Solubility:

Whenever we refer solubility as a means of separation it essentially implies precipitating the constituent of interest from the solution. While the solubility limit of the solute in the solvent exceeds, a material appears as a precipitate. If this procedure is carried out within an appropriately chosen solvent and in a controlled manner, it could lead to crystallization. Precipitation could be brought about in a number of ways out of that some important ones are discussed below.

Solvent precipitation is achieved by adding another miscible solvent to the solution such that the solubility of the component of interest is reduced and the material appears as a precipitate in the mixed solvent. The precipitation brought about by a chemical reaction is well known. Right from the beginning in a chemistry laboratory, you have been precipitating different ions by adding appropriate reagents. A typical example is precipitating Ba²⁺ by adding SO₄^2-. There are many selective reagents known which can precipitate one ion in the presence of other ions. A detailed discussion about this is given in Units 14 and 15 of the Course on 'Basic Analytical Chemistry'. Therefore, some organic compounds can be precipitated by suitable adjustment of pH. It is probable to precipitate weakly basic organic compound from an aqueous solution by making the solution more basic. Similarly, organic acids can be precipitated by making the solution more acidic by strong acids.