Purification of Colloidal Solutions

Colloidal solutions when prepared; generally contain excessive amount of electrolytes and some other soluble impurities. While the presence of traces of electrolyte is essential for the stability of the colloidal solution, larger quantities coagulate it. It is, therefore, necessary to reduce the concentration of these soluble impurities to a requisite minimum.

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 of diffusionthrough a suitable membrane.

The principle is based upon the fact that colloidal particles cannot pass through animal membrane (bladder) or parchment or cellophane membrane while the ions of the electrolyte can pass through it.

The apparatus used for this purpose is called dialyser.

A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The impurities slowly diffused out of the bag leaving behind pure colloidal solution.

The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag.

Dialysis can be used for removing HCl from the ferric hydroxide sol.

Electro-dialysis:

Ordinarily, 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.

The colloidal solution is placed in a bag of suitable membrane while pure water is taken outside.

Electrodes are fitted in the compartment. The ions present in the colloidal solution migrate out to the oppositely charged electrodes.

Ultra–filtration: 

Sol particles directly pass through ordinary filter paper because their pores are larger (more than 1μ) than the size of sol particles (less than 200 μ).

If the pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin of colloidion and subsequently hardened by soaking in formaldehyde, the treated filter paper may retain colloidal particles and allow the true solution particles to escape. Such filter paper is known as ultra-filter and the process of separating colloids by using ultra – filters is known as ultra-filtration.

Ultra–centrifugation: 

The sol particles are prevented from setting out under the action of gravity by kinetic impacts of the molecules of the medium.

The setting force can be enhanced by using high speed centrifugal machines having 15,000 or more revolutions per minute. Such machines are known as ultra–centrifuges.

Coagulation or Flocculation or Precipitation

The process of settling of colloidal particles is called coagulation or precipitation of the sol.

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 to form aggregates (or coagulate) and settle down under the force of gravity.

Coagulation of lyophobic sols

1. By electrophoresis:

The colloidal particles move towards oppositely charged electrodes, get discharged and precipitated.

2. By mixing two oppositely charged sols:

When oppositely charged sols are mixed in almost equal proportions, their charges are neutralised. Both sols may be partially or completely precipitated. For example mixing of hydrated ferric oxide (+ve sol) and arsenious sulphide (–ve sol) bring them in the precipitated forms. This type of coagulation is called mutual coagulation or meteral coagulation.

3. By boiling:

When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the molecules of dispersion medium. This reduces the charge on the particles and ultimately they settle down to form a precipitate.

4. By persistent dialysis:

On prolonged dialysis, the traces of the electrolyte present in the sol are removed almost completely and the colloids become unstable.

5. By addition of electrolytes:

When excess of an electrolyte is added, the colloidal particles are precipitated. The reason is that colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralization leading to their coagulation.

The ion responsible for neutralisation of charge on the particles is called the coagulating ion or flocculating ion. A negative ion causes the precipitation of positively charged sol and vice versa.

Hardy schulze rule:

The coagulation capacity of different electrolytes is different. It depends upon the valency of the flocculating ion. According to Hardy Schulze rule,

“greater the valency of the active ion or flocculating ion, greater will be its coagulating power”

For example to coagulate negative sol, the coagulation power of cations is in the order Al³⁺ > Mg²⁺ > Na⁺.

Similarly, to coagulate a positive sol such as Fe(OH)₃, the coagulating power of different anions is in the order [Fe(CN)₆]^4- > PO₄^3- > SO₄^2- > Cl^-

Coagulation or flocculation value

“The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a sol is known as flocculation value.” or “The number of millimoles of an electrolyte required to bring about the coagulation of one litre of a colloidal solution is called its flocculation value.”

Coagulation value or flocculating value ∝ 1coagulating power Coagulation value or flocculating value ∝ 1coagulating power.