Colloidal solutions are referred to as mixture that is formed by a mixture of substances that are regularly suspended within a fluid. Colloidal Solutions are heterogeneous systems and have the diameter of dispersed particles lying between \(1\;{\rm{nm}} – 1000\;{\rm{nm}}\). The particles present within the colloidal solution are thus larger and referred to as colloidal particles. Although colloidal particles are quite larger in size, they are not large enough to be seen with the naked eye. However, they can be seen with the help of an ultra-microscope. Colloidal Solutions can easily pass through ordinary filter papers but cannot get through an animal membrane.

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Colloidal Solutions: Purification

The colloidal solutions are rendered unstable by the presence of different impurities. These impurities can be removed by suitable methods, which are discussed in details below.

1. Dialysis
The process of separating a crystalloid from a colloid by diffusion through a parchment membrane is known as dialysis, and the apparatus used to this effect is known as a dialyzer. There are two types of dialyzers.
(a) Graham’s dialyzer: An impure colloidal solution is taken in the dialyzer made up of parchment or cellophane paper. It is suspended in a beaker containing running distilled water. The rate of flow of water is adjusted to keep the level of water constant. The impurities in the form of crystalloid then pass through the parchment member leaving behind the colloidal solution in the pure form inside the dialyzer (parchment bag).
For example, sodium silicate is allowed to react with dilute \({\rm{HCl}}\). Then, the mixture has the Properties of Colloidal Solutions. This Colloidal solution is taken in the parchment bag and immersed in water to get a colloidal solution of silicic acid.
\({\rm{N}}{{\rm{a}}_2}{\rm{Si}}{{\rm{O}}_3} + 2{\rm{HCl}} \to {{\rm{H}}_2}{\rm{Si}}{{\rm{O}}_3} + 2{\rm{NaCl}}\)
In this case, \({\rm{NaCl}}\) and unused \({\rm{HCl}}\) pass through the membrane leaving behind a pure colloidal solution of silicic acid.

(b) Electro-dialyzer: The dialysis process is ordinarily quite slow, but electric field application can hasten it. The impure colloidal solution is taken in a parchment bag suspended in a vessel through which freshwater continuously passes. The electrolyte ions in the bag migrate faster under the influence of an electric field towards the oppositely charged electrodes leaving behind the colloidal solution in the bag. The process is known as electrodialysis.

2. Ultrafiltration
The separation of colloids from crystalloids can also be done by ultrafiltration. Ordinary filter paper has pores larger than \(1\mu \) i.e. \({10^{ – 6}}\;{\rm{m}}\) through which colloidal solution and the impurities can easily pass. The pores of the filter paper can be made smaller by soaking it in a solution of gelatine or collodion and then hardening them by soaking them in formaldehyde.

The pores thus become very small so as not to allow the colloidal particles to pass through them. Only the crystalloids can pass through such filter paper leaving behind the colloidal solution. This type of filter paper is known as ultrafilter, and this process is known as ultrafiltration.

This process is beneficial in removing soluble impurities from the colloidal solution. Cellophane membranes are the best ultrafilters.

Physical Properties of Colloidal Solution

1. Heterogeneous character: Colloidal sols form a heterogeneous mixture consisting of particles of the dispersed phase and dispersion medium. The phenomena of the Tyndall effect, electrophoresis and electro-osmosis confirm the heterogeneity of colloidal systems.
2. Stability: Colloidal sols are quite stable. Only a few colloidal particles of comparatively larger size may settle but very slowly.
3. Filterability: Ordinary filter paper cannot be used to remove the dispersed phase because the size of filter paper’s pores is bigger than the size of colloidal particles, which can easily pass through the pores of the ordinary filter paper. Animal membrane or parchment paper does not allow the colloidal particles to pass through it. It forms the basis of separating the colloid particles from those of the crystalloids in the dialysis process.
4. Visibility: Even with the help of the most powerful microscope, the colloidal particles cannot be seen. However, in recent times U.V rays or cathode rays are used for seeing these particles. The electron microscope may also be used for this purpose.
5. Colour: The colour of the colloidal particles is not always the same as the colour of the substance taken in bulk. For example, colloidal sulphur is colourless, whereas sulphur is yellow. Colloidal gold is red, whereas gold is golden yellow.

Colligative Properties in Colloidal Solution

Colligative properties are those which depend upon the number of solute particles that are present in the given mass of solvent. These properties include relative lowering of vapour pressure, depression inzing point, elevation in boiling point, etc. Colloidal particles are not simple molecules; these are a physical aggregation of molecules. So the number of particles present in the sol is always less than the number of particles present in the true solution. Hence all the colloidal dispersions show a low value of colligative properties.

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

The colloidal sols may be classified as positively charged and negatively charged sols based on the nature of the charge.

(ii) Electrophoresis
Due to a particular type of electrical charge, the colloidal particles 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 the charge present on them. For example, if the colloidal particles carry a positive charge, they move towards the cathode when subjected to an electric field and vice versa. This phenomenon is called electrophoresis and may be defined as follows.

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

The phenomena of electrophoresis indicate that the colloidal particles carry a particular type of charge. Thus, the property can be used to find the nature of the charge carried by colloidal particles in a colloidal dispersion.

