+2 UNIT 7 PAGE-3

PURIFICATION OF COLLOIDAL SOLUTION
The colloidal solutions prepared by the above methods usually contain impurities especially electrolytes which can destabilise the sols. These impurities must be eliminated to make the colloidal solutions stable. The following methods are commonly used for the purification of colloidal solution.
(i) Dialysis : The process of separating the electrolytes in the colloidal state from those present in the true solution by means of diffusion through a semi-permeable membrane is called dialysis. The apparatus used for dialysis is called dialyser. Dialysis is based on the fact that colloidal particles are retained by a semi-permeable membrane ( e.g. parchment , cellophane membrane etc.) while the ions of the electrolyte pass through it. The impure colloidal solution is taken in a parchment bag and placed in a vessel containing running water (Fig).

The impurities move slowly out of the bag leaving behind pure colloidal dispersion.
(ii) Electrodialysis : This is a modified form of dialysis. Ordinary dialysis is quite slow, but it can be quickened by applying an electric field. The process is then called electrodyalysis.
(iii) Ultrafiltration : The pores of an ordinary filter paper are large enough to allow the passage of both impurity particles as well as colloidal particles. Therefore , an ordinary filter paper cannot be used for removing the impurities of the electrolytes from an impure sol. However, if the pore size of ordinary filter paper is reduced, it can be used for separating the impurities from impure sols. This is achieved by treating an ordinary filter paper with collodion or gelatine followed by its hardening by dipping it in formaldehyde solution. This treatment reduces the pore size and enables it to check the passage of colloidal particles through it. Filter papers thus obtained are called ultrafilters. Filtration through ultrafilters is called ultrafiltration.
In utrafiltration, the ultra-filter is supported over a wire mesh and impure sol is poured over it. The impurity particles (electrolytes) pass through the ultrafilter while larger colloidal particles are retained. The process is very slow. However, it can be expedicted by applying pressure on sol side or by using a suction pump on filtrate side. By using a series of graded ultrafilters, impurities of different size can be removed and it is even possible to separate colloidal particles of different size from one another.

Ultrafiltration
PROPERTIES OF COLLOIDAL SOLUTIONS
1. Colligative properties : Colloidal solutions show the colligative properties viz., relative lowering of vapour pressure, elevation in boiling point, depression in freezing point and osmotic pressure. However, due to high average molecular masses of colloidal particles, mole fraction of dispersed phase is very low. Hence , the values of colligative properties observed experimentally are very small. Only osmotic pressure measurements are used in determining the molecular mass of polymers.
2. Mechanical properties
(a) Brownian movement : When a colloidal solution is viewed under an ultramicroscope, the colloidal particles are seen continuously moving zig-zag path. The property was discovered by a botanist Robert Brown (1827) when he observed that pollen grains suspended in water exhibit random zig-zag motion. The property was named as Brownian movement.





Brownian movement
The continuous rapid zig-zag movement of colloidal particles in the dispersion medium is called Brownian movement.
Cause of Brownian movement : Brownian movement is due to the unequal bombardments of the moving molecules of dispersion medium on colloidal particles. The moving molecules of the dispersion medium continuously attack on colloidal particles . The moving molecules of the dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them. Since the chances of their collisions are unequal , the net driving force on a colloidal particle forces it to move in a particular direction. As the particle moves in that direction, other molecules of the medium again collide with it and the particle changes its direction. The process continues. This results in a random zig-zag movement of the colloid particle.
The Brownian movement decreases with increase in the size of colloidal particle. This is why suspensions do not exhibit this type of movement. Brownian movement plays an important role in imparting stability to a sol. This is because Brownian movement opposes the gravitational forces acting on colloidal particles and prevents them from getting settled down.
(ii) Diffusion : The sol particles diffuse from high concentration region to low concentration region. However, the rate of diffusion of colloidal solution is less than that of true solutions.
(iii) Sedimentation : The particles of colloidal solution are in a state of constant motion. Under the influence of gravity, the sol particles tend to settle under very slowly. The rate of sedimentation can be increased by the use of ultracentrifuge.
3. Optical properties
i) Tyndall Effect : Tyndall observed that when a beam of light is passed through a colloidal solution, placed in a dark room, the path of the beam gets illuminated by a bluish light. This is due to the scattering of light by colloidal particles(molecules and ions do not scatter light). The phenomenon of scattering of light by colloidal particles is known as Tyndall effect and the illuminated path is known as Tyndall cone.
The visibility of dust particles in a semi darkened room when a beam of sunlight enters is an example of Tyndall effect. Tyndall effect is not observed in a solution as the particles are too small in size to cause any scattering.

