Question

Gelatin is best protective colloid because its gold number is-
(A)- maximum
(B)- minimum
(C)- zero
(D)- infinite

Answer 1

. The efficiency of a colloid to protect a colloidal solution from precipitation can be measured by Gold number. It is given by the relationship - smaller the gold number value, higher is the protective power of the colloid.

Complete step by step answer:
The term Gold number is used in colloidal chemistry which is defined as the minimum amount of protective colloid (in milligrams) that prevents a color change from red to violet/blue (or to prevent the coagulation) of 10ml gold sol by the addition of 1ml of NaCl solution. The coagulation of gold sol results in particle size, which is evident by a color change from red to violet/blue. This term was used by Zsigmondy.
- Lyophobic sols are not so stable and get precipitated in addition to electrolytes. However certain lyophilic colloids like gelatin, gums make coagulation of lyophobic colloids difficult in addition to electrolytes as they cover the particles of lyophobic colloids. This process is known as Protection and the lyophilic colloids are termed as protective colloids.
- Also, greater the gold number, the lower is the protective power of the colloid and vice-versa, as a large amount of colloid is required to prevent coagulation/precipitation.
Protective power $\propto \dfrac{1}{gold~number}$
Since gelatin is the best protective colloid, hence its gold number is minimum.
So, the correct answer is “Option B”.

Additional Information:
Gold number of some colloids are:
Gelatin - 0.005-0.01
Haemoglobin – 0.03-0.07
Egg albumin – 0.15-0.25
Potato starch- 20-25
Dextrin- 125-150
From the above data,we can say that gelatin has a very low value of gold number as compared to other colloids, and hence is a good protective colloid.

Note: The ion which is responsible for neutralization of charge on the colloidal particles is known as the coagulating or Flocculating ion. And the minimum of an electrolyte (millimoles) that must be added to one litre of a colloidal solution leads to complete coagulation is known as coagulation value or Flocculation Value.