Purification of Colloidal Solutions: The capacity of dissolved compounds to permeate into water over a porous membrane was examined by Thomas Graham \((1861)\). Crystalline compounds including sugar, urea, and sodium chloride flowed through the membrane, whereas glue, gelatin, and gum arabic did not. He called the former crystalloids and the later colloids (kolla \( = \) glue and eidos \( = \) similar in Greek).

The difference in behaviour between ‘crystalloids’ and ‘colloids,’ according to Graham, is attributable to particle size. It was later discovered that any substance, regardless of its nature, may be transformed into a colloid by subdividing it into colloidal-sized particles. After preparation, the resultant sol usually contains significant concentrations of electrolytes in addition to colloidal particles. These electrolytes must be eliminated in order to obtain pure sol.

What are Colloids?

The solute particles are disseminated in the solvent as single molecules or ions in a true solution, such as sugar or salt in water. As a result, the scattered particles’ diameters range from \({\rm{1}}\mathop {\rm{A}}\limits^{\rm{o}} \) to \({\rm{10}}\mathop {\rm{A}}\limits^{\rm{o}} \). The scattered particles in a suspension, such as sand mixed into water, are an aggregation of millions of molecules. The diameter of these particles is of the order \(2000\mathop {\rm{A}}\limits^{\rm{o}} \) or more.

Colloidal solutions, also known as colloidal dispersions, are a hybrid of real solutions and suspensions. To put it another way, the diameter of the dispersed particles in a colloidal dispersion is larger than the diameter of the solute particles in a real solution but smaller than that of a suspension. A colloidal solution, colloidal dispersion, or simply a colloid is a system in which the diameter of the particles of a substance dispersed in a solvent ranges from roughly \(10\mathop {\rm{A}}\limits^{\rm{o}} \) to \(2000\mathop {\rm{A}}\limits^{\rm{o}} \). The colloidal state is defined as a substance with particle sizes in the colloidal range.

The form of the colloidal particles does not have to be corpuscular. In fact, these can take the form of rods, discs, thin films, or lengthy filaments. The diameter of matter in the form of corpuscles is a measure of particle size. However, for a substance to be classified as colloidal, one of the dimensions (length, breadth, and thickness) must be in the colloidal range. In a larger sense, colloidal dispersion is defined as a system containing at least one dimension (length, breadth, or thickness) of dispersed particles in the range of \(10\mathop {\rm{A}}\limits^{\rm{o}} \) to \(2000\mathop {\rm{A}}\limits^{\rm{o}} \).

Types of Colloidal Systems

A colloidal system is made up of two phases, as we’ve seen. The dispersed phase refers to the substance that is scattered as colloidal particles. The Dispersion medium is the second continuous phase in which colloidal particles are dispersed. For example, in a colloidal solution of copper in water, the dispersed phase is copper particles, while the dispersion medium is water. A colloidal system is made up of a dispersed phase and a dispersion medium, as previously indicated. There are eight different types of colloidal systems since the dispersed phase, or the dispersion medium might be a gas, liquid, or solid. Because the two gases would produce a uniform molecular mixture, colloidal dispersion of one gas in another is not possible. The various types of colloidal systems are listed in Table:

Preparation of Sols

Simply warm the solid with the liquid dispersion medium, such as starch and water, to make lyophilic sols. Lyophobic sols, on the other hand, require particular preparation. These techniques are divided into two groups:

(a) Dispersion methods, which break down bigger macro-sized particles to colloidal size.
(b) Aggregation methods, which include aggregating single ions or molecules to form colloidal-size particles.

Purification of Sols

This purification of sols can be accomplished by three methods:

(a) Dialysis
(b) Electrodialysis
(c) Ultrafiltration

Dialysis

Animal membranes (bladder), as well as parchment paper and cellophane sheet membranes, feature extremely small pores. Ions (or small molecules) can travel through these pores, but not huge colloidal particles. The ions diffuse across the membrane when a sol containing dissolved ions (electrolyte) or molecules is placed in a bag of permeable membrane dipped in pure water. The electrolyte concentration outside the membrane tends to be zero when using a constant flow of fresh water. As a result, ion diffusion into pure water remains constant. Almost all of the electrolytes in the sol can be easily extracted in this manner.

Dialysis is the process of removing ions (or molecules) from a solution by diffusion via a permeable membrane. Dialysis equipment is referred to as a Dialyser.

Example. Hydrochloric acid will be combined with a ferric hydroxide sol (red) generated by hydrolysis of ferric chloride. The outside water will produce a white precipitate with silver nitrate if the impure sol is placed in the dialysis bag for some time. After a long time, it will be discovered that nearly all the hydrochloric acid has been eliminated, leaving only pure red sol in the dialyser bag.