(ii) Visibility : The ultra microscope developed by Zsigmondy , is used to visualise the colloidal particles. This technique depends upon Tyndall effect. A beam of light is allowed to pass through the sol and the scattered beam is viewed through a microscope placed at right angles to the incident beam. This arrangement is known as ultramicroscope. In doing so, we can observe spots of light moving irregularly against a dark background. It may be noted that, we do not see the actual particles, but only the light scattered by them.
4. Electrical properties : The particles of the colloidal solutions possesses electrical charge. The presence of charge is responsible for the stability of these solutions. It may be noted that only the sol particles carry some charge, while the dispersion medium has no charge. For example, the colloidal solutions of gold, arseneous sulphide(As2S3) are negatively charged, while those of Fe(OH)3 and Al(OH)3 have positive charge. In the case of silver chloride sol, the particles may either be positively charged or negatively charged.

Electrophoresis
The presence of the charge on the sol particles and its nature whether positive or negative can be determined with the help of a phenomenon known as electrophoresis. In this experiment, the colloidal particles move towards positive or negative electrodes depending upon their charge under the influence of electrical field. The phenomenon of movement of colloidal particles under an applied electric field is called electrophoresis.
If a particle accumulate near the negative electrode, the charge on the particle is positive. On the other hand, if the sol particles accumulate near the positive electrode, the charge on the particle is negative.
The apparatus used consists of a U-tube with two platinum electrodes in each limb. Take a sol of As2S3 in a U-tube. The intensity of colour of the sol in both the arms is the same. Now pass the current through the sol. After some time, it is observed that the colour of the sol near the positive electrode become intense than the initial colour. This indicates that the As2S3 particles are negatively charged and they move towards oppositely charged (positive) electrode and accumulate here.
Similarly, when electric current is passed through positively charged Fe(OH)3 sol , it is observed that they move towards negatively charged electrode and get accumulate there.
Thus, by observing the direction of movement of colloidal particles, the sign of the charge carried by the particles can be determined.
Electro-osmosis
The migration of colloidal particles under the influence of an electric field can be prevented by placing suitable semipermeable membranes on either side of the U-tube. At the same time, when an electric field is applied, the dispersion medium is seen to migrate in a direction opposite to the direction in which the sol particles would be moved, if they were free to do so. Thus, the movement of dispersion medium under the influence of an electric field, when the sol particles are prevented from moving is called electro-osmosis.

Electro-osmosis
Origin of Charge
(i) Due to frictional electrification : The rubbing of dispersed phase particles with those of dispersion medium may result in some charge on colloidal particles.
(ii) Due to Dissociation of surface molecules : Sol particles may get charge due to dissociation of surface molecules. For example, an aqueous solution of soap dissociates into ions :

The cation (Na+) pass into solution, while the anions have a tendency to form aggregates (micelle) due to weak attractive forces present in the hydrocarbon chain. Thus, the anions which are of colloidal dimensions get negative charge.
(iii) Selective adsorption of ions : It has been observed that a small amount of electrolyte is essential for the stability of the sols. It is therefore believed that the charge on the colloidal particles is due to preferential adsorption of positive or negative ions from the electrolyte. Thus, if positive ions are adsorbed by sol particles, then the sol particles get positive charge and vice versa. Generally sol particles adsorb only those ions, preferentially which are common with their own lattice ions. Examples :
(a) The Fe(OH)3 sol prepared by the hydrolysis of FeCl3 has positive charge due to the preferential adsorption of Fe3+ ions.

