discussion of the fundamental theory and its uses is referred to such books as Talbot and Blanchard's Electrolytic Dissociation Theory (Macmillan Company), or Alexander Smith's Introduction to General Inorganic Chemistry (Century Company).

The electrolytic dissociation theory, as propounded by Arrhenius in 1887, assumes that acids, bases, and salts (that is, electrolytes) in aqueous solution are dissociated to a greater or less extent into ions. These ions are assumed to be electrically charged atoms or groups of atoms, as, for example, H⁺ and Br^- from hydrobromic acid, Na⁺ and OH^- from sodium hydroxide, 2NH₄⁺ and SO₄^-- from ammonium sulphate. The unit charge is that which is dissociated with a hydrogen ion. Those upon other ions vary in sign and number according to the chemical character and valence of the atoms or radicals of which the ions are composed. In any solution the aggregate of the positive charges upon the positive ions (cations) must always balance the aggregate negative charges upon the negative ions (anions).

It is assumed that the Na⁺ ion, for example, differs from the sodium atom in behavior because of the very considerable electrical charge which it carries and which, as just stated, must, in an electrically neutral solution, be balanced by a corresponding negative charge on some other ion. When an electric current is passed through a solution of an electrolyte the ions move with and convey the current, and when the cations come into contact with the negatively charged cathode they lose their charges, and the resulting electrically neutral atoms (or radicals) are liberated as such, or else enter at once into chemical reaction with the components of the solution.

Two ions of identically the same composition but with different electrical charges may exhibit widely different properties. For example, the ion MnO₄^- from permanganates yields a purple-red solution and differs in its chemical behavior from the ion MnO₄^-- from manganates, the solutions of which are green.