Compare the Precipitation Values of Sodium Chloride Barium Chloride and Aluminum Chloride for Arsenious Sulfide Sol

Colloidal solutions are made of a dispersion medium and dispersed phase. The dispersion medium is the solvent, and the dispersed phase is the solute. Coagulation or flocculation of a colloidal solution is a condition wherein the colloids break, separating the solute and solvent from each other. The dispersed phase no longer remains suspended in the dispersion medium. All the colloidal particles coagulate with each other and form a precipitate, which gets settled down. This precipitation is brought about by various electrolytes which have a charge opposite to the charge of the colloidal solution. Common electrolytes used are various salts such as Sodium chloride, Aluminium chloride, Barium chloride etc.

Table of Contents

• Aim

• Apparatus required

• Theory

• Procedure

• Observation

• Result

Aim

To compare the precipitation values of sodium chloride, barium chloride and aluminium chloride for arsenious sulfide sol.

Apparatus Required

1. Conical flasks (100ml)

2. Beurette

3. Beaker

4. Arsenious sulfide

5. 0.1M Sodium chloride solution

6. 0.01M Barium chloride solution

7. 0.001M Aluminium chloride solution

Theory

Arsenious sulfide sol is lyophobic in nature and has colloidal particles of arsenious sulfide in water. The arsenious solution is a negatively charged sol, and hence a positively charged sol or a positively charged electrolyte is used to precipitate the As2S3 colloidal particles. The coagulation value is directly proportional to the valency of the coagulating ion. Thus univalent ions such as Na+ have less coagulating power as compared to trivalent ions such as Al3+. Hence sodium salts will be required in large quantities as compared to aluminium salts, which will be required in less quantity. Arsenious sulfide sol is a yellow-coloured colloidal sol which on precipitation gives a yellow-coloured As2S3 precipitate.

Procedure

1. Take three conical flasks and label them as A, B, and C.

2. Add 20 ml of arsenious sulfide sol to each of the conical flasks.

3. Take a burette and fill it with 0.1M NaCl solution.

4. Take the conical flask containing 20 ml of arsenious sulfide sol and dropwise add the 0.1M NaCl solution. Shake gently and continue adding the NaCl solution till a yellow-colored precipitate of arsenious sulfide is just formed.

5. Note the volume of 0.1M NaCl solution required to cause the precipitation of arsenious sulfide sol.

6. Repeat the same procedure with 0.01M Barium chloride solution and 0.001M Aluminium chloride solution.

7. Note down the volume consumed by both the salt solution and tabulate the results.

Observations 

Result

1. The precipitation values of NaCl. BaCl2 and AlCl3 for As2S3 sol are____,____ and ____respectively.

2. The precipitation values are in the order of NaCl>BaCl2>AlCl3.

3. The coagulating powers of these electrolytes are in the order of AlCl3>BaCl2>NaCl.

Precautions

1. Apparatus should be cleaned before and after the experiment.

2. To detect the start of precipitation directly look vertically down the flask.

3. Do not shake the contents of the flask vigorously.

4. Add the electrolyte slowly and dropwise so as to find the exact volume where precipitate is formed.

5. Handle arsenious oxide and hydrogen sulfide with precaution.

Lab Manual Questions

1. What is known as Coagulation value?

Ans: The minimum concentration of a particular electrolyte (in millimoles) which is necessary to cause coagulation of 1 litre of a colloidal solution is known as the coagulation value of that electrolyte. Higher the coagulation value lower is the coagulating power.

2. Why is the valency of barium 2?

Ans: Barium is an alkaline earth metal. In its outermost cell or the valence cell it contains 2 electrons, hence the valency of barium is 2. When it donates these two electrons it forms Ba2+ ions.

3. What is known as coagulating power?

Ans: Coagulating power is the amount of electrolyte which is required to cause coagulation. According to Hardy Schulze rule, more the charge on the oppositely charged ion, more is the coagulating power of that ion. Hence for coagulating a negatively charged sol a positively charged ion is required having greater charge or having more positive ions in its valence shell. For eg. Al3+, Ba2+ etc.

4. Give examples of positively and negatively charged sols.

Ans: Positively charged sols include: - Haemoglobin, Metal hydroxides, basic dyes etc.

Negatively charged sols include: - Metal sols, Metal sulfides, gold sols, blood, acidic dyes etc.

Viva Questions

1. Which is a commonly used coagulant in clearing water?

Ans: The commonly used coagulant is alum or aluminium sulfate. 

