To Study the Dialysis of a Starch Sol Containing Sodium Chloride Through Cellophane or Parchment Paper

There can be various impurities present in colloidal sols during the process of their preparations. These impurities in colloidal sols are responsible for the destruction of the colloids and cause the colloidal particles to precipitate. Hence, impurities, such as electrolytes, acids, etc., must be removed to prevent this. This removal is done through the process of Dialysis.

Dialysis is the process of removing impurities from a colloidal solution through a semipermeable membrane and via diffusion. This process makes the colloidal solutions stable.

Table of Contents

• Aim

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

Aim

To study the dialysis of a starch sol containing sodium chloride through a cellophane or parchment paper.

Apparatus Required

1. Beaker(250ml) 

2. Funnel with long stem

3. Cellophane or parchment membrane

4. Iron stand

5. Test-tubes

6. Starch sol containing sodium chloride

7. AgNO3 solution

8. Iodine solution

Theory

Dialysis of a colloidal solution is based on the principle of movement of electrolytes through the cellophane membranes. Electrolytes, such as Na+, Cl- etc., easily move through the parchment or cellophane membrane and dissolve in the water surrounding the parchment paper. Colloidal particles cannot pass through the cellophane membrane and remain inside the bag made of cellophane paper. This way, colloids are purified from the impurities and separated from each other.

Procedure

1. Take a parchment membrane and fold it into the shape of a bag.

2. Tie the bag to the long stem of the funnel with the help of a thread or rubber band.

3. Pour the starch sol containing sodium chloride through the funnel until the parchment bag is filled two-thirds.

4. Take a 400 ml beaker containing three-fourths of distilled water.

5. Place the parchment bag into this beaker and fix the funnel with the help of the clamp of the iron stand.

6. Keep this apparatus standing for about 30 mins.

7. Post 30 mins, take 1 ml of distilled water with a pipette or dropper and pour it into two test tubes.

8. Put a few drops of iodine solution in one test tube and observe the changes. Put a few drops of AgNO3 solution in another test tube and observe.

Observations 

1. In addition, no colour change was seen in the test tube's iodine solution. 

2. In addition, to the silver nitrate solution in the test tube, white coloured ppt of AgCl was observed.

Result

1. Since no colour change was observed upon adding iodine solution, it confirms the absence of starch in distilled water.

2. Since white-coloured ppt of AgCl was observed upon adding silver nitrate solution, it confirms the presence of chloride ions in distilled water.

This image shows the process of purification of colloids

Precautions

1. Do not overfill the parchment bag with the colloidal solution.

2. The parchment bag should be intact without any holes in it.

3. Use distilled water.

4. Change the distilled water at regular intervals.

Lab Manual Questions

1. Which chemical can detect the presence of starch?

Ans: The Iodine solution is used to detect the presence of starch. Iodine in contact with starch turns the solution into a blue-black colour. In the absence of starch, no colour is seen.

2. What is a cellophane membrane?

Ans: Cellophane membranes are transparent membranes which are regenerated from cellulose. They have high hydrophobicity and mechanical strength; they are biodegradable and act as a gas barrier.

3. What is the coagulation value?

Ans: The minimum concentration of an electrolyte in millimoles required for coagulation in one litre of a colloidal solution is known as the coagulation value.

4. How to infer that dialysis is completed?

Ans: To understand if dialysis is completed, the following should be done-

• Take the cellophane bag, dip it in the beaker containing the fresh distilled water, and wait for 10-15 mins.

• Post that, take the distilled water sample from the beaker and again test for the presence of Cl- ions. If Cl- ion's test is positive, dialysis is still incomplete, and if this test is negative, dialysis is over.

Viva Questions

1. Which test is done for the detection of starch in a solution?

Ans: A starch test is performed to detect the presence of starch in a solution.

2. What are the qualities of a semi-permeable membrane?

Ans: Semi-permeable membranes are partially permeable membranes which allow the passage of only some selected entities.

