Question

For coagulation of arsenious sulphide sol, which one of the following salt solutions will be most effective?
A.${\text{AlC}}{{\text{l}}_{\text{3}}}$
B.${\text{NaCl}}$
C.${\text{BaC}}{{\text{l}}_{\text{2}}}$
D.${\text{N}}{{\text{a}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$

Answer 1

Coagulation refers to the process of aggregating together the colloidal particles so as to convert them into large sized particles which ultimately settle as precipitate.
Coagulation is brought about by ions which have charge opposite to that of the colloidal particles.

Complete step by step answer:
-In a colloidal system, the term dispersed phase is used to denote the substance which is distributed in the dispersion medium in the form of colloidal particles while the term dispersion medium is used to denote the medium in which the substance is dispersed in the form of colloidal particles.
-Arsenious sulphide, ${\text{A}}{{\text{s}}_{\text{2}}}{{\text{S}}_{\text{3}}}$ sol is a negatively charged sol and so its coagulation will be brought about by oppositely charged ions, i.e., positively charged ions.
-The flocculation value of an electrolyte or the precipitation value of an electrolyte is the minimum amount of the electrolyte in millimoles which must be added to one litre of a colloidal solution so as to bring about complete coagulation or precipitation.
-The effective ions of the electrolyte which brings about coagulation contain charge opposite to that of the colloidal particles. The coagulating power is inversely proportional to the flocculation value of the electrolyte and directly proportional to the valency of the flocculating ion.
-Since coagulating power increases with the increase in charge of the coagulating ion, the coagulation of negative arsenious sulphide sol by trivalent cations will be more effective than divalent cations which in turn will be more effective than monovalent cations.
Now, the ${\text{AlC}}{{\text{l}}_{\text{3}}}$ salt contains the trivalent ${\text{A}}{{\text{l}}^{{\text{3 + }}}}$ cation, the ${\text{NaCl}}$ and ${\text{N}}{{\text{a}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ salts have the monovalent ${\text{N}}{{\text{a}}^{\text{ + }}}$ cation and the ${\text{BaC}}{{\text{l}}_{\text{2}}}$ salt has the divalent ${\text{B}}{{\text{a}}^{{\text{2 + }}}}$ cation. Hence, the ${\text{AlC}}{{\text{l}}_{\text{3}}}$ salt containing the trivalent ${\text{A}}{{\text{l}}^{{\text{3 + }}}}$ cation will be the most effective for coagulation of arsenious sulphide sol.

Hence option A is correct.

Note:
Two colloidal sols having oppositely charged dispersed phases can bring about mutual coagulation. This means if one colloidal sol has positively charged dispersed phase and the other has negatively charged dispersed phase, the charge provided by one colloidal sol can be neutralized by the charge on the other sol resulting in mutual coagulation.
For example, since the dispersed phase in colloidal iron (III) hydroxide is positively charged and that in colloidal gold is negatively charged, the charge on one sol will be neutralized by the charge on the other sol and so there will be mutual coagulation.