Question
What is the function of NH4Cl in group V analysis?
Solution
Cations of group V (Ba2+, Sr2+, Ca2+) are precipitated as their carbonates on adding ammonium carbonate to their solution containing NH4Cl and NH4OH.
Due to common ion effect of NH4+ ions, the degree of dissociation of (NH4)2CO3 is suppressed, so the concentration of 
ions decreases. Thus. ionic product of carbonates of group V cations decreases. Even the low concentration of 
ions is sufficient for the ionic product of these ions and cations of the fifth group to exceed the solubility product of corresponding carbonates. Hence, the carbonates of the fifth group get precipitated.
Other group cations (Na+, K+ and NH4+ ) are either soluble or their solubility product (in the case of Mg2+) is higher.