Representation of Fe(OH)3 sol

(b) The negative charge on As2S3 sol particles is due to the preferential adsorption of S2 on the surface of sol particles.

(c) When AgNO3 solution is added to aqueous KI solution , a negatively charged sol of AgI is formed. This is due to selective adsorption of I ions from the dispersion medium.
AgI + I  [AgI] I
dispersion (negative sol)
medium
On the other hand , when KI is added to AgNO3 solution, a positively charged sol of AgI is formed. This is due to the selective adsorption of Ag+ ions present in the dispersion medium.
AgI + Ag+  [AgI] Ag+
dispersion (positive sol)
medium
Coagulation of Colloidal solutions
The stability of colloidal state is due to the existence of similar charges on the sol particles. For this a small amount of electrolyte is essential. However, in the presence of a large excess of electrolyte, the charge on sol particles get neutralised. As a result, these neutralised particles comes closer, grow in size and form precipitate. This is called coagulation. The coagulation of colloids can be brought about by any one of the following methods.
(i) By the action of electrolytes : In a colloidal system, the sole particles bear some charge. When an electrolyte is added to the sol, the colloidal particles take up ions carrying opposite charge from electrolyte. This results in neutralisation of the charge on the particles leading to their coagulation. For example, if aluminium sulphate solution is added to As2S3 sol, the Al3+ ions are attracted by the negatively charged sol particles. This causes neutralisation of the charge and hence precipitation of the colloidal solution occurs.
HARDY-SCHULZ RULE
The coagulation capacity of electrolyte depends on the valence of the active ion or flocculating ion. This is expressed by Hardy-Schulz law which states : ‘the greater the valence of the flocculating ion, the greater will be its coagulating power’. Thus the coagulation of the negatively charged As2S3 sol, the coagulating power of positive ions decrease in the order :
Al3+ > Ba2+ > Na+.
Similarly, the coagulation of positive sol of Fe(OH)3, the flocculating power of negative ions decreases in the order :
[Fe(CN)6]4 > PO43 > SO42 > Cl.
However, the rule is only approximate. The minimum concentration (in millimoles per litre) of an electrolyte which is required to cause the flocculation of a sol is known as flocculation value of the electrolyte. The flocculation values ( in millimoles per litre) for the negativesly charged As2S3 sol and positively charged Fe(OH)3 sol are given in TABLE.
For Negatively charged As2S3 sol
Electrolyte Flocculating ion Flocculation value(millimoles/litre)
NaCl Na+ 52
KCl K+ 50
HCl H+ 30
MgCl2 Mg2+ 0.72
BaCl2 Ba2+ 0.69
ZnCl2 Zn2+ 0.68
AlCl3 Al3+ 0.093


For Positively charged Fe(OH)3 sol
Electrolyte Flocculating ion Flocculation value(millimoles / litre)
KBr Br 138
HCl Cl 132
KNO3 NO3 132
K2CrO4 CrO42 0.315
K2SO4 SO42 0.210
K2C2O4 C2O42 0.238
K3[Fe(CN)6] [Fe(CN)6]3 0.096

(ii) By mutual precipitation : When two oppositely charged sols [eg Fe(OH)3 and As2S3 sols] are mixed in equimolar proportions, they mutually neutralise their charges and precipitate out.
(iii) By persistent dialysis : The stability of colloidal solution is due to the presence of traces of electrolytes. On prolonged dialysis, these traces of electrolytes are removed and particles of the dispersed phase will be coagulated.
PROTECTIVE COLLOIDS
The lyophilic colloids are more stable than lyophobic colloids. This is due to heavy solvation of lyophilic sol particles. Lyophilic colloids possess the property of protecting lyophobic colloids from precipitation by the action of electrolytes. The process of protecting lyophobic colloidal solution from coagulation by the action of electrolytes due to the previous addition of some lyophilic colloid is called protection. The lyophilic colloids used for such purposes are called protective colloids.
For example, the addition of gelatine ( a lyophilic colloid) to a gold sol (lyophobic sol) protects the latter from being coagulated on addition of sodium chloride solution.
The exact mechanism of protection is not very clearly understood. It may be due to the formation of a protective coating of lyophilic colloid around the lyophobic sol particles which prevent it from coming in contact with the ions of the electrolyte.

