Devarda's Alloy
Definition and meaning of Devarda's Alloy in chemistry.
Devarda's alloy is a fine powder of copper, aluminum, and zinc used to reduce nitrate and nitrite ions to ammonia in strongly alkaline solution for quantitative nitrogen analysis.
In more detail
When added to a sample dissolved in hot sodium hydroxide, aluminum in the alloy is oxidized to aluminate ion while nitrate or nitrite nitrogen is reduced to ammonia gas, which distills off and is trapped for titration. Copper and zinc do not themselves reduce nitrate efficiently but activate and roughen the aluminum surface, giving a faster, more reproducible reaction than aluminum alone. This makes Devarda's alloy essential for determining nitrate-nitrogen in soils, fertilizers, and waters, since standard Kjeldahl digestion converts only organic and ammoniacal nitrogen to ammonium, leaving nitrate untouched.
Key facts
| Typical composition | ~50% Cu, 45% Al, 5% Zn by mass |
|---|---|
| Named after | Arturo Devarda, Italian chemist |
| Primary use | Reduces NO3⁻/NO2⁻ to NH3 in alkaline solution |
| Field | Analytical Chemistry |
A soil extract containing dissolved KNO3 is made alkaline with NaOH and treated with Devarda's alloy; on heating, 3NO3⁻ + 8Al + 5OH⁻ + 18H2O → 8[Al(OH)4]⁻ + 3NH3, and the liberated ammonia is distilled into boric acid and titrated to determine the nitrate content.
Frequently asked questions
Why use an alloy instead of pure aluminum to reduce nitrate?
The copper and zinc in Devarda's alloy activate and roughen the aluminum surface, making the reduction faster and more reproducible than with aluminum powder alone.
How does Devarda's alloy relate to the Kjeldahl method?
Standard Kjeldahl digestion converts organic and ammoniacal nitrogen to ammonium but not nitrate; treating the digest (or a separate aliquot) with Devarda's alloy reduces nitrate-nitrogen to ammonia so it too can be distilled and measured.