Select the correct statements about the test for ammonium ion.

AThe test is done on the original solution, since ammonium salts are added during the scheme.
BNessler reagent ( $K X_{2} [HgI X_{4}]$ in alkali) gives a brown precipitate/colour of the iodide of Millon's base, $HgO \cdot Hg (NH X_{2}) I$ .
CWarming with $NaOH$ releases $NH X_{3}$ , which turns moist red litmus blue.
DThe group III filtrate can be used for the Nessler test with the same significance as the original solution.

Answer: A, B, C

NH X_{4} X^{+} + OH X^{-} NH X_{3} + H X_{2} O

explains the litmus test, and

NH X_{3}

reacts with alkaline

[HgI X_{4}] X^{2 -}

to form the brown Millon-base iodide. Because

NH X_{4} Cl

and

NH X_{4} OH

are used as reagents in the scheme, the later filtrates always contain

NH X_{4} X^{+}

, so statement 4 is false.