Protective action of lyophilic colloid particles
GOLD NUMBER
The lyophilic colloids differ in their protective powers. Zsigmondy introduced the term gold number to measure the protective powers of different colloids. This is defined as : ‘ The number of milligrams of the protective colloid that will just prevent the coagulation of 10 ml of a gold sol on the addition of 1 ml of 10% sodium chloride solution.’
The coagulation of the gold sol is indicated by colour change from red to blue when the particle size just increases. The gold number of a few protective colloids are given below :


Protective colloid Gold number Reciprocal
Gelatin 0.0050.01 200100
Haemoglobin 0.03  0 .14 3314
Egg albumin 0.12  0.2 10  5
Dextrin 6  20 0.160.05
Potato starch 20  25 0.05  0.04
Evidently , the smaller the value of gold number, the greater is the protecting power of the protective collloid. Therefore, the reciprocal of gold number is a measure of the protective power of a colloid. Thus in the above table, gelatin is the best protective colloid (reciprocal value of 200 - 100 ).
EMULSIONS
Emulsions are the colloidal solutions of two immiscible liquids in which one liquid acts as the dispersed phase and the other as the dispersion medium.
In emulsion, the suspended droplets (dispersed phase) are larger than the particles of dispersion medium. There are two types of emulsions :
(i) Oil - in - water (o - w) emulsions : In this type of emulsions, oil is the dispersed phase(small amount) and water is the dispersion medium(excess). E.g. milk is an emulsion of soluble fats in water and hence casein acts as an emulsifier. Another example is vanishing cream. Such emulsions are called aqueous emulsions.
(ii) Water-in-oil (w-o) Emulsions : In this type of emulsions, water is the dispersed phase and oil is the dispersion medium. E.g. cod liver oil, butter, cold cream etc. have particles of water dispersed in oil. Such types of emulsions are called oily emulsions.

Types of Emulsions
Identification of Emulsions
The following tests may be employed to distinguish between the two types emulsions.
(i) Dry test : Some oil soluble dye is added to the emulsion, if the background becomes coloured, the emulsion is water-in-oil type and if the coloured droplets are seen , the emulsion is oil-in water type.
(ii) Dilution test : If the emulsion can be diluted with water, this indicates that water is the dispersion medium and emulsion is of oil-in-water type. In case the added water forms a separate layer, the emulsion is water-in-oil type.
Preparation of Emulsions
Emulsification : The process of making an emulsion from water and oil is called emulsification. Emulsions can be prepared by vigourously shaking a mixture of two immiscible liquids or by propagating ultrasonic vibrations through course suspension.
Emulsifier
The emulsion prepared by mixing only two immiscible liquid is quite unstable and liquids separate after some time. To obtain stable emulsions, it is necessary to add small amount of suitable outside reagent called emulsifier or emulsifying agent. These are usually long chain compounds having both polar and non-polar ends. When an emulsion is formed , the polar part is attracted towards water and the non-polar part towards oil , i.e., an emulsifier reduces the interfacial tension between water and oil and thus helps in the mixing of two liquids, e.g. soap, gum, gelatin, detergents etc. Emulsifying properties of soaps and detergents are exploited in washing clothes and crockery. They emulsify the grease along the dirt and carry them away in the wash water.
Digestion of fats in the intestines is aided by emulsification. A little of the fat forms a sodium soap with the alkaline solution of intestine, and this soap emulsifies the rest of the fat. This makes it easier for digestive enzymes to carry out their functions.
Industrial Applications
(i) Emulsions find many industrial applications. For example, the concentration of the sulphide ore of a metal by froth floatation process involves the use of some oil like pine oil. The oil forms emulsion with ore particles. When air is bubbled through the emulsion, it rises to the surface as foam and is skimmed off.
(ii) Many pharmaceutical preparations like cod liver oil, lotions, ointments etc. are emulsions.
(iii) The cleaning action of soap : The cleaning action of soaps and synthetic detergents is based on their ability to form micelles.

