151. Define the terms colloid and colloidal state. How does the theory of selective adsorption explain the origin of charge on colloidal particles?

Answer:

Colloids and colloidal state: In the light of present day researches, Thomas Graham's classification of substances into colloids and crystalloids has been found to be wrong. Graham thought of colloids as substances. Now-a-days the term is applied not so much to the substance but rather to a state in which that substance exists. The same substance can behave as a crystalloid in one solvent and a colloid in another.
Origin of charge on colloidal particles:
According to the modern theory of electrical double layer (proposed by Helmholtz) the charge on colloidal particles is due to the preferential adsorption of common ion on their surface and consequently an electrical double layer is formed, in which adsorbed ions form one layer; while the remaining oppositely charged ions form the second diffused layer. Examples:

(i) When freshly precipitated ferric hydroxide is peptized with dilute FeCI₃, a +ve sol is formed, due to the adsorption of Fe³⁺ ions of FeCl₃ on the surface of Fe(OH)₃ colloid particles, while the negative CI^– ions form the diffused layer. This may be shown as:

$$\begin{gathered} \left[\mathrm{Fe}(\mathrm{OH}{)}_3\right]{\mathrm{Fe}}^{3+} : 3{\mathrm{CI}}^{-} \\ + \mathrm{ve} \mathrm{sol} \mathrm{solution} -\mathrm{ve} \mathrm{diffused} \mathrm{layer} \end{gathered}$$


(ii) As₂S₃ sol prepared by passing H₂S gas through As₂O₃ solution., is – ve due to preferential adsorption of common ions S^2– on its surface, while the H⁺ ions forming the diffused layer. This may be shown as:

$$\begin{gathered} \left[{\mathrm{A}}_2{\mathrm{S}}_3\right]{\mathrm{S}}^{2-} : 2{\mathrm{H}}^{+} \\ +\mathrm{ve} \mathrm{sol} \mathrm{solution} -\mathrm{ve} \mathrm{diffused} \mathrm{layer} \end{gathered}$$