2. What are the 4 clotting factors in humans required for blood coagulation?

Ans: Blood clotting factors XII, XI, IX and VIII are the 4 clotting factors which help in blood coagulation in humans.

3. What is oil in water emulsions?

Ans: Oil in water is a type of emulsion wherein oil is the dispersed phase and water is dispersion medium eg. Milk, vanishing cream etc.

4. Is arsenious oxide toxic?

Ans: Yes, arsenious oxide is toxic and low doses can cause liver and kidney problems, also it is very dangerous if it comes in contact with eyes.

5. Human blood is which type of colloidal solution?

Ans: Blood is a type of colloid wherein the plasma protein is the liquid or dispersion medium and the blood cells are the solids which act as the dispersed phase. Since blood is made up of a liquid and solid component it is a sol type of colloidal solution.

6. How to prepare emulsions?

Ans: Emulsions are liquid-liquid colloidal solutions and are obtained by mixing two liquids vigorously. To keep these liquids in emulsions for a long time, emulsifiers such as soaps, proteins, detergents etc are used.

7. Enlist different methods used for coagulating colloidal solutions.

Ans: The different methods used for coagulating are-

• Mutual Precipitation

• Electrophoresis

• Multiple Dialysis

• Heating

8. What are lyophobic sols?

Ans:  Colloidal sols where the solid dispersed phase has less affinity towards the liquid dispersion medium are known as lyophobic sols. For eg. Aluminium hydroxide, Ferric hydroxide, Arsenious sulfide etc.

9. What is clotting factor 3 seen in humans?

Ans: Clotting factor 3  is known as Tissue factor(TF) or Thromboplastin and is the primary initiator of the extrinsic coagulation pathway.

10. What are clouds?

Ans: Clouds are formed when lots of water vapour present in air condenses to form small water droplets.It is an aerosol type of colloidal solution where in the dispersed phase is liquid and dispersion medium is air.

Practical Based Questions (MCQs) 

1. What is the valency of barium in barium chloride?

1. 1

2. 2

3. 5

4. 7

Ans: 2

2. The most effective electrolyte in coagulation of arsenious sulfide are____

1. Barium ions

2. Carbon ions

3. Aluminium ions

4. Sodium ions

Ans: Aluminium ions

3. Aluminum chloride salt is___

1. Acidic salt

2. Basic salt

3. Neutral salt

4. All of the above

Ans: Acidic salt

4. For the coagulation of 100 ml of arsenious sulfide, 5 ml of 1M of sodium chloride is used, then what will be the coagulation value of sodium chloride?

1. 50 m mol L-1

2. 5 m mol L-1

3. 0.5 m mol L-1

4. 500 m mol L-1

Ans: 50 m mol L-1

5. Common emulsifiers are

1. Proteins

2. Gum

3. Agar-Agar

4. All of the above

Ans: All of the above

6. Coagulating power of an electrolyte is ______to coagulating value.

1. Directly proportional

2. Inversely proportional

3. More than

4. Less than

Ans: Inversely proportional

7. In water in oil emulsions, water is_____and oil is____

1. Dispersion medium, dispersed phase

2. Solvent, solute

3. Dispersed phase, dispersion medium

4. Both B and C

Ans: Dispersed phase, dispersion medium

8. Define Demulsification.

1. It is a process of making an emulsion

2. It is a process of making an emulsifier

3. It is a process of breaking the emulsion

4. It is a process of making a true solution

Ans: It is a process of breaking the emulsion

9. State the true statement.

1. Soaps and detergents can become good true solutions.

2. Disinfectants such as Lysol and Dettol are oil in water type of emulsions when in water.

3. Salts of sodium ions have the highest coagulating power in coagulating negative sols.

4. Arsenious solution is positively charged.

Ans: Disinfectants such as Lysol and Dettol are oil in water type of emulsions when in water.

10. Decreasing order of coagulating power for positively charged sols is-

1. PO3-4 > SO2-4 > Cl-

2. Al3+ > Ba 2+ > Na+

3. Cl- > SO2-4 > PO3-4

4. Na+ > Ba 2+ > Al3+

Ans: PO3-4 > SO2-4 > Cl-

Conclusion

From the above experiment we understand that precipitation of negative sols occurs by positive ions and vice-versa. The precipitation value is also known as coagulating value or flocculation value and is the minimum amount of coagulant required to precipitate 1 litre of a colloidal solution. Higher the charge more is the capacity of that ion to precipitate a colloidal solution. Therefore, in the above experiment arsenious sulfide was precipitated most efficiently by AlCl3 as compared to the other two salts.