Nitrite `(NO_(2)^(-))` interferes in the 'ring-test' of nitrate `(NO_(3)^(-))`. Some of the following reagents can be used for the removal of nitrite.
(P) Potassium dichromate(s)
(Q) `CO(NH_(2))_(2)` (urea)
(R) `NH_(2)SO_(3)H` (Sulphamic acid)
(S) Zinc/sodium hydroxide

Options

A. P,Q
B. P,Q,S
C. Q,R
D. Q,R,S

Answer

C

Explanation

`NH_(2)-overset(O)overset(||)(C)-NH_(2)+NO_(2)^(-)+H^(+)rarr N_(2)uarr +CO_(2) uarr +H_(2)O`
`H_(2)N-overset(S)overset(||)(C)-NH_(2)+HNO_(2)rarrN_(2)uarr+ H^(+) +SCN^(-)+H_(2)O`
Note: No nitrate is formed in 3rd reaction, and it is therefore an excellent method for complete removal of nitrite. Traces of nitrates are formed with `NH_(4)CI`, urea and thiourea.