why do we add NH4Cl before NH4OH in analysis of group 3 cations?
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In salt analysis NH4Cl is added in sufficient amount before adding NH4OH because otherwise the cations of higher group may get precipitated in group III.
This is because NH4Cl is strong electrolyte and decomposes completely. The NH4OH however is a weak base and does not ionize completely. Thus due to presence of common ion NH4 + in NH4Cl, it suppresses the ionization of weak base NH4OH in order to decrease the OH- concentration so that higher group cations will not get precipitated.
Common ion effect:
NH4Cl → NH4 + + Cl-
NH4OH ↔ NH4 + + OH-
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