Preparation & Purification Of Colloidal Solutions - Practice Questions & MCQ

Electrical disintegration or Bredig’s Arc method

This process involves dispersion as well as condensation. Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared by this method. In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium as shown in figure. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.

The process which involves the conversion of fresh precipitates into colloidal solution is called peptization and the electrolyte added is called peptizing agent or dispersing agent. For example, in a freshly precipitated solution of ferric hydroxide when a small amount of electrolyte ferric chloride is added changes into a colloid.

Some examples involved:

• Fresh precipitates of CdS, HgS is peptized by H₂S.
• Fresh precipitates of Stannic acid is peptized by HCl or NH₃.

Cause of Peptization: On adding the electrolyte to the freshly precipitated substance, the particles of the precipitate preferentially absorb, a particular type of ions of the electrolyte and get dispersed due to electrostatic repulsion. This produces particles of the colloid.

Colloidal dispersion can be prepared by chemical reaction leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to formation of sols.

The properties exhibited by colloidal solutions are described below:

• Colligative Properties: Colloidal particles being bigger aggregates, the number of particles in a colloidal solution is comparatively small as compared to a true solution. Hence, the value of colligative properties (osmotic pressure, lowering in vapour, pressure, depression in freezing point, etc.) are of small order compared to true solution of same concentration.
• Tyndall effect: If is a homogeneous solution placed in dark is observed in the direction of light, it appears clear and if it is observed from a direction at right angle to the direction of light beam, it appears perfectly dark.
  
• Colour: The colour of colloidal solution depends on the wavelength of light scattered by the dispersed particles. The wavelength of light, in turn, depends on the size and nature of the particles.
• Brownian movement: When colloidal solutions are viewed under a powerful ultramicroscope, the colloidal particles appear to be in a state of continuous zig-zag motion all over the field of view. This is known as Brownian movement.
  
• Electrophoresis: The existance of charge on colloidal particles is confirmed by electrophoresis experiment. The movement of colloidal particles under an applied electric potential is called electrophoresis.
• Coagulation or precipitation: The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If somehow the charge is removed the particles will come nearer to each other and settle down under gravity.

The existence of charge on colloidal particles is confirmed by electrophoresis experiment. When electric potential is applied across two platinum electrodes dipping in a colloidal solution, the colloidal particles move towards one or the other electrode. The movement of colloidal particles under an applied electric potential is called electrophoresis. Positively charged particles move towards the cathode while negatively charged particles move towards the anode.
When electrophoresis, i.e., movement of particles is prevented by some suitable means, it is observed that the dispersion medium begins to move in an electric field. This phenomenon is termed electroosmosis.

The colloidal sols are stable due to the presence of electric charges on the colloidal particles. Because of the electrical repulsion, the particles do not come close to one another to form precipitates.

• The removal of charge by any means will lead to the aggregation of particles for precipitation is known as coagulation and the minimum amount of an electrolyte required to cause precipitation of one liter of a colloidal solution is called coagulation value or flocculation value of the electrolyte for the sol.
• The reciprocal of coagulation value is regarded as the coagulating power.

Colloidal particles always carry an electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either positive or negative.

The presence of equal and similar charges on colloidal particles is largely responsible in providing stability to the colloidal solution, because the repulsive forces between charged particles having same charge prevent them from coagulating or aggregating when they come closer to one another.
The charge on the sol particles is due to one or more reasons, such as, due to electron capture by sol particles during electrodispersion of metals, due to preferential adsorption of ions from solution and/or due to formulation of electrical double layer. 
The sol particles acquire positive or negative charge by preferential adsorption of positive or negative ions. When two or more ions are present in the dispersion medium, preferential adsorption of the ion common to the colloidal particle usually takes place.

The combination of the two layers of opposite charges around the colloidal particle is called a Helmholtz electrical double layer. According to modern views, the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed a diffused layer. The formation of double layer. Since the separation of charge is a seat of potential, the charges of opposite signs on the fixed and diffused parts of the double layer result in a difference in potential between these layers in the same manner as the potential difference is developed in a capacitor. This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.
If two particles of insoluble material (precipitate) do not have double layers they can come close enough and attractive van der Waals forces pull them together. When particles possess a double layer, the overall effect is that particles repel each other at large distances of separation. This repulsion prevents its close approach. They remain dispersed and the colloid is stabilised.
The addition of more electrolytes to sol suppresses the diffused double layer and reduces the zeta potential. This decreases the electrostatic repulsion between particles to a large extent and colloid precipitates. That is why colloid is particularly sensitive to oppositely charged ions.

According to it, "Higher the valency of the active electrolyte ion, the more is its power to precipitate the sol."

• The coagulation power of cations for negatively charged colloids is:
  Si⁺⁴ > Al³⁺ > Mg²⁺ > Na⁺
• The coagulation power of anion for positively charged sol is:
  [Fe(CN)₆]^4- > PO₄^3- > SO₄^2- > Cl^-.

Protection involves the protection of lyophobic colloid from Coagulation by using a lyophilic colloid which is called protective colloid and the protection power of lyophilic colloids can be expressed in terms of Gold number which is explained as "Gold number is the number of milligrams of the protective colloid needed to prevent the coagulation of a standard 10 ml gold sol when 1ml of 10% solution of sodium chloride is added to it."

For example:

• Gelatin is added in the preparation of ice cream to protect colloid particles from being coagulated.
• Protargols and argyrols are used in eye drops.

NOTE: Smaller the gold number greater will be the protection power.

The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution. The purification of colloidal solution is carried out by the following mehods:

• Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means diffusion through a suitable membrane.
  
• Electrodialysis: The process of dialysis is quite slow. It can be made faster by applying an electric field if the dissolved substance in the impure colloidal solution is only an electrolyte. The process is then named electrodialysis.
  
• Ultrafiltration: Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles. It is a slow process.