Electrodialysis

Dialysis is carried out under the influence of an electric field in this process. Between the metal screens that support the membranes, the potential is applied. Ion migration to the opposite electrode is accelerated as a result of this. As a result, dialysis is significantly hastened. Electrodialysis is clearly not designed to remove non-electrolyte pollutants like sugar and urea.

Ultrafiltration

Sols flow through regular filter paper, which has holes that are too large to hold colloidal particles. The pore size is greatly lowered if the filter paper is treated with collodion or regenerated cellulose such as cellophane. An ultrafilter is a type of customised filter paper. Ultrafiltration is the process of separating the sol particles from the liquid medium and electrolytes by filtration through an ultrafilter. Ultrafiltration is a time-consuming procedure. To accelerate it, gas pressure (or suction) must be added. Slime is formed when colloidal particles are left on the ultrafilter. To recover the pure sol, mix the slime into a fresh medium. By using graded ultrafilters, the technique of ultrafiltration can be employed to separate sol particles of different sizes.

Summary

1. A colloidal solution, colloidal dispersion, or simply a colloid is a system in which the diameter of the particles of a substance dispersed in a solvent ranges from roughly \(10\mathop {\rm{A}}\limits^{\rm{o}} \) to \(2000\mathop {\rm{A}}\limits^{\rm{o}} \). The colloidal state is defined as a substance with particle sizes in the colloidal range.
2. A colloidal system is made up of two phases, as we’ve seen. The dispersed phase refers to the substance that is scattered as colloidal particles. The Dispersion medium is the second continuous phase in which colloidal particles are dispersed.
3. Dialysis is the process of removing ions (or molecules) from a solution by diffusion via a permeable membrane. Dialysis equipment is referred to as a Dialyser.
4. Dialysis is carried out under the influence of an electric field in electrodialysis. Between the metal screens that support the membranes, the potential is applied. Ion migration to the opposite electrode is accelerated as a result of this. As a result, dialysis is significantly hastened.
5. Ultrafiltration is the process of separating the sol particles from the liquid medium and electrolytes by filtration through an ultrafilter. Ultrafiltration is a time-consuming procedure.

FAQs on Purification of Colloidal Solutions

Q.1. What are colloids?
Ans: Colloidal solutions, also known as colloidal dispersions, are a hybrid of real solutions and suspensions. To put it another way, the diameter of the dispersed particles in a colloidal dispersion is larger than the diameter of the solute particles in a real solution but smaller than that of a suspension. A colloidal solution, colloidal dispersion, or simply a colloid is a system in which the diameter of the particles of a substance dispersed in a solvent ranges from roughly \(10\mathop {\rm{A}}\limits^{\rm{o}} \) to \(2000\mathop {\rm{A}}\limits^{\rm{o}} \).

Q.2. How are colloids prepared?
Ans: Simply warm the solid with the liquid dispersion medium, such as starch and water, to make lyophilic sols. Lyophobic sols, on the other hand, require particular preparation. These techniques are divided into two groups:

(a) Dispersion Methods, which break down bigger macro-sized particles to colloidal size.
(b) Aggregation Methods, which include aggregating single ions or molecules to form colloidal-size particles.

Q.3. Explain the dialysis method for the purification of colloids.
Ans: Dialysis is the process of removing ions (or molecules) from a solution by diffusion via a permeable membrane. Dialysis equipment is referred to as a Dialyser. Hydrochloric acid will be combined with a ferric hydroxide sol (red) generated by hydrolysis of ferric chloride. The outside water will produce a white precipitate with silver nitrate if the impure sol is placed in the dialysis bag for some time. After a long time, it will be discovered that nearly all the hydrochloric acid has been eliminated, leaving only pure red sol in the dialyser bag.

Q.4. What is electrodialysis?
Ans: Dialysis is carried out under the influence of an electric field in this process. Between the metal screens that support the membranes, the potential is applied. Ion migration to the opposite electrode is accelerated as a result of this. As a result, dialysis is significantly hastened. Electrodialysis is clearly not designed to remove non-electrolyte pollutants like sugar and urea.

Q.5. What is the ultrafiltration method of purification of colloids?
Ans: Ultrafiltration is the process of separating the sol particles from the liquid medium and electrolytes by filtration through an ultrafilter. Ultrafiltration is a time-consuming procedure. To accelerate it, gas pressure (or suction) must be added. Slime is formed when colloidal particles are left on the ultrafilter. To recover the pure sol, mix the slime into a fresh medium. By using graded ultrafilters, the technique of ultrafiltration can be employed to separate sol particles of different sizes.

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