Question

In an experiment, addition of $4.0$ mL of $0.005M$ $BaC{l}_2$ to $16.0$ mL of arsenious sulphide sol just causes the complete coagulation in $2$ h. The flocculating value of the effective ion is:

• A. $3.0$
• B. $2.0$
• C. $1.0$
• D. $0.5$

Answer 1

The correct option is C $1.0$

Coagulation/Flocculation :

The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids.

However, when an electrolyte is added in larger concentration; the particles of the sol take up the ions which are oppositely charged and thus get neutralised. The neutral particles then start aggregating giving particles of larger size which are then precipitated. This process of aggregation of colloidal particles into an insoluble precipitate by the addition of some suitable electrolyte is known as coagulation.

Coagulation value or Flocculation value :

It needs to be noted that the coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum concentration of

electrolyte in millimoles required to cause coagulation of one litre of colloidal solution is called coagulation value. It is express in terms of millimoles/litre.

$A{s}_2{S}_3$ sol is negatively charged owing to preferential adsorption of $S^{2-}$ ions. Cation would be the effective ion in coagulation.
Flocculating value $=$ minimum milli mol of the effective ion per liter of sol

Coagulation value = millimolesof electrolyte / volumeof sol in litre

$=\frac{4\times 0.005\times {10}^3}{4+16}=1.0$