Purification of Colloids- Dialysis, Ultrafiltration, Ultracentrifugation, Practice Problems and FAQs

Have you thought that the water you drink is always that pure? The water that we drink contains a number of unwanted impurities. To remove those impurities, it is passed through several filters to purify it and make it fit for drinking. Do you ever think of how these impurities are being removed? If a normal kitchen strainer would have worked, life would be much easier. There would be no need for large-scale water treatment plants. Which process is used to filter impurities from the water such that water must contain minerals required for our body? Let's see what all techniques are used to purify the colloidal solutions at the industrial level.

Table of contents

• Why do we need to purify colloids?
• Dialysis
• Electrodialysis
• Ultrafiltration
• Principle, techniques of purification, Application
• Practice problems
• Frequently Asked Questions-FAQs

Why do we need to purify colloids?

Electrolytes and other soluble contaminants are typically present in high concentrations in colloidal solutions. Electrolytes must be present in trace amounts for the colloidal solution to be stable, but excessive amounts cause coagulation. Therefore, the contaminants must be kept to a minimum in order to maintain balance.  The techniques used to purify colloids are given as follows:

1. Dialysis: 

• It is based on the principle of diffusion. 
• In the process of dialysis, dissolved substances are removed from a colloidal solution through a suitable semi-permeable membrane. Only those membranes can be used that allow a true solution, containing ions or smaller molecules to pass through it and not the colloidal particles. Therefore, animal membrane (bladder), parchment paper or cellophane sheets are used for this purpose. 
• The apparatus used for the separation of the colloidal solution is called a dialyser. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which freshwater is continuously flowing The molecules and ions diffuse through the membrane(selectively permeable membrane) into the outer water and the pure colloidal solution is left behind.

Applications: Dialysis is most commonly used to remove undesired tiny molecules like salts, reducing agents, or dyes from bigger macromolecules like proteins, DNA, or polysaccharides. 

2. Electrodialysis

• It is based on the principle that when an electric field is applied through an electrolytic solution, the ions move towards the oppositely charged electrodes. 
• Electrodialysis is used if the impurities present in the colloidal solution contain only an electrolyte. This alteration makes the process faster as compared to simple dialysis. 
• The colloidal solution is placed in a bag of the suitable membrane, while pure water is taken outside. Electrodes are fitted in the compartment. The ions present in the colloidal solution migrate out to the oppositely charged electrodes penetrating the membrane while colloidal particles remain inside the solution due to their larger size. 

Applications: It is used in the purification of blood in the artificial kidney machine. Dialysis involves the process of removing the waste and extra fluid from one's blood that the kidneys can no longer remove themselves.

3. Ultrafiltration

• Ultrafiltration is the process of separating colloidal particles from solvent and soluble solutes by specially prepared filters. The filter used for this purpose should be permeable to all substances except the colloidal particles. Thus, Ultra-filter paper is used for this purpose. 
• Ultrafiltration is a selective separation method for concentrating and purifying medium to high molecular weight components such as plant and dairy proteins, polysaccharides, and enzymes. It is used in the production of potable water. 
• The colloidal particles left on the ultra-filter paper are then stirred with a fresh dispersion medium to get a pure colloidal solution, protein concentration, etc. Smaller lactose, water, mineral, and vitamin molecules pass through the barrier, whereas larger protein and fat molecules (essential ingredients in cheese production) are kept and concentrated. In sugar refining, polysaccharides, proteins, and colloidal contaminants are removed using this process.

Preparation of Ultra-filter paper

It is prepared by soaking the filter paper in a collodion solution. Collodion is a 4% solution of nitrocellulose in a mixture of alcohol and ether. It is then hardened by formaldehyde and dried. 

4. Ultracentrifugation 

• It is based on the principle of sedimentation, according to which denser particles settle down faster than less dense ones due to gravity.
• In the ultracentrifugation technique, the sample is made to rotate at a very high speed. As a result of which larger and denser particles move faster and settle down in form of pellets at the bottom of the tube. On the other hand, the smaller and less dense particles move slower and remain floating on the surface.

Applications: Used in quantitative analysis of macromolecules in solution, determination of properties of biomolecules like proteins and nucleic acids.

Practice problems

Q 1. Name the membrane used in simple dialysis.

(A) Cellophane sheet
(B) Animal membrane
(C) Ultra-filter paper
(D) Both (A) and (B)

Answer: (D)
Membranes like animal membrane (bladder) or parchment paper or cellophane sheet which allow a true solution, containing ions or smaller molecules to pass through it and not the colloidal particles are used in dialysis. 

Q. 2. Which compound is used in the hardening of filter paper when dipped in a collodion solution in the ultrafiltration method?

(A) Acetaldehyde
(B) Formaldehyde
(C) Formic acid
(D) None of the above

Answer: (B)
Ultrafilter paper is prepared by soaking the filter paper in a collodion solution when is then hardened by formaldehyde and dried. 

3. Which of the following technique works under the principle of gravity?

(A) Ultrafiltration
(B) Ultracentrifugation
(C) Dialysis
(D) Electrodialysis

Answer: (B)
Ultracentrifugation is based on the principle of sedimentation, according to which denser particles settle down faster than less dense ones due to action of gravity.     

Q. 4. Which of the following is the application of ultrafiltration?

(A) Water Treatment
(B) Desalination
(C) Both (A) and (B)
(D) None of the above

Answer: (C) 
To make potable water, ultrafiltration can be used to remove particles and macromolecules from raw water. It has been utilised to replace current secondary (coagulation, flocculation, sedimentation) and tertiary (sand filtration and chlorination) filtering systems in water-treatment plants. Also, it is used to remove contaminants from sea water in order to make it potable for drinking purposes.

Frequently Asked Questions-FAQs

Q 1. Why formaldehyde is used as an external agent to hardened filter paper soaked in collodion in ultrafiltration?

Answer: Urea-formaldehyde resin, a thermosetting resin, is used to prepare ultrafiltration membranes because of its excellent mechanical properties and filtration performance.

Q 2.  How ultrafiltration method is used in the production of potable water?

Answer: Ultrafiltration is used in water purification in which water is forced through a semipermeable membrane. Water and low-molecular-weight solutes filter through the membrane at one side, while suspended particles and high-molecular-weight solutes remain on the other side of the membrane. Most organic compounds and viruses, as well as a variety of salts, can be removed with ultrafiltration. It removes 90-100% of pathogens and does not require chemicals.

Q 3.  What is the size of colloidal particles to pass through parchment paper?

Answer: Colloidal particles have sizes in the range   and parchment paper allows particles of the size approximately 10^-12 to pass through it. Therefore, colloidal particles in the range  can pass through the parchment paper.

Q 4. What is the size of the particles that can be separated using ultracentrifugation? 

Answer: Particles of the size range  can be used for separation using ultracentrifugation technique.

Related topics