Electrical Properties of Colloidal Solutions

Some important electrical properties of colloidal solutions are as follows: 

(i) Presence of electrical charge on colloidal particles and stability of sols:One of the most important properties of colloidal solutions is that colloidal particles posses a definite type of electrical charge. In a particular colloidal solution, all the colloidal particles carry the same type of charge, while the dispersion medium has an equal but opposite charge. Thus, the charge on colloidal particles is balanced by that of the dispersion medium and the colloidal solution as a whole is electrically neutral. For example, in a ferric hydroxide sol, the colloidal ferric hydroxide particles are positively charged, while the dispersion medium carries an equal and opposite negative charge. 

The stability of a colloidal solution is mainly due to the presence a particular type of charge on all the colloidal present in it. Due to the presence of similar and equal charges, the colloidal particles repel one another and are thus unable to combine together to form larger particles. This keeps them dispersed in the medium and the colloidal remains stable. This is why sol particles do not settle down even on standing for a long time. 

Based on the nature of charge, the colloidal sols may be classified as positively charged and negatively charged sols. Some common examples of these sols are given below. 

Positively charged sols: 

Metallic hydroxide sols e.g., Fe(OH)3, Al(OH)3, Cr(OH)3, etc., TiO2 sol, haemoglobin, sols of basic dyes such as methylene blue etc. 

Negatively charged sols: 

Metal sols e.g., Au, Ag, Cu, Pt etc. sols, metal sulphide sols e.g., As2S3, CdS etc. sols; starch sol, sols of acid dyes such as Congo red etc. 

Origin of charge on colloidal particles: 

There are several views regarding the origin of charge on colloidal particles. According to these views, colloidal particles acquire charge due to the following reasons. 

(a) Due to dissociation of the adsorbed molecular electrolytes: Colloidal particles have a strong tendency to adsorb reactant or product molecules. The molecules thus adsorbed on the surface of colloidal particles may undergo dissociation/ionization and may impart charge to them. For example, during the preparation of sulphide sols (e.g., As2S3 sol), H2S molecules get adsorbed on colloidal particles. H2S molecules thus adsorbed undergo ionization and release H+ions into the medium. Consequently, colloidal particles are left with negative charge.

(b) Due to the dissociation of molecules forming colloidal aggregates: The molecules responsible for the formation of aggregates of colloidal dimensions may themselves undergo dissociation/ionisation resulting in the development of charge on the colloidal particles formed by their aggregation. For example, the soap molecules (RCOONa) dissociate to give RCOO- and Na+ ions. RCOO- ions aggregate together to form micelles which carry negative charge as explained earlier. 

c) Due to preferential adsorption of ions from solutions: The colloidal particles have a tendency to preferentially adsorb a particular type of ions from the solution. A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice. This preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles. 

For example, when a ferric hydroxide sol is prepared by the hydrolysis of ferric chloride in warm water, the colloidal particles of Fe(OH)3 formed have a tendency to adsorb preferentially the Fe3+ ions present in the solution. This is because Fe3+ ions are common to the lattice of Fe(OH)3 particle. The Fe3+ ions thus adsorbed impart positive charge to the colloidal particles present in the sol. 


           Fe(OH)3     +           Fe3+           -->          Fe(OH)3    :      Fe3+ 
         (colloidal            (ions common         preferential adsorption of Fe3+ ions
          particle)          to the lattice of           (colloidal particle acquires 
                               colloidal particle)                positive charge)


Similarly, during the preparation of AgCl sol using excess of KCl solution, the Cl ions are preferentially adsorbed and the colloidal particles acquire negative charge. However, if an excess of AgNO3 is used, Ag+ ions get preferentially adsorbed and the colloidal particles acquire positive charge. 

  AgCI          +        CI-                 -->                  AgCI : CI- 
(colloidal)            (Chloride ions present              Preferential adsorption of CI- ions 
particle)           in excess in the solution)       (Collodial particle acquires negative charge)
 

  AgCI          +          Ag+                -->                AgCI : Ag+ 
(colloidal)             (Silver ions present in          Preferential adsorption of Ag+ions 
particle)              excess in the solution)     (Collodial particle acquires positive charge)



(ii) Electrophoresis: Due to the presence of a particular type of electrical charge, the colloidal particles present in a colloidal dispersion move towards a particular electrode under the influence of an electric field. The direction of movement of the colloidal particles is decided by the nature of charge present on them. If the colloidal particles carry positive charge, they move towards cathode when subjected to an electric field and vice versa. The phenomenon is called electrophoresis and may be defined as follows. 

                                  before-electrophoresis-and-after-electrophoresis 
                      
                   (a) Before electrophoresis (b) After electrophoresis 

The movement of colloidal particles towards a particular electrode under the influence of an electric field is called electrophoresis. 

The phenomenon of electrophoresis clearly indicates that the colloidal particles carry a particular type of charge. The property can be used to find the nature of charge carried by colloidal particles in a colloidal dispersion. 

Electrophoresis is an important phenomenon and finds several applications in industry. 

(iii) Electro-osmosis: When the movement of colloidal particles under the influence of the applied electric field is checked with the help of a suitable membrane (semi permeable membrane), the dispersion medium moves in a direction opposite to the direction in which the colloidal particles would have otherwise moved. This phenomenon is called electro-osmosis and may be defined as follows. 

