Question

Minimum amount of protective colloid added to $10mL$ of a standard gold sol to prevent a colour change on the addition of $1mL$ of 10% $NaCl$ solution is called

• A. Flocculation value
• B. Gold number
• C. Protective power
• D. None of the above

Answer 1

The correct option is B Gold number

Flocculation value :
Flocculation value is the minimum concentration of electrolyte in millimoles required to cause coagulation of one litre of colloidal solution.
$\text{Flocculation value}=\frac{\text{Millimoles of electrolyte}}{\text{Size of particle volume of sol in litre}}$

Gold number:
Gold number is the minimum amount of protective colloid in milligrams which when added to $10mL$ of a standard gold sol just sufficient to prevent a colour change from red to blue on the addition of $1mL$ of 10% $NaCl$ solution.

$\text{Gold number}=\frac{\text{Weight of lyophilic sol in mg}\times 10}{\text{Volume of gold sol in mL}}$

$\text{Protective power of protective colloids}\propto \frac{1}{\text{Gold number}}$