Difference Between Precipitation and Co-precipitation

Precipitation vs Co-precipitation
 

In analytical chemistry, precipitation is an important technique to separate out a compound/ material out from a solution. Insolubility, purity, easiness to filter, un-reactivity with atmospheric substances are some of the significant features of a precipitate, which allows them to be used for analytical purposes.

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 precipitated 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 easily filtered. 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 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, super-saturation can occur. (Supersaturated solution is an unstable solution which contains a higher solute concentration than a saturated solution.)

Co-precipitation

“Co-precipitation is a process in which normally soluble compounds are carried out of solution by a precipitate.” There are four types of co-precipitation as surface adsorption, mixed- crystal formation, occlusion and mechanical entrapment. Surface adsorption takes place for precipitates with larger surface areas. Specially coagulated colloids contaminate by this method. In mixed- crystal formation, one of the ions in the crystal lattice is replaced by another ion. Surface adsorption and mixed- crystal formation are equilibrium processes, whereas the other two are kinetic phenomena. When a crystal is growing rapidly, contaminant can trap inside the growing crystal and this is known as occlusion. Mechanical entrapment is the mechanism where some amount of solution is trapped inside the crystals. This happens when two growing crystals are close together, so that they grow together.

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