Question

In the measurement of gold number, the useful electrolyte is
A.${\rm{AuC}}{{\rm{l}}_{\rm{3}}}$
B. NaCl
C.${\rm{AlC}}{{\rm{l}}_{\rm{3}}}$
D.${\rm{FeC}}{{\rm{l}}_{\rm{3}}}$

Answer 1

We know that the minimum weight of a protective colloid in milligram needed to prevent coagulation of 10 ml of standard hydro gold sol on adding 1 ml of 10% sodium chloride to it is termed as gold number.

Complete step by step answer:
Let's first understand colloids in detail. A heterogeneous system in which dispersion of one substance as very fine particles in another substance takes place is termed a colloidal system . The size of particles in a colloid is larger than solutions.
Now, we discuss protective colloids. A colloid which is used to prevent precipitation of a lyophobic sol on adding a lyophilic sol is termed as protective colloid.
Now, come to the question. Here, we have to identify the electrolyte used in measuring gold number. Gold number represents the mass of protective colloid that prevents the coagulation of 10 ml standard gold sol on adding 10% sodium chloride to it. So, sodium chloride (NaCl) is the important electrolyte in measuring gold number.

Thus, option B is the correct answer.

Note: When particles of a lyophilic sol are added to a lyophobic sol, it absorbs particles of lyophilic sol. Thus, a coating of lyophilic sol is formed around lyophobic sol. The hydrophobic colloid, therefore, behaves as a hydrophobic sol and is less precipitated by electrolytes. One example is, if gelatin (hydrophilic colloid) is added to a gold sol (hydrophobic sol), the letter is protected. The protected gold sol is no longer precipitated in addition to NaCl.