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Crystallization

  • Last Updated : 21 Jul, 2021

Crystallization is a natural process that happens when the materials solidify from a liquid, or as they precipitate out of a liquid or gas. This process can be carried out by causing a physical change, like a change in temperature, or a chemical change like acidity. The crystallization process is carried out on the basis of the size and shapes of the molecules involved, and their chemical properties. Crystals can be made out of 1 species of the atom, different species of ions, or even huge molecules like proteins. Some big molecules have a difficult time going through the crystallization process, as their internal chemistry is not symmetrical or interacts with itself to avoid crystallization.

The unit cell is known as the smallest unit of the crystal. It is the base form of atoms or molecules upon which more units can be attached. Think of this as a children’s building block, to which other blocks can be joined. Crystallization goes on as if the blocks are getting attached in all directions. Some of the materials form different shaped crystals, which results in great variation in shape, size, and color of various crystals.

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Types of Crystallization

Cooling Crystallization

Saturated solution at 30°C by cooling it to 0°C the precipitation occurs when a large crystal mass is precipitated after hydration of water.

Evaporative Crystallization

At constant temperature the precipitation of crystals by increasing the solute concentration above the solubility threshold. The solute/solvent mass ratio is increased using the technique of evaporation.

Factors Affecting Crystallization 

  • Presence of another substance: Sodium chloride crystallized from aqueous solutions produces cubic crystals. If sodium chloride is crystallized from a solution containing a small amount of urea, the crystals obtained will have octahedral faces. 
  • Solvent: The solvent with moderate solubility is preferred for crystallization. The presence of benzene can help crystal growth. Avoid highly volatile solvents.
  • Nucleation: Crystals initially form via “nucleating events”. After a crystallite has nucleated it must grow. Nucleation sites are necessary for the formation of crystals. Excess nucleation sites cause smaller average crystal sizes. 
  • Crystal Growth: Crystals grow by the ordered deposition of the solute molecules onto the surface of a pre-existing crystal. Crystal growth is facilitated by the environment changing slowly over time. Keep crystal growth vessel away from sources of mechanical agitation (e.g. vibrations). Set-up away from vacuum pumps, hoods, doors, drawers, and so on.
  • 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 crystallization higher chance of lower quality Crystals.

Importance of Crystallization:



  • Purification of drugs
  • Improve bioavailability of the drug and choose the most stable form
  • A crystalline powder is easily handled, stable, possesses good flow properties, and has an attractive appearance.

Application of Crystallization

Crystallization is a key component of almost all processes in the manufacturing of small molecule pharmaceuticals. It is essential in both processing and pharmaceutical development. Since the properties of a solid material (polymorphism) can dramatically affect the process or the product’s compliance and effect (dissolution rate for example), monitoring and controlling the isolation of solids for the various applications through crystallization is of paramount interest.

So crystallization is used in:

  • Purification of drug
  • Better processing characteristics
  • Ease for handling
  • Improved physical stability
  • Sustained release
  • Improved bioavailability
  • Preparation of organic and inorganic API 8- separation of API from galenical extracts
  • Manufacturing of pure API by high yield

Theory of Crystallization:

The three major stages in the process of Crystallization are:

  1. Super saturation of the solution: It can be done in three ways: Heating the solution, Cooling the solution, Salting out.
  2. Nucleation: This takes place in several steps. During their random motion, the atoms/ molecules/ ions will come closer to one another and forms aggregates called Clusters. These clusters will combine to form an EMBRYO. In this stage, only the lattice formation begins. This embryo’s combine to form NUCLEI. From nuclei crystals are formed.
  3. Crystal Growth: Once the crystals are formed, nuclei formation stops and crystal growth begins.

Process of Crystallization

The crystallization process consists of two major events. One is nucleation and the other is crystal growth which both are driven by thermodynamic properties also as chemical properties. Nucleation is the primary step where the solute molecules or atoms dispersed within the solvent start to collect into clusters, that become stable under the present operating conditions. These stable clusters constitute the nuclei. Therefore, the clusters got to reach a critical size so as to become stable nuclei. It’s at the stage of nucleation that the atoms or molecules arrange during a defined and periodic manner that defines the crystal structure.

Crystal growth refers to the increase in the size of the nuclei which is able to achieve the critical cluster size. Many compounds have the power to crystallize with some having different crystal structures, a phenomenon called polymorphism. Certain polymorphs can be metastable, meaning that although it’s not in thermodynamic equilibrium, it’s kinetically stable and requires some input of energy to initiate a change to the equilibrium phase.

Examples of Crystallization:

Some common examples of crystallization are listed below:

  • The crystallization of water to form ice cubes and snow.
  • The crystallization of honey when it is placed in a jar and exposed to suitable conditions.
  • The formation of stalagmites and stalactites (especially in caves).
  • The deposition of gemstone crystals.

What are the advantages of crystallization?

The key advantages of crystallization are listed below.



  • A product of high purity can be obtained from one single step via the process of crystallization.
  • The dry products formed from crystallization can be directly packaged and stored.
  • The energy requirements and the operating temperatures of this process are relatively low.

Sample Questions

Question 1. What is Efflorescence?

Answer:

Efflorescence: Crystalline hydrated salts which on exposure to the atmosphere lose their moisture (water of crystallisation) partly or completely to the atmosphere and change into the amorphous state. Examples: Washing soda (Na2CO3.10H2O), Glauber salt (Na2SO4.10H2O).

Question 2. Define water of crystallization, give two examples with their chemical formulae?

Answer:

The definite amount of water associated with hydrated crystals which is an integral part of the crystal is known as water of crystallisation.  

Example:

  • Blue vitriol CuSO4.5H2O
  • Glaubler salt Na2SO4.10H2O

Question 3. Does anhydrous crystals of sodium chloride contain water? How are crystals containing water of crystallization different from anhydrous crystals of sodium chloride?

Answer:

Anhydrous substance does not contain water molecule. These obtained from hydrated salts by carefully removing water of crystallization from them.



NaCl is an anhydrous salt but if some impurity like salts of group II is present, they absorb moisture from the air because they are hygroscopic substances that’s why NaCl looks moist. But it’s an anhydrous salt.

 Let us take an example so that you will clear your concept about this,

CuSO4.5H2O + heat        →         CuSO4 + 5H2O

                                                            Blue                                              White

                                           (Salt with water of crystallization)          (anhydrous salt)

Question 4. Name a decahydrated crystalline salt. Give its molecular formula.

Answer:

Washing soda (Na2CO3.10H2O) is a decahydrate crystalline salt.

Question 5. Difference between Crystallization and Evaporation?

Answer:

Crystallization

Evaporation

Crystallization is the process of formation of solid Crystals Precipitating from a solution, melt or more rarely deposited directly from a gas.Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid.
Is used when the soluble solid does not decompose upon strong heatingIs used when the soluble solid contains water of crystallisation.
Drawback: any soluble impurities may also be present after evaporation.Advantage: pure solid is collected in the form of crystals.
Example: Seawater Evaporating and forming into clouds.Example: Dry Ice.

Question 6. What is the difference between Crystallization and Recrystallization?

Answer:

Recrystallization is done to crystals formed from a crystallization method.

Crystallization is a separation technique. Recrystallization is used to purify the compound received from crystallization.

Question 7. Insoluble impurities from solution during crystallization are removed by?

Answer:

Filtration is the process of separating small particles of solid matter from a liquid, by causing the liquid to pass through the pores of some substance, called a filter.

Insoluble impurities from solution during crystallization are removed by filtration as the insoluble solid particles passed through the filter.




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