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Coagulation of colloidal solution is the method to settle down the particles of colloidal solution and form aggregates or precipitates. Coagulation is a process of converting the liquid in some form of gel, semi-solid or solid state. On other hand, the colloidal solution is the mixture of one or more substances which are microscopically dispersed insoluble particles. Colloidal solution contains the charging carrying dispersed phase and uncharged dispersion medium.
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Key Terms: Coagulation, Precipitation, Flocculation, Colloids, Electrolyte, Dialysis, Lyophilic, Lyophobic, Charges, Electrophoresis
Techniques for Coagulation of Colloidal Solution
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Coagulation is done on colloidal solutions by affecting the charge they occur. There are many techniques used for it which are discussed below:
Coagulation by Electrophoresis
Electrophoresis is the process in which the charge particles of the solution move because of the electric field produced. In coagulation of colloidal solution, the charge particles contained in the solution are forced to move toward the opposite charge. As the process the continues, the charge particle gets discharged and are later collected from the bottom.
Coagulation by Heating or Boiling
Heat coagulation is commonly used mechanism in converting the solution in semi-solid state. When the colloidal solution is kept for boiling, then the absorption layer of the solution is disturbed due to movement of the charge particles as they collide with each other as well as with surface which leads to lower down of charge and settlement of the particles.
Coagulation by Mixing Opposite Charged Solution
In this method, the solution which contains opposite charge as that of the colloidal solution is added in same amount so that the charge collides and nullifies their effect and forms precipitates. For example, the colloidal solution has particles with positive charge of +10 units. So, the solution added in colloidal solution should be of -10 units charge which will result in coagulation.
Fe(OH)3 (positively charged solution) + As2S3 (negatively charged solution) → precipitates
Coagulation by Persistent Dialysis
Electrolytes are the substances which produce electric current because of the movement of the ions (positive and negative particles). The stability of colloidal solution is due to the presence of small amounts of electrolytes. In this method, the solution goes through the persistent dialysis which removes the electrolytes from the colloidal solution. Due to this, the solution becomes unstable and precipitates.
Coagulation by Persistent Dialysis
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Coagulation of Lyophilic Colloids
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Lyophilic colloids are the types of colloids in which the dispersed phase and dispersed medium have a very strong affinity with the liquid. For example- gum, protein, starch etc. They are difficult to coagulate because of the stability which is due to presence of charges. Their property of high solvation results in formation of the layer when particles are mixed in dispersed medium.
They are also known as reversible solutions due to their ability to reconstitute on adding dispersion medium after separation. So, to coagulate or precipitate the lyophilic colloid solution, it needs to be unstable by attacking the charge and its behaviour of solvation. They can be coagulated either by adding the electrolyte or by adding the oppositely charged solvent in the colloidal solution.
Coagulation of Lyophobic Colloids
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Lyophobic colloids are the types of colloids in which the dispersed phase and dispersed medium have a little or no affinity with the liquid so the colloid is not readily found. Thus, the difference between lyophobic and lyophilic colloids is because of their stability. Lyophobic colloids are stable because of the charges they carry. They can be coagulated with the addition of electrolyte which will remove the charges they have and precipitate the solution. Moreover, they can also be coagulated easily by heating them because of their less stability.
Things to Remember
- Coagulation is also known as flocculation or precipitation.
- Colloid substances are found in eight different physical states.
- Gas dispersion phase and gas dispersion medium do not form colloidal solutions.
- The stability of the colloidal particles is due to the charges.
- Coagulation of colloidal solution is done by four methods.
- Coagulation by adding electrolyte or mixing opposite charge solution is most common.
- Lyophilic colloids are more stable than the lyophobic colloids.
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Sample Questions
Ques: How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium? (3 Marks)
Ans: Classification on the basis of physical state of the dispersed phase and dispersed medium is one of the three categories that differentiate the colloidal solutions.