In solution soap dissociates to give carboxylate ion (RCOO ) and sodium(Na+) ion. The carboxylate ion (RCOO ) is composed of a non-polar part(R-) of long hydrocarbon chain called tail and a polar part of COO, called head. Dirt present in clothes is oily in nature. When dirty clothes are soaked with soap solution, the tail part(it is hydrophobic in nature and hence has affinity to oil or grease) of soap and dissolves in the grease deposit and form micelle (Fig above). At the same time, the water soluble carboxylate ion (head) make hydrophilic surface around this sphere and render the entire micelle of grease water-soluble ( in the figure the solid circles represent the polar group and the wavy lines represent alkyl portions.). Thus soap helps in forming a stable emulsion of oil and water by acting as an emulsifier. Finally, when the cloth is rinsed with water, the soluble micelles are removed and thus the cloth becomes free from dirt.
APPLICATIONS OF COLLOIDS
Colloids play a vital role in our daily life. The applications in general are based on the presence of the charge on the colloidal particles.
(i) Smoke precipitation : Smoke is a colloidal dispersion of carbon particles. Smoke is made free from carbon particles by passing in between metal electrodes of a Cottrell precipitator. The electrodes are maintained at a high difference of potential. The carbon particles gets discharged and precipitate, while gases come out from the chimney.

Cottrel smoke precipitator
(ii) Sewage disposal : Sewage water contains charged colloidal particles of dirt, rubbish etc. and these do not settle down easily. The particles can be removed by discharging them at electrodes. Dirty water is passed through the tunnel fitted with metallic electrodes which are maintained at high potential difference. The particles migrate to the oppositely charged electrode, lose their charge and get coagulated. The deposited matter is used as a manure and the water left behind is used for irrigation.
(iii) Colloidal medicines : The colloidal medicines are more effective and are easily assimilated. A few important medicines are colloidal solutions of gold, manganese, sulphur etc. Ferric chloride is quite effective to stop bleeding as it can coagulate the blood due to its charged nature.
(iv) Foods : Many of our foods are colloidal nature. For example, milk is an emulsion of fat dispersed in water. Gelatin is added to ice cream as a protective agent to preserve its smoothness.
(v) Industrial goods : Soap is a colloidal electrolyte. Paints, varnishes, enamels, resins, gums etc are colloidal in nature. Latex from rubber is a suspension of negatively charged colloidal particles of rubber.
(vi) Purification of water : The drinking water can be purified by the precipitation of suspended particles. For this small amount of potash alum is added. The aluminium ions neutralise the negative charge on the mud particles and they get coagulated.
(vii) Formation of delta : The deltas at the mouth of great rivers are formed by the charged clay particles present in river water by the action of ions present in sea water. The river water contains colloidal particles of sand and clay which carry negative charge. The sea water contains a number of positive ions such as Na+, Mg2+, Ca2+ ion etc. When the river water comes in contact with sea water, the negative charge present on colloidal sand and clay particles get neutralised by the positively charged ions (present in sea water) and they get coagulated. The coagulated sand and clay particles settle down and take the shape of delta in due course of time (Fig).

