# Testing for nitrate ions with NaNO3 in Devarda's test

When we use Devarda's test on $\ce{NaNO3}$ solution (we add $\ce{NaOH}$ and $\ce{Al}$ powder, heat the mixture, and produce ammonia), what is the equation of the process?

I understand that $\ce{Al}$ reduces $\ce{NO3-}$ to $\ce{NH4+}$ then $\ce{NH4+}$ and $\ce{OH-}$ have a reversible reaction to produce $\ce{NH3}$, and water, but I'm not sure how to summarize this in an equation. In particular, what happens to the aluminium?

• Aluminium oxidizes, as you may guess. To find its final form, keep in mind that $\ce{Al}$ is amphoteric. – Ivan Neretin May 3 '16 at 19:22

## 1 Answer

Quoting from Svelha G., Vogel's Qualitative Inorganic Analysis,

A solution of nitrate ion is boiled with zinc dust/aluminium powder and sodium hydroxide solution to get positive result. Excellent result is obtained by the use of Devarda's alloy (45% Al, 5% Zn, 50% Cu) but the ammonium ions must be removed by boiling the solution with sodium hydroxide solution or almost evaporating it to dryness before addition to metal.

The overall reaction is:

$$\ce{3NO3- + 8Al + 5OH- + 18H2O ->3NH3 ^ + 8[Al(OH)4]-}$$