In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :

Explanation:-

Role of Ammonium Chloride in Qualitative Analysis of Iron Group III

• During qualitative analysis, specifically in the precipitation of the iron group (Group III cations), ammonium chloride (NH4Cl) is added before adding ammonium hydroxide (NH4OH).
• Ammonium chloride provides a source of ammonium ions (NH4⁺), which play a crucial role in controlling the concentration of hydroxide ions (OH^-).
• This is important because a higher concentration of OH^- ions might cause the precipitation of Group IV cations (like Zn²⁺, Mn²⁺) in addition to the iron group cations.
• NH4⁺ ions from NH4Cl suppress the ionization of NH4OH, thereby maintaining a moderate concentration of OH^- ions just enough to precipitate the Group III hydroxides without causing the formation of unwanted hydroxides of subsequent groups.

NH4OH \(\rightleftharpoons\) NH4+ + OH-

NH4Cl → NH4+ + Cl

Due to common ion effect of NH4+,

[OH-] decreases in such extent that only group-III cation can be precipitated , due to their very low Ksp in the range of 10–38.

In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to decrease concentration of –OH ion