Zn can reduce \(\mathrm{NO}_{3}^{-}\) to \(\mathrm{NH}_{3}(\mathrm{g})\) in basic
solution. (The following equation is not balanced.)
$$\begin{aligned}\mathrm{NO}_{3}^{-}(\mathrm{aq})+\mathrm{Zn}(\mathrm{s})+&
\mathrm{OH}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})
\longrightarrow
\\\&\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{2-}(\mathrm{aq})+\mathrm{NH}_{3}(\mathrm{g})\end{aligned}$$
The \(\mathrm{NH}_{3}\) can be neutralized with an excess of
\(\mathrm{HCl}(\mathrm{aq})\) Then, the unreacted HCl can be titrated with NaOH.
In this way a quantitative determination of \(\mathrm{NO}_{3}^{-}\) can be
achieved. A 25.00 mL sample of nitrate solution was treated with zinc in basic
solution. The \(\mathrm{NH}_{3}(\mathrm{g})\) was passed into \(50.00
\mathrm{mL}\) of \(0.1500 \mathrm{M} \mathrm{HCl} .\) The excess \(\mathrm{HCl}\)
required \(32.10 \mathrm{mL}\) of \(0.1000 \mathrm{M} \mathrm{NaOH}\) for its
titration. What was the \(\left[\mathrm{NO}_{3}\right]\) in the original sample?