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Preparation and Purification of Colloids

  • Last Updated : 09 Nov, 2021

Solutions are systems that are homogeneous. When sand is mixed with water, it forms a suspension that settles over time. Colloidal dispersions, or simply colloids, are a large group of systems that exist between the two extremes of suspensions and solutions. A colloid is a heterogeneous system in which one substance is dispersed as very fine particles in a different substance known as the dispersion medium.

Colloids and colloidal systems are necessary for life to exist. They are also extremely useful, if not indispensable, in a variety of commercial and industrial settings. They are present in every cell, the blood, and all bodily fluids. Colloidal science, on the other hand, is still in its infancy, and the number of qualified experts is small in comparison to other fields of science. Furthermore, most colloidal research has focused on industrial processes.

Colloids 

  • The diameter of colloids ranges from 0.001 to 0.1 micron. Since a micron is one-millionth of a metre. Thus, a colloid ranges in size from four-millionths of an inch to four hundred millionths of an inch at the smaller end of the scale. This means that the smallest colloids are roughly 10 times the size of a hydrogen atom.
  • Colloids do not settle, but coarser particles in the dispersion size range do. Dispersed colloids differ from “particles” in molecularly dispersed systems in that they cannot pass through the fine pores of passive membranes. Colloids diffuse slowly due to their size.
  • A colloidal system must have the following three properties in addition to particle size in order to be distinguished from other dispersions:
    • It must be heterogeneous or made up of dissimilar constituents, such as silver and water.
    • It must be multiphasic, that is, solid/liquid, gas/liquid, and so on.
    • In order to be insoluble in the solution or suspension, the particles must be insoluble in the solution or suspension.

Each of these characteristics interacts with the others to give a colloidal system its distinct characteristics. One fascinating aspect is that, even if particle size and concentration vary, as long as the majority of particles are within the proper range, the system will retain its colloidal properties, even if it is not ideal.

Preparation of Colloids

  • Chemical Methods

Colloidal dispersions can be created through chemical reactions that result in the formation of molecules such as double decomposition, oxidation, reduction, or hydrolysis. These molecules then clump together, resulting in the formation of sols.

  • Electrical disintegration or Bredig’s Arc method

This procedure includes both dispersion and condensation. This method can be used to create colloidal sols of metals such as gold, silver, and platinum. An electric arc is struck between electrodes of the metal immersed in the dispersion medium in this method. The intense heat generated vaporizes the metal, which then condenses to form colloidal particles.

  • Peptization

Peptization is the process of converting a precipitate into colloidal sol by shaking it in the presence of a small amount of electrolyte with dispersion medium. 

The peptizing agent is a name given to the electrolyte used for this purpose. In general, this method is used to convert a freshly prepared precipitate into a colloidal sol. During peptization, the precipitate adsorbs one of the electrolyte’s ions on its surface. This results in the formation of positive or negative charges on precipitates, which eventually break up into smaller particles the size of a colloid.

Purification of Colloids

When colloidal solutions are prepared, they typically contain an excess of electrolytes and other soluble impurities. While trace amounts of electrolyte are required for colloidal solution stability, larger amounts coagulate it. As a result, the concentration of these soluble impurities must be kept to a bare minimum. Purification of colloidal solution refers to the process of reducing the number of impurities to a bare minimum. The purification of the colloidal solution is accomplished using the methods listed below.

  • Dialysis

It is the removal of a dissolved substance from a colloidal solution via diffusion through a suitable membrane.

The membrane can be used for dialysis because particles in a true solution can pass through the animal membrane (bladder), parchment paper, or cellophane sheet but not colloidal particles. Dialyzers are the devices used for this purpose. A suitable membrane bag containing the colloidal solution is suspended in a vessel through which freshwater flows continuously. The molecules and ions diffuse through the membrane into the surrounding water, leaving only pure colloidal solution behind.

  • Electro-dialysis

Normally, the dialysis process is quite slow. If the dissolved substance in the impure colloidal solution is only an electrolyte, it can be accelerated by applying an electric field. The procedure is then known as electrodialysis. While pure water is taken outside, the colloidal solution is placed in a bag with a suitable membrane. The compartment is outfitted with electrodes. Ions in the colloidal solution migrate to the electrodes with opposing charges.

  • Ultrafiltration

The process of separating colloidal particles from the solvent and soluble solutes present in the colloidal solution using specially prepared filters that are impermeable to all substances except the colloidal particles is known as ultrafiltration.

Because the pores in ordinary filter paper are too large, colloidal particles can pass through. To stop the flow of colloidal particles, the pores of filter paper can be reduced in size by impregnating them with collodion solution. The standard collodion solution is a 4% nitrocellulose solution in a mixture of alcohol and ether. To make ultra-filter paper, soak the filter paper in a collodion solution, harden it with formaldehyde, and finally dry it. Thus, colloidal particles are separated from the rest of the materials by using ultra-filter paper. Ultrafiltration is a time-consuming process. Pressure or suction is used to accelerate the process. To obtain a pure colloidal solution, the colloidal particles left on the ultra-filter paper are stirred with a fresh dispersion medium (solvent).

Sample Questions

Question 1: Give an example of multimolecular colloids.

Answer:

Multimolecular colloids are formed by the aggregation of relatively small sol particles. As an example, consider Ag sol..

Question 2: Give examples of macromolecular colloids.

Answer:

These are typically biomolecular particles, such as enzymes or proteins, that aggregate to form sols when immersed in a suitable dispersion medium.

Question 3: Why dialysis is not the best method for the purification of colloids?

Answer:

Dialysis is the separation of ionic impurities dissolved in sol by a semi-permeable membrane. All ionic impurities are removed by prolonged dialysis, and sol particles are gathered together, causing neutralised lead to precipitate.

Question 4: Which phenomenon of colloids involves the formation of a delta?

Answer:

The formation of the delta is aided by coagulation or flocculation. The river contains clay particles that coagulate when combined with seawater and electrolyte due to the presence of opposite ions.

Question 5: Name the substance that is used to reduce the pore size of filter paper during ultrafiltration.

Answer:

Because of the large pore size, normal filter paper cannot be used in ultrafiltration. The pores are reduced in size by using collodion solution, which is a 4% cellulose nitrate solution in an alcohol ether mixture.

Question 6: Differentiate between lyophilic and lyophobic sols?

Answer:

There is a strong interaction between the dispersed phase and the dispersion medium in lyophilic sols, which is highly stable and resistant to coagulation. Lyophobic sols are unstable weak unstable Van Der Waals forces of attraction between dispersed phase and dispersion medium due to these forces these are irreversible and ready to coagulate.

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