Methods of Preparation:

Dispersion and Condensation

Methods

• Massive materials are disintegrated into fine colloidal particles while preparing colloids. Dispersion methods are used to break up this materials into colloidal size.

Dispersion methods

Mechanical dispersion

Peptization Bredig’s arc dispertion methods

In dispersion methods, colloidal particles are obtained by breaking largeparticles of a substance in the presence of a dispersion medium. Since the solsformed are unstable, they are stabilized by adding mechanical energy input.

1. Mechanical dispersion• The colloid mill consists of two

metal discs, close together and rotating at high speed (7000 revolutions per minute) in opposite directions. By the process of such grinding, the suspension particles are torn off to colloidal sizes.

• This method is used to make colloidal graphite (a lubricant) and certain types of printing ink.

2. Peptization

• The process of converting a precipitate into a colloidal sol by shaking it with the dispersion medium, in the presence of a small amount of electrolyte

• The electrolyte = peptizing agent.

• This method is used to convert a freshly prepared precipitate into a colloidal sol.

• The precipitate absorbs one of the ion of the electrolyte onto its surface. The ion absorbed on the surface is generally common with those of the precipitate. Absorption of ion results in the development of positive or negative charge on precipitates and which ultimately break it up into colloidal size particles.

• For e.g., a precipitate of silver iodide already formed can be dispersed by the addition of potassium iodide. Here potassium iodide is the peptizing agent.

3. Bredig’s arc dispersion method

• This method is suitable for the preparation of metallic sols such as gold sol or silver sol.

• Water containing a little of alkali like KOH (in order to stabilise the resulting sol) is taken in a vessel. It is kept cold by immersing the container in ice water bath.

• An electric arc is struck between two gold (or silver) electrodes (about 2 mm apart).

• Heat produced by the spark of high voltage causes a small amount of gold to vaporize and condense to particle of colloidal size. The particle is filtered to obtain colloidal solution of gold.

Condensation methods

Exchange of solvent

Double decomposition

Hydrolysis Oxidation

Reduction

The materials originally in true solution(particle diameter <10-9) where chemical reactions are used to produce colloidal solutions. Concentration of the solutions used is controlled to prevent the growth of the colloidal particles and consequence precipitations.

Exchange of solvent

• A process for recovering an organic acid or a metal salt.

• Colloidal solution of certain substances such as sulphur, phosphorus, which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of water.

• For example, alcoholic solution of sulphur (sulphur-ethanol solution) on pouring into water gives milky colloidal solution of sulphur because sulphur is less soluble in water than in ethanol.

Double Decomposition

• Reaction between two chemical substances (usually salts in solution) that results in the exchange of a constituent from each compound to create two different compounds.

• E.g: A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenic (III) oxide in water.

As2O3 + 3H2S → As2S3 + 3H2O• This method produce sol that used in the preparation

of photographic film, paper and plates.

Hydrolysis

• Hydrolysis literally means reaction with water. It is a chemical process in which a molecule is split into two parts by the addition of a molecule of water. One fragment of the parent molecule gains a hydrogen ion (H + ) from the additional water molecule.

• Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chlorides.

FeCl3 + 3H2O → Fe(OH)3 + 3HCl

Colloidal sol

Similarly silicic acid sol is obtained by the hydrolysis of sodium silicate.

Oxidation

• Defined as the loss of at least one electron or addition of O2 when two or more substances interact.

• E.g: A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising agent like HNO3, Br2, SO2 etc.) through a solution of hydrogen sulphide in water.

2H2S + O2 (or any other agent) → 2H2O + 2S

Reduction • Defined as the addition of at least one electron or loss of O2 when

substances come into contact with each other.• A number of metals such as silver, gold and platinum, have been

obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as formaldehyde, acetyldehyde, hydrogen peroxide, stannous chloride etc 2AuCl3 + 3SnCl2 → 3SnCl + 2Au

Gold sol

2AuCl3 + 3Fe2+ Au + 3Fe3+ Gold sol • The gold sol, thus prepared, has a purple colour and is called

purple of cassius.• A colloidal solution of silver is brown.

ANY QUESTION??

Questions for you

• 1.How many method in preparing lyophobic sol?• 2.How many dispersion method is there?, list all

of it…• 3.How many condensation method is there? List

all of it…

THANK YOU

THE END

BY:NURHIDAYAH BINTI MAT SARIT

NURFAHIMA BINTI MUSTAFA AZMYSITI IYLIA ZARITH BINTI HASAN

SITI NUR FATHIRAH BINTI JUNAIDI-UITM NEGERI SEMBILAN-