Difference Between Crystallization and Precipitation

Crystallization vs Precipitation
 

Crystallization and precipitation are two similar concepts, which are used as separation techniques. In both methods, the end product is a solid and its nature can be controlled by manipulating different variables throughout the process.

Precipitation

Precipitates are solids consisting of particles in a solution. Sometimes solids are a result of a chemical reaction in a solution. These solid particles will eventually settle down due to their density, and it is known as a precipitate. In centrifugation, the resulting precipitate is also known as the pellet. The solution above the precipitate is known as the supernatant. The particle size in the precipitate changes from occasion to occasion. Colloidal suspensions contain tiny particles, which do not settle down, and cannot be filtered easily. Crystals can be easily filtered, and they are larger in size.

Though many scientists have researched about the mechanism of precipitate formation, the process has not fully understood yet. However, it has been found that the particle size of the precipitate is influenced by precipitates’ solubility, temperature, reactant concentrations and rate at which reactants are mixed. Precipitates can be formed in two ways; by nucleation and particle growth. In nucleation, a few ions, atoms or molecules come together to form a stable solid. These small solids are known as nuclei. Often, these nuclei form on the surface of suspended solid contaminants. When this nucleus is further exposed to the ions, atoms or molecules, additional nucleation or further growth of the particle can happen. If nucleation continues to take place, a precipitate containing a large number of small particles results. In contrast, if growth predominates, a smaller number of larger particles are produced. With the increasing relative super saturation, the rate of nucleation increases. Normally, precipitation reactions are slow. Therefore, when a precipitating reagent is added slowly to a solution of an analyte, a super saturation can occur. (Supersaturated solution is an unstable solution which contains a higher solute concentration than a saturated solution.)

Crystallization

Crystallization is the process of precipitating crystals from a solution due to changes in solubility conditions of the solute in the solution. This is a separation technique similar to regular precipitation. The difference in this method from the normal precipitation is that, the resulted solid is a crystal. Crystalline precipitates are more easily filtered and purified. The crystal particle size can be improved by using dilute solutions and adding the precipitating reagent slowly while mixing. The quality of the crystal and the improvement in filterability can be obtained from the dissolution and re-crystallization of the solid. Crystallization can be seen in nature too. It is most often carried out artificially for various types of crystal production and purification.