Views: 15
Factors Affecting Separation Of Metals And Compounds
All precipitation gravimetric analysis share two important attributes.
First, the precipitate must be of low solubility, of high purity, and of known composition if its mass is to accurately reflect the analyte’s mass.
Second, the precipitate must be easy to separate from the reaction mixture.
In addition to having a low solubility, the precipitate must be free from impurities. Because precipitation usually occurs in a solution that is rich in dissolved solids, the initial precipitate is often impure and it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation.
The greatest source of impurities is the result of chemical and physical interactions occurring at the precipitate’s surface. A precipitate is generally crystalline with a well-defined lattice of cations and anions.
Those cations and anions at the precipitate’s surface carry, respectively, a positive or a negative charge because they have incomplete coordination spheres.
The presence of these partial charges makes the precipitate’s surface an active site for the chemical and physical interactions that produce impurities. We must remove these impurities before determining the precipitate’s mass.
The common impurity are those that occurs due to inclusion, occlusion and surface adsorbates .
Inclusion occurs when a potential interfering ion whose size and charge is similar to a lattice ion is substituted into the lattice structure and chemically becomes part of the precipitate lattice.
An inclusion is difficult to remove since it is chemically part of the precipitate’s lattice. The only way to remove an inclusion is through reprecipitation.
After isolating the precipitate from its supernatant solution, we dissolve it by heating in a small portion of a suitable solvent. We then allow the solution to cool, reforming the precipitate. We can repeat the process of reprecipitation until the inclusion’s mass is insignificant. An inclusion usually does not decrease the amount of analyte that precipitates ..
Occlusions form when interfering ions become trapped within the growing precipitate. Unlike inclusions, which are randomly dispersed within the precipitate, an occlusion is localized within the precipitate’s lattice structure or within aggregates of individual precipitate particles. An occlusion usually increases a precipitate’s mass. Occlusions can be minimized maintaining the precipitate in equilibrium with its supernatant solution (mother liquor) for an extended time.
This process is called a digestion. During digestion ensures that the occlusion is reexposed to the supernatant solution.
Surface adsorbates occurs after a completely formed precipitate continue to attract contaminating ions from the solutions on to its external surface. We can minimize surface adsorption by decreasing the precipitate’s available surface area and can be removed by washing the precipitate.
Inclusions, occlusions, and surface adsorbates are examples of Co-precipitates and post-precipitates. We must distinguish between two important types of precipitation.
Co-precipitation relates to the inclusion and occlusion of foreign substances during the process of crystal growth from the primary particles whereas Post precipitation is concerned with adsorption at the surface of the particles exposed to the solution. Appreciable errors rnay also be introduced by precipitations and post-precipitation.
Post-precipitation differs from Co-precipitation in several respects:
(a) The contamination increases with the time that the precipitate is left in contact with the mother liquor in post-precipitation, but usually decreases in Co-precipitation.
(b) With post-precipitation, contamination increases the faster the solution is agitated by either mechanical or thermal means. The reverse is usually true with Co-precipitation.
(c) The magnitude of contamination by post-precipitation rnay be much greater than in Co-precipitation.