The movement of dispersion medium under the influence of an electric field in the situation when the movement of colloidal particles is prevented with the help of a suitable membrane is called electro-osmosis. 

The electro-osmosis can be demonstrated with the help of the apparatus shown in figure. The colloidal solution is placed between two partitions made by semi permeable membranes. The outer compartments consisting of platinum electrodes and side tubes are filled with water. On passing electric current, water level begins to rise in one of the side tubs and falls in the other. The phenomenon can be explained as follows. We have already seen that the colloidal particles and dispersion medium carry charges which are equal but opposite in nature. Under the influence of an electric field, both have a tendency to move towards the oppositely charged electrodes. Semi permeable membranes do not allow the passage of colloidal particles. However, dispersion medium can pass through them. Therefore during electro-osmosis, colloidal particles are checked and it is the dispersion medium that moves towards the oppositely charged electrode.



               electro-osmosis 
 
                                                   Electro-osmosis

(iv) Coagulation or flocculation: The stability of a sol is due to the charge present on the colloidal particles. Due to similar charges, colloidal particles repel one another and are unable to combine together to form larger particles. However, if the charge on colloidal particles is destroyed, they are free to come nearer and grow in size. When the particles become sufficiently large, they get precipitated. This phenomenon is termed as coagulation or flocculation. The coagulation of colloidal solution can be achieved by the addition of an electrolyte. It is to be noted that a small amount of electrolyte is necessary for the stability of a sol because the ions of the electrolyte get adsorbed on colloidal particles and impart them some charge. However, when an electrolyte is added in substantial amount the positively charged ions of the electrolyte neutralize the charge on colloidal particles and compel the sol to get coagulated. Coagulation may be defined as follows.

The phenomenon involving the precipitation of a colloidal solution on addition of an electrolyte is called coagulation or flocculation. 

Hardy-Schulze rule:
 The coagulation capacity of an electrolyte depends upon the valence of ion responsible for causing coagulation. As we have seen above, the ion responsible for causing coagulation is the one which carries charge opposite to that present on colloidal particles. For example, a positively charged sol gets coagulated by the negatively charged ions of the added electrolyte. From a study of the coagulation behavior of various electrolytes towards a particular sol, Hardy and Schulze suggested a general rule known as Hardy-Schulze rule. The rule can be stated as follows. 

The greater is the valence of the oppositely charged ion of the electrolyte added to a colloidal solution, the faster is the coagulation of the colloidal solution. 

Thus, higher the charge on oppositely charged ion greater is its coagulating power. For example, the coagulation power of different cations for coagulating a negatively charged sol of As2S3 follows the order. 

                                       Al3+ > Ba2+ > Na+ 

Similarly, for the coagulation of a positively charged sol such as Fe(OH)3, the coagulating power of different anions follows the order. 

                     [Fe(CN)6]4-   >   PO43-  >  SO42-    >   Cl- 


Flocculation value:
 The coagulating power of an electrolyte is usually expressed in terms of its flocculation value which may be defined as follows. 

The minimum concentration (in millimoles per litre) of an electrolyte required to cause the coagulation of a sol is called the flocculation value of the electrolyte. 

The flocculation values (in millimoles per litre) for the coagulation of negatively charged As2S3 sol and positively charged Fe(OH)3 sol are given in the Table below.

Table: Flocculation Values of Some Common Electrolytes 

 

For Negatively charged As2S3 Sol

For Positively charged Fe(OH)3 Sol

Electrolyte

Flocculating ion

Flocculation value (millimoles/litre)

Electrolyte

Flocculating ion

Flocculation value (millimoles/litre)

NaCl

Na+

52

KBr

Br-

138

KCl

K+

50

HCl

Cl-

132

HCl

H+

30

KNO3

NO-3

132

MgCl2

Mg2+

0.72

K2CrO4

CrO42-

0.315

BaCl2

Ba2+

0.69

K2SO4

SO42-

0.210

ZnCl2

Zn2+

0.68

K2C2O4

C2O42-

0.238

AlCl3

Al3+

0.093

K3[Fe(CN)6]

Fe(CN)6]3-

0.096


 

It is to be noted that a smaller flocculation value indicates the greater coagulating power of the electrolyte. Thus. 

                 Coagulating power µ 1/(Flocculation value) 

Some other methods for causing coagulation: The most commonly used method for causing coagulation in a colloidal solution is the addition of an electrolyte as described above. However, the coagulation of colloidal solution can also be achieved by any of the following methods. 

(a) By electrophoresis: In electrophoresis, the charged colloidal particles migrate to the oppositely charged electrode and get discharged. This results in the coagulation of the colloidal solution. 

(b) By mixing two oppositely sols: When two sols carrying opposite charges are mixed together in suitable proportions, the colloidal particles of one sol neutralize the charge present on the particles of the other sol and both get coagulated. 

(c) By persistent dialysis: We have already seen that a small amount of electrolyte is essential to make a sol stable. When a sol is subjected to persistent dialysis, the traces of electrolyte also pass out through the membrane. In the absence of electrolyte, sol becomes unstable and gets coagulated.

 

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