Preparation of colloidal solutions
(1) Preparation of lyophilic sols : The colloidal solutions of lyophilic colloids like starch, glue, gelatin etc., can be readily prepared by dissolving these substances in water either in cold or on warming. Solutions of colloidal electrolytes such as soaps and dye stuffs can also be prepared similarly.
(2) Preparation of lyophobic sols : To get a substance in colloidal form either the substance in bulk is broken down into fine particles of colloidal dimension (1Å to 103 Å ) or increasing the size of molecular particles as to form larger aggregates. In some cases, a third substance as to form larger aggregates. In some cases, a third substance is usually added to increase the stability of the sol. These substances are called stabilizers. Thus, there are two ways by which the lyophobic sols can be prepared :
(i) Dispersion methods : By splitting coarse aggregates of a substance into colloidal size.
(ii) Condensation methods : By aggregating very small particles (atoms, ions or molecules) into colloidal size.
1. Mechanical dispersion
1. Exchange of solvents
2. Electro-dispersion state
2. Change of physical
3. Ultrasonic dispersion
3. Chemical methods:
(i) Double decomposition
(1) Mechanical dispersion : Solid material is first finely ground by usual methods. It is then mixed with dispersion medium which gives a coarse suspension. The suspension is now introduced into the colloid mill. The simplest form of colloid mill consists of two metal discs held at a small distance apart from one another and capable of revolving at a very high speed (about 7000 revolutions per minute) in opposite directions. The particles are ground down to colloidal size and are then dispersed in the liquid . A stabilizer is often added to stabilize the colloidal solution. Colloidal graphite (a lubricant) and printing ink,are made by this method. Tannin is used as a stabilizer in the preparation of colloidal graphite and gum arabic in lampblack colloidal solution (Indian ink).
(2) Electro-dispersion (Bredig's arc method) : This method is suitable for the preparation of colloidal solutions of metals like gold, silver, platinum, etc. An arc is struck between the metal electrodes under the surface of water containing some stabilizing agents such as a track of KOH. The water is cooled by immersing the container in an ice bath. The intense heat of the arc vaporises some of the metal which condenses under cold water.
Note : (1) This method is not suitable when the dispersion medium is an organic liquid as considerable charring occurs.
(2) This method comprises both dispersion and condensation.
(3) Ultrasonic dispersion : The sound waves of high frequency are usually called ultrasonic waves. These waves can be produced when quartz crystal discs are connected with a high frequency generator. The application of ultrasonic waves for the preparation of colloidal solutions was first introduced by wood and Loom is, in 1927. Various substances like oils, mercury, sulphur, sulphides and oxides of metals can be dispersed into colloidal state very easily with the help of ultrasonic waves.
(4) Peptization : The dispersion of a freshly precipitated material into colloidal solution by the action of an electrolyte in solution is termed peptization. The electrolyte used is called a peptizing agent.
A few examples of sols obtained by peptization are :
(i) Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride solution at once forms a dark reddish brown solution. Ferric chloride acts as a peptizing agent.
(ii) Freshly prepared stannic oxide on treatment with a small amount of dilute hydrochloric acid forms a stable colloidal solution of stannic oxide.
(iii) Freshly precipitated silver chloride can be converted into a colloidal solution by a small amount of hydrochloric acid.
(iv) Cadmium sulphide can be peptized with the help of hydrogen sulphide.
The process of peptization thus involves the adsorption of suitable ions (supplied by the electrolyte added -- particularly a common ion) and electrically charged particles then split from the precipitate as colloidal particles.
Important peptizing agents : Sugar, Gum, Gelatin & Electrolytes.
Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing Fe3+ or OH¯ or FeCl3
Fe(OH)3 + xFe 3+ → Fe (OH)3.xFe 3+
Precipitate Peptizing agent colloid
(1) By exchange of solvents : If a solution of sulphur or phosphorus prepared in alcohol is poured into water, a colloidal solution of sulphur or phosphorus is obtained due to low solubility of sulphur or phosphorus is obtained due to low solubility in water. Thus, there are a number of substances whose colloidal solutions can be prepared by taking a solution of the substance in one solvent and pouring it into another solvent in which the substance is relatively less soluble.
(2) By change of physical state : Colloidal solutions of certain elements such as mercury and sulphur are obtained by passing their vapour through cold water containing a stabilizer (an ammonium salt or a citrate)
(3) Chemical methods : The chemical methods involve chemical reactions in a medium in which the dispersed phase is sparingly soluble. A condition of supersaturation is produced but the actual precipitation is avoided. Some familiar reactions used are :
(a) Double decomposition : (i) Arsenious sulphide sol : A 1% solution of arsenious oxide is prepared is hot water. The solution is cooled, filtered and is then gradually in hot water saturated with hydrogen sulphide. This is continued till an intense yellow-coloured solution is obtained. Excess of H2S is removed by bubbling hydrogen through the solution.
As2O3 + 3H2S As2S3 + 3H2O
(ii) Antimony sulphide sol : A 0.5% solution of potassium antimonyl tartrate is added drop by drop to water saturated with H2S, whilst H2S is being passed through the solution. Orange coloured solution of antimony sulphide is obtained.
(b) Oxidation : A colloidal solution of sulphur is obtained by passing H2O into a solution of sulphur dioxide.
2H2S + SO2 2H2O + 3S
Sulphur sol can also be obtained when H2S is bubbled through an oxidising agent (bromine water or nitric acid).
(c) Reduction : Colloidal solutions of metals like gold, silver, platinum, lead, etc., can be obtained when their salts solutions are acted upon by reducing agents.
2AuCl3 + 3SnCl2 3SnCl4 + 2Au
Organic reducing agents such as formaldehyde, phenyl hydrazine, tannic acid, etc., can also be used.
AgNO3 + tannic acid Silver sol
AuCl3 + tannic acid Gold sol
(d) Hydrolysis : Colloidal solutions of some salts can be prepared by hydrolysis. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.]
FeCl3 + 3H2O
The colloidal solution of silicic acid is also obtained by hydrolysis of dilute solution of sodium silicate with 4N hydrochloric acid which is added drop by drop with constant stirring.