Analytical Chemistry Question 75
Question: In the precipitation of the iron group in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to
[AIIMS 1980; NCERT 1976; DPMT 1983; CPMT 1971, 73, 77,78, 80, 81, 83, 86; KCET 1999]
Options:
A) Decrease concentration of $ O{H^{-}} $ ions
B) Prevent interference by phosphate ions
C) Increase concentration of $ C{l^{-}} $ ions
D) Increase concentration of $ NH_4^{+} $ ions
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Answer:
Correct Answer: A
Solution:
As NH4Cl is a strong electrolyte. It supresses the ionization of NH4OH, so the concentration of $ O{H^{-}} $ ions in the solution is decreased, but it is sufficient to precipitate the III group basic radicals because the solubility product of III group hydroxides is lower than IV, V and VI group hydroxides. As $ \begin{align} & NH_4OH \\ & NH_4Cl\to \\ \end{align} $
$ $
$ \begin{align} & +O{H^{-}} \\ & \underset{\begin{smallmatrix} Common \\ ioneffect \end{smallmatrix}}{\mathop{+C{l^{-}}}} \\ \end{align} $
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