Colloidal State

Colloidal State

(1) The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists like Tyndall, Hardy, Zsigmondy, N.R. Dhar, S.S. Bhatnagar and others.

(2) Thomas Graham classified the soluble substances into two categories depending upon the rate of diffusion through animal and vegetable membranes or parchment paper.

(i) Crystalloids : They have higher rate of diffusion and diffused from parchment paper.

Examples : All organic acids, bases and salts and organic compounds such as sugar, urea etc.

(ii) Colloids (Greek word, kolla, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment paper.

Examples : Starch, gelatin, gums, silicic acid and hdemoglobin etc.

(3) The above classification was discarded i.e., the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any substance can be brought into colloidal state.

(4) The colloidal state depends on the particle size. If is regarded as intermediate state between true solution and suspension.

 Table: Features of the three types of solutions

Property

Suspension

Colloid solution

True solution

Nature

Heterogeneous

Heterogeneous

Homogeneous

Particle size

> 100 nm

1 nm – 100 nm

< 1 nm

Separation by

(i) Ordinary filtration

(ii) Ultra- filtration

 

Possible

Possible

 

Not possible

Possible

 

Not possible

Not possible

Settling of particles

Settle under gravity

Settle only on centrifugation

Do not settle

Appearance

Opaque

Generally transparent

Transparent

Tyndall effect

Shows

Shows

Does not show

Diffusion of particles

Does not diffuse

Diffuses slowly

Diffuses rapidly

Brownian movement

May show

Shows

Negligible

 

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