On the ionic solubility-product

Abstract

The theory of a constant ionic solubility-product was first advanced by Nernst, from the analogy of the laws governing dissociation in gases. According to the principle of mass action, if the dissociation pressure of a system in equilibrium be changed by the introduction of an excess of either of the dissociated gases, combination takes place between them until equilibrium is restored. Thus, on the addition of either NH 3 or HCl to a system consisting of ammonium chloride in equilibrium with its dissociation products, ammonium chloride is produced. For each temperature, the product of the pressures of NH 3 and HCl is a constant. Similarly, Nernst found that the solubility of an electrolyte in water was reduced by the addition of any salt containing a common ion. The variations in the solubility of silver acetate (a sparingly soluble salt) in water containing known amounts of silver nitrate or sodium acetate were investigated, and the results obtained confirmed the view that a corresponding equilibrium law was here applicable. If we confine ourselves to substances of the simplest type RX, the law may be stated thus: "At a given temperature the solubility of a sparingly soluble electrolyte is dependent upon a constant, which is proportional to the product of the concentrations of the ions of the electrolyte." The fundamental assumption made in the application of this law is that in a saturated solution of an electrolyte the undissociated salt in solution plays the part of an intermediary between the ions and the solid, being in equilibrium with the ions on the one hand and with the solid on the other. Consequently, so long as the solvent remains the same, the concentration of the undissociated salt is constant, however much the ionic concentrations may be separately varied by the introduction of other electrolytes; and the product of these ionic concentrations is also a constant.