3. Can cellophane be considered similar to plastic?

Ans: No, cellophane is obtained from natural resources, example-wood. Plastic is obtained from oil, which is non-biodegradable.

4. Give various examples of colloids

Ans: Common examples of colloids are Milk, starch solutions, Mayonnaise, Ice-creams, Smoke, Fog, etc.

5. What is electro-dialysis?

Ans: The process of separating impurities from the colloidal solution under the influence of an electric field is known as electrodialysis.

6. How are coagulating value and rate of coagulation of a colloid related to each other?

Ans: Coagulating value and rate of coagulation are directly proportional to each other; hence, the greater the valency of the coagulating ion, the higher the coagulation rate.

7. What is smoke composed of? 

Ans: Smoke is a solid-aerosol colloidal solution wherein solid dust particles are dispersed in a gaseous air medium.

8. What is centrifugation?

Ans: Centrifugation is a method to separate colloidal particles from colloidal solutions.

9. Explain colloidal solutions

Ans: Colloidal solutions are heterogeneous solutions wherein the dispersed phase is immiscible with the dispersion medium; hence, the dispersed phase remains suspended in the dispersion medium.

10. What is chemical coagulation?

Ans: Chemical coagulation is a method of separating colloidal particles from the solution by destabilising the charges present on their surface.

Practical Based Questions (MCQs) 

1. Coagulation is also known as____

1. Sedimentation

2. Precipitation

3. Flocculation

4. Suspension

Ans: Flocculation

2. What is the common name of sodium chloride?

1. Black salt

2. Table salt

3. Rock salt

4. Red salt

Ans: Table salt

3. AgNO3 is known as____

1. Silver nitrate

2. Silver nitrite

3. Argon nitrite

4. Aurum chloride

Ans: Silver nitrate

4. Fog is a type of_____where in _____is a dispersion medium.

1. Sol, gas

2. The Colloidal solution, liquid

3. Emulsion, Liquid

4. The Colloidal solution, gas

Ans: The Colloidal solution, gas

5. Cellophane membrane is an example of a____

1. Natural semi-permeable membrane

2. Artificial semi-permeable membrane

3. The animal’s completely permeable membrane

4. Plant completely permeable membrane

Ans: Artificial semi-permeable membrane

6. Cellophane is made of____

1. Lignin

2. Pectin

3. Cellulose

4. Gum

Ans: Cellulose

7. The coagulation values in millimoles per litre of electrolytes which are required for coagulation of Fe(OH)3 are as follows- NaCl=0.22, KCl=51, BaCl2=0.69, MgSO4=52. The correct order of flocculating power is

1. KCl> BaCl2>NaCl> MgSO4

2. NaCl> MgSO4 > KCl>BaCl2

3. BaCl2>NaCl> KCl> MgSO4

4. NaCl> BaCl2> KCl> MgSO4

Ans: NaCl> BaCl2> KCl> MgSO4

8. Cellophane is_____

1. Non-biodegradable

2. Recyclable

3. Biodegradable

4. Hazardous

Ans: Biodegradable

9. A millimole is _____

1. One-thousandth of a mole

2. One-hundredth of a mole

3. Ten times a mole

4. Double a mole

Ans: One-thousandth of a mole

10. A liquid-liquid colloidal solution is____

1. Foam

2. Emulsion

3. Sol

4. Aerosol

Ans: Emulsion

Conclusion

From the above experiment, it can be concluded that dialysis is a method of purification of colloidal sols, and it helps remove various impurities such as NaCl, HCl and other electrolytes. When iodine solution was added to distilled water, it showed a negative result, meaning that colloidal particles such as starch, egg albumin, gums etc., cannot move through the parchment membrane. In contrast, Na+ or Cl- electrolytes can move through the cellophane membrane. Hence, in this way, a colloidal sol is purified and is made devoid of all impurities.