Formation of delta

(vii) Building roads : Asphalt emulsified in water is used for building roads without the necessity of melting the asphalt.
(viii) Blue of sky : The sky is the empty space around the earth and as such it has no colour. It appears blue due to the scattering of light by colloidal dust particles present in the air (Tyndall Effect).
QUESTIONS
1. Explain the term adsorption.
2. Differentiate between adsorption and absorption.
3. Distinguish between the terms adsorption and absorption.
4. How is adsorption classified ?
5. Differentiate between physical adsorption and chemical adsorption. Why are all adsorptions exothermic ?
6. What are the factors on which adsorption of a gas on solid depends.
7. What is an adsorption isobar ? Explain.
8. List the main differences between physical and chemical adsorption and depict graphically the variation in extent of adsorption with temperature at constant pressure for both types of adsorption.
9. How does the adsorption of a gas on a solid depend upon ; (i) gas pressure (ii) temperature ?
10. Mention any four applications of adsorption phenomenon.
11. What is the role of adsorption in heterogeneous catalytic reactions ?
12. Give four examples of heterogeneous catalytic reactions.
13. Differentiate between homogeneous catalysis and heterogeneous catalysis with one example each.
14. What is homogeneous catalytic reaction. Indicate a chemical reaction involving a homogeneous catalyst.
15. What do you mean by ‘activity’ and ‘selectivity’ of catalysts ?
16. What is the shape-selective catalysts ?
17. What are zeolites ? Describe some of their features.
18. Give two applications of zeolites.
19. Write a note on adsorption on solids from solutions.
20. Write a note on structure of zeolites ?
21. Define crystalloid, colloids and colloidal state.
22. How are colloids classified ?
23. What are lyophobic and lyophilic sols. Give one example each. Give their chief distinguishing characteristic.
24. Give four points of difference between lyophobic sols and lyophilic sols. What are multimolecular and macromolecular colloids ?
25. What are associated colloids and in what way do they differ from the other two types of colloids.
26. What are micelles ? Give an example of a micellar system. How do they differ from a normal colloidal solution
27. Discuss the stability of micelles.
28. What are the different methods employed for the preparation of lyophilic and lyophobic colloids.
29. How will you prepare a colloidal solution of arsenious sulphide ?
30. Describe a chemical method each for the preparation of sols of sulphur and platinum in water.
31. Describe a chemical method each for the preparation of sols of gold and ferric hydroxide.
32. Describe the preparation of a colloidal solution of ferric chloride by peptisation.
33. Illustrate with an example, the formation of lyophobic sol.
34. Explain the term peptisation.
35. What happens when a freshly prepared ferric hydroxide is shaken with a little amount of dilute solution of ferric chloride ?
36. Write notes on : (a) dialysis b) electrodialysis c)Ultrafiltration
37. How are colloidal solutions purified ?
37. Write notes on : (a) Tyndall effect b) electrophoresis (c) Brownian movement (d) electrophoresis
39. What is meant by coagulation of sols ? How is it brought about ?
40. What happens when a solution of sodium chloride is added to ferric hydroxide sol ? Explain
41. Write whether ferric hydroxide sol is multimolecular or macromolecular colloid.
42. Explain the phenomenon which takes place when ferric hydroxide sol is added to a gold sol. Write whether the gold sol is macromolecular or multimolecular colloid.
43. Explain the term electroosmosis.
44. Write a note on Hardy-Schulze rule.
45. Electrolytes can cause coagulation as well as precipitation of solution. Comment on this statement.
46. How will you explain the origin of charge on colloidal particles ?
47. Explain why deltas are formed when rivers and sea water meet.
48. Write a note on emulsions
49. What are emulsifying agents.
50. Compare the characteristics of water-in-oil and oil-in-water emulsions.
51. Mention two uses of emulsions.
52. Write a note on cleaning action of soaps.
53. What is a gel ? Give one example.
54. What is the difference between a sol and a gel.
55. Explain the terms emulsification and de-emulsification.
56. Name the ‘state’ of the following substances : milk, collodion, cheese, butter gum.
57. Why is lyophilic colloidal solution more stable than a lyophobic one.
58. What are protective colloids ? Justify the use of gelatin as a protective colloid.
59. Write a note on gold number.
60. Give a few important applications of colloids.
61. Give four uses of emulsions.
62. Describe some features of catalysis by zeolites.
63. What is demulsification ? Name two demulsifiers.
64. Action of soap is due to emulsification and micelle formation. Comment.
65. What do you understand by ‘activation of adsorbent’ ? How is it achieved ?