Dispersed phase and dispersed medium are either solid, liquid or gas which gives eight types of colloidal solution as one type i.e., gas dispersed phase and gas medium is homogenous mixture which does not form colloidal solution. For example, smoke is a colloidal solution with solid phase and gas dispersed medium.
Ques: Why are hydrophobic sols easily coagulated? (2 Marks)
Ans: Hydrophobic solutions such as oil are the compounds which repel the water molecules. They are stable because of the presence of charge particles in the solution. They can easily coagulate if the charge particles are removed by addition of the electrolyte or by mixing the solution containing the opposite charges.
Ques: Which of the following does not cause coagulation of colloidal solution? (2 Marks)
(a) Filtration
(b) Nonelectrolyte
(c) Electrolyte
(d) All
Ans: (b)
Explanation: Coagulation or precipitation of colloidal solution is due to the settlement of the particles. As the colloidal solution contains charged particles which is the reason for their stability. So, to settle down the particles, charge on them should be nullified. The addition of nonelectrolyte will fail to perform the procedure for settlement. Hence, nonelectrolyte cannot cause coagulation.
Ques: What is Tyndall effect? (2 Marks)
Ans: When the light passes through the colloidal solution then it shifts from its normal path and scatters, the phenomenon of scattering of light is known as Tyndall effect. This is due to the presence of the colloidal particles in the solution which do not allow the light to pass through them. The ray of light colloids with the colloidal particles and make them visible through the solution.
Ques: What is Hardy Schulze law of coagulation? (2 Marks)
Ans: Hardy Schulze law of coagulation gives the idea of the amount of electrolyte needed in the process to coagulate the colloidal solution. It states that the quantity of electrolyte required to coagulate a definite amount of colloidal solution depends upon the valency of the coagulating ion. Moreover, the valency of coagulating ions is directly proportional to the coagulation power i.e., the greater the valence of coagulating ions represents the greater power of coagulating.
Ques: What can be the best method to coagulate a colloidal particle? (2 Marks)
Ans: Coagulation of colloidal particles is done by the neutralization of electrical charges contained by the solution. The best method to coagulate is either lowering the zeta potential of the colloids or by mixing the solution containing a large number of opposite charge ions. Moreover, the method of absorption or refining is also done to precipitate the colloidal solution.
Ques: How is electrophoresis helpful in causing coagulation of colloidal solution? (2 Marks)
Ans: The colloidal solution consists of dispersed phase and dispersed medium. In which, the dispersion possesses electric charge whereas the dispersion medium has no charge. In the process of electrophoresis, the colloidal particle carrying charges moves towards the opposite charge which discharges the colloidal particle and settles them at the bottom of the tank. Thus, the colloidal particles form precipitates.
Ques: Give an example of each method of coagulating the colloidal solution? (3 Marks)
Ans: Example of coagulation of colloidal particles by electrophoresis is rubber which gets coagulated from latex on cloth or metal.
Example of coagulation by boiling: Egg albumin from egg gets coagulated if boiled.
Example of coagulation of colloidal particle by mutual coagulation:
Fe(OH)3 (positively charged solution) + As2S3 (negatively charged solution) → precipitates
Example of coagulation by persistent dialysis: When BaCl2 is added to the negative solution of As2S3, the solution gets precipitated because of the Ba2+ ion.
Ques: What is coagulation value? (2 Marks)
Ans: Addition of electrolytes in the colloidal solution is commonly used to coagulate them. So, the amount of electrolyte added to 1 litre of the colloidal solution to convert it into precipitates is called coagulation value. Moreover, the coagulation is inversely proportional to the coagulating value i.e., more the coagulation value means less coagulation.
Ques: Can lyophilic solutions be coagulated? (2 Marks)
Ans: The colloids in which the dispersed phase and dispersed medium have strong affinity with liquid are known as lyophilic solutions. They are not easily coagulated because they form the protection layer due to their property of solvation. They can be coagulated either by adding the electrolyte or by adding the oppositely charged solvent in the colloidal solution.
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