Preparation of Colloids
Many lyophillic substances are made in their colloidal form by warming with water. Rubber forms colloidal solution with benzene. Soap spontaneously forms a colloidal solution by just mixing with water.
In general, colloidal are prepared by the following methods.
i. Dispersion methods: In this method larger particles are broken to colloidal dimension.
ii. Condensation method: In this method, smaller atom or molecules are converted into larger colloidal sized particles.
Using a colloid mill, the solid is ground to colloidal dimension. The colloid mill consists of two metal plates rotating in opposite direction at very high speed of nearly 7000 revolution / minute.
The colloidal particles of required colloidal size is obtained by adjusting the distance between two plates.
By this method, colloidal solutions of ink and graphite are prepared.
A brown colloidal solution of platinum was first prepared by George Bredig in 1898. An electrical arc is struck between electrodes dispersed in water surrounded by ice. When a current of 1 amp /100 V is passed an arc produced forms vapours of metal which immediately condense to form colloidal solution. By this method colloidal solution of many metals like copper, silver, gold, platinum, etc. can be prepared Alkali hydroxide is added as an stabilising agent for the colloidal solution.
Svedberg modified this method for the preparation of non aqueous inflammable liquids like pentane, ether and benzene, etc using high frequency alternating current which prevents the decomposition of liquid.
Sound waves of frequency more than 20kHz (audible limit) could cause transformation of coarse suspension to colloidal dimensions.
Claus obtained mercury sol by subjecting mercury to sufficiently high frequency ultrasonic vibrations.
The ultrasonic vibrations produced by generator spread the oil and transfer the vibration to the vessel with mercury in water.
By addition of suitable electrolytes, precipitated particles can be brought into colloidal state. The process is termed as peptisation and the electrolyte added is called peptising or dispersing agent.
AgCl …HCL→ AgCl
Precipitate …HCL→ Colloid
When the substance for colloidal particle is present as small sized particle, molecule or ion, they are brought to the colloidal dimension by condensation methods. Here care should be taken to produce the particle with colloidal size otherwise precipitation will occur. Various chemical methods for the formation of colloidal particles.
Sols of some non metals are prepared by this method.
(a) When hydroiodic acid is treated with iodic acid, I2 sol is obtained.
HIO3 +5HI → 3H 2O + I2 (Sol)
(b) When O2 is passed through H2Se, a sol of selenium is obtained.
H 2 Se + O2 → 2H2 O + Se(sol)
Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For example: Gold sol is prepared by reduction of auric chloride using formaldehyde.
2AuCl 3 +3HCHO+3H2O → 2Au(sol)+6HCl+3HCOOH
Sols of hydroxides of metals like chromium and aluminium can be produced by this method.
FeCl3 +3H 2 O → Fe(OH)3 +3HCl
For the preparation of water insoluble sols this method can be used.
When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
As2 O 3 + 3H2 S → As 2 S3 +3H 2 O
When few drops of an acid is added to a dilute solution of sodium thiosulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system depending on their growth within the size of colloidal dimensions.
S2O32-+2H+ → S (sol) + H2O + SO2
Colloidal solution of few substances like phosphorous or sulphur is obtained by preparing the solutions in alcohol and pouring them into water. As they are insoluble in water, they form colloidal solution.
P in alcohol + water→ Psol.