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

(iii) Electro Osmosis
On placing a colloidal solution under the influence of an electric field, the particles of the dispersion medium (electrically charged) move towards the oppositely charged electrode, provided the colloidal particles are not allowed to proceed through the semipermeable membrane, as shown in the figure. This phenomenon is called electro-osmosis.

Electro-osmosis may be defined as a phenomenon in which the molecules of the dispersion medium are allowed to move under the influence of an electric field. In contrast, colloidal particles are not allowed to move.

(iv) Coagulation and 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 cannot combine to form larger particles. However, if the charge on colloidal particles is destroyed, they can come nearer and grow in size. When the particles become sufficiently large, they get precipitated. This phenomenon is termed coagulation and flocculation. The addition of an electrolyte with an opposite charge can achieve the coagulation of a colloidal solution.

It is to be noted that a small amount of electrolyte is necessary for the stability of sol because the ions of the electrolyte get adsorbed on colloidal particles and impart some charge. However, when an electrolyte is added substantially, the electrolyte’s oppositely charged ions 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 the addition of an electrolyte is called coagulation or flocculation.

Hardy – Schulze Rule

The coagulation capacity of an electrolyte depends upon the valency of electrolyte responsible for causing coagulation.  The electrolyte responsible for causing coagulation is the one that carries a charge opposite to that present on colloidal particles. Thus, for example, a positively charged sol gets coagulated by the negatively charged ions of the added electrolyte. From a study of the coagulation behaviour 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 the valency of the oppositely charged ion of the electrolyte added to a colloidal solution, the faster is the coagulation of the colloidal solution.

Thus, the higher the charge on the oppositely charged electrolyte greater is its coagulation power. For example, the coagulation power of different cations for coagulating a negatively charged sol of \({\rm{A}}{{\rm{s}}_2}\;{{\rm{S}}_3}\) follows the order
\({\rm{A}}{{\rm{l}}^{3 + }} > {\rm{B}}{{\rm{a}}^{2 + }} > {\rm{N}}{{\rm{a}}^ + }\)

Similarly, for the coagulation of a positively charged sol such as \({\rm{Fe}}{({\rm{OH}})_3}\), the coagulating power of different anions follows the order.
\({\left[ {{\rm{Fe}}{{({\rm{CN}})}_6}} \right]^{4 – }} > {\rm{PO}}_4^{3 – } > {\rm{SO}}_4^{2 – } > {\rm{C}}{{\rm{l}}^ – }\)

Flocculating 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 millimole 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 \({\rm{A}}{{\rm{s}}_2}\;{{\rm{S}}_3}\) sol and positively charged \({\rm{Fe}}{({\rm{OH}})_3}\) sol is given below.

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

Coagulating power \(\alpha \frac{1}{{{\rm{ Flocculation Value }}}}\)

Summary

Colloids are mixtures in which one or more substances are dispersed as relatively large solid particles or liquid droplets throughout a solid, liquid, or gaseous medium. The particles of a colloid remain dispersed and do not settle due to gravity, and they are often electrically charged. 

In this article, we learnt about the purification of colloids, properties of colloidal solutions and electrical properties of colloidal solutions.

Frequently Asked Questions on Properties of Colloidal Solutions

Frequently asked questions related to properties of colloidal solutions are listed as follows:

Q.1. What is a colloidal solution?
Ans: The type of solution in which the size of the solute particle is in the range of \(1\) to \(1000\;{\rm{nm}}\) is called a colloid. A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase or colloidal particles) in another substance called dispersion medium particles.

Q.2. What is the composition of the colloidal solution?
Ans: The two components of a colloidal solution are dispersed phase and dispersing medium.

Q.3. What are the different methods of purification of colloids?
Ans: Colloidal contains several electrolytic impurities. The following method is used to purify colloids:
1. Dialysis (by using semipermeable membrane)
2. Ultra-filtration (by using ultra-fine quality filter papers)
3. Ultra-centrifugation.

Q.4. Write any three characteristics of the colloidal solution?
Ans: The characteristics of colloidal solutions are:
1. They are large enough to scatter a beam of light and make the path visible.
2. Size is too small to be seen by the naked eye.
3. Colloids are a heterogeneous mixture consisting of two phases: dispersed phase and dispersion phase.

Q.5. What are the important physical properties of colloids?
Ans:  The important physical properties of Colloids are as follows:
1. Heterogeneous character: Colloidal sols form a heterogeneous mixture consisting of the dispersed phase and dispersion medium. The phenomena of the Tyndall effect, electrophoresis and electro-osmosis confirm the heterogeneity of colloidal systems.
2. Stability: Colloidal sols are quite stable. Only a few colloidal particles of comparatively larger size may settle but very slowly.
3. Filterability: Ordinary filter paper cannot be used to remove the dispersed phase because the size of filter paper’s pores is bigger than the size of colloidal particles, which can easily pass through the pores of the ordinary filter paper. Animal membrane or parchment paper does not allow the colloidal particles to pass through it. Instead, it forms the basis of separating the colloid particles from those of the crystalloids called dialysis.
4. Visibility: Even with the help of the most powerful microscope, the colloidal particles cannot be seen. However, in recent times U.V rays or cathode rays are used for seeing these particles.
5. Colour: The colour of the colloidal particles is not always the same as the colour of the substance taken in bulk. For example, colloidal sulphur is colourless, whereas sulphur is yellow.

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