First slide

Colloids

Question

Identify the correct decreasing order of coagulating value of the following electrolytes when each of them is separately added to Haemoglobin sol(Assume all electrolytes are strong electrolytes).

Moderate

A

${AlF}_{3} > {Na}_{2} {SO}_{4} > {Ba}_{3} {({PO}_{4})}_{2}$
B

${Ba}_{3} {({PO}_{4})}_{2} > {Na}_{2} {SO}_{4} > {AlF}_{3}$
C

${AlF}_{3} > {Ba}_{3} {({PO}_{4})}_{2} > {Na}_{2} {SO}_{4}$
D

${Na}_{2} {SO}_{4} > {Ba}_{3} {({PO}_{4})}_{2} > {AlF}_{3}$

Solution

Haemoglobin is a positive sol. According to Hardy Schulze rule “A positive sol is most effectively coagulated by an anion with higher valency”.
$∴$ coagulating power of ${PO}_{4}^{3 -} > {SO}_{4}^{2 -} > F^{-}$ in the coagulation of Hb sol.
Number of milli moles of electrolyte added to one litre of colloid to protect it from coagulation in two hours is called flocculating value or coagulating value.
Coagulation power $\propto$ $\frac{1}{Coagulation value}$ ;
Thus coagulating value of ${PO}_{4}^{3 -} < {SO}_{4}^{2 -} < F^{-}$ ….for a positive sol

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