2. CRYSTALLIZATION
• It is defined as a spontaneous arrangement of the particle into
a repetitive order.
• In other words a process in which crystals are formed either
from something that has been melted or from a solution is
usually termed as crystallization.
• The process of crystallization involves adsorption of solutes at
growing crystal surfaces or planes.
• A crystal is defined as a solid particle,which is formed by the
solidification process under suitable environment in which
structural units are arranged by a fixed geometric pattern.
3. MECHANISM OF
CRYSTALLIZATION
• The formation of crystal from solution involves three steps:
A. Supersaturation
B. Nucleation
C. Crystal growth
4. SUPERSATURATION
• Supersaturation can be achieved by the following methods:
A. Evaporation of the solvent from the solution.
B. Cooling of the saturated solution.
C. Addition of a substance, which is more soluble in solvent
than the solid to be crystallized.
5. NUCLEATION
• Nucleation is the birth of very small bodies of molecules.
• Some clusters may become so big, they may arrange
themselves in lattice arrangement.These bodies of
aggregates are termed as embryos.
• Embryos are unstable and they may break into clusters
again.These stable structures together form a nuclei.
6. TYPES
OF
NUCLEATION
A.Primary Nucleation: This type of nucleation is
homogenous(spontaneous).
B. Secondary Nucleation(induced by crystals): It is of
heterogenous type(induced by foreign particles).
7. CRYSTAL
GROWTH
• Every crystal is sorrounded by a layer of liquid known as
stagnant layer.
• Once the crystals are formed,nuclei formation stops and
crystal growth begins.
• From the bulk solution a solute particle diffuse through this
stagnant layer and then reaches the surface of the crystal.
• These particles then organize themselves in the crystal lattice.
• This phenomenon continues at the surface at a slow rate.
9. PRESENCE OF ANOTHER
SUBSTANCE
• Sodium chloride(NaCI) crystallized from aqueous solutions
produces cubic crystals.
• If NaCI is crystallized from a solution containing a small
amount of urea,the crystals obtained will have octahedral
faces.
10. SOLVENT
CONSIDERATIONS
• The solvent with moderate solubility is prefer for
crystallization.
• Presence of benzene can help crystal growth.
• Avoid highly volatile solvents.
11. NUCLEATION
• Crystals initially form via ‘nucleating events’.
• After a crystallite has nucleated it must grow.
• Nucleation sites are necessary for formation of crystal.
• Excess nucleation sites cause smaller average crystal size.
12. CRYSTAL GROWTH
• Keep crystal growth vessel away from sources of mechanical
agitation(eg:vibrations)
• Set-up away from vacuum pumps, hoods, doors, drawers and
so on.
• Leave samples alone for one week.
13. RATE OF COOLING/TIME
• Quality crystals grow best over time in near equilibrium
conditions.
• The longer the time, the better the crystals
• Faster crystallization is not as good as slow crystallization.
• Faster is the crystallization, higher is the chances of lower
quality crystals.
