Given below are a few electrolytes, indicate which one among...

Given below are a few electrolytes, indicate which one among them will bring about the coagulation of a gold sol quickest and in the least of concentration? (a) $\mathrm{NaCl}$ (b) $\mathrm{MgSO}_{4}$ (C) $\mathrm{Nl}_{2}\left(\mathrm{SO}_{4}\right)_{3}$ (d) $\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$

Given below are a few electrolytes, indicate which one among them will bring about the coagulation of a gold sol quickest and in the least of concentration?
(a) $\mathrm{NaCl}$
(b) $\mathrm{MgSO}_{4}$
(C) $\mathrm{Nl}_{2}\left(\mathrm{SO}_{4}\right)_{3}$
(d) $\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$

Key Concepts

Colloidal Stability

Colloidal stability refers to the ability of particles suspended in a medium to remain dispersed without agglomerating. In gold sols, the stability is achieved by the presence of surface charges on the gold nanoparticles which generate repulsive forces among them, preventing coagulation. Any disruption of these forces, such as by the addition of suitable electrolytes, can lead to the aggregation and eventual coagulation of the colloidal particles.

Electrolyte Effect on Colloids

Electrolytes, when added to a colloidal system, provide ions that can screen or neutralize the surface charges of the colloidal particles. The presence of these counter ions reduces the repulsive forces between the particles by compressing the electrical double layer, thereby promoting the aggregation or coagulation of the colloid. The efficiency of this process depends on the concentration and nature of the ions, particularly their charge.

Double Layer Compression

The electrical double layer around colloidal particles consists of a layer of adsorbed ions and a diffuse layer of counter ions, which together stabilize the colloid through repulsive interactions. When electrolytes are added, especially those containing multivalent ions, the double layer becomes compressed. This compression reduces the effective range of repulsive forces between particles, making it easier for them to come close enough to aggregate.

Schulze-Hardy Rule

The Schulze-Hardy rule states that the coagulating power of an ion is highly dependent on its valence; ions with higher charges are much more effective at inducing coagulation than those with lower charges. This means that even in lower concentrations, multivalent ions can significantly reduce the repulsive forces between colloidal particles, leading to quicker coagulation. This principle explains why electrolytes with higher valency often lead to earlier and more efficient coagulation of colloidal sols.

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