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Devarda's alloy (CAS # 8049-11-4), is an alloy of aluminium (44% – 46%), copper (49% – 51%) and zinc (4% – 6%). Devarda's alloy is used as reducing agent in analytical chemistry for the determination of
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
s after their reduction to ammonia under alkaline conditions. It is named for Italian chemist
Arturo Devarda Arturo is a Spanish and Italian variant of the name Arthur. People *Arturo Álvarez (footballer, born 1985), American-born Salvadoran footballer *Arturo Álvarez (footballer, born 1959), Mexican footballer *Arthuro Henrique Bernhardt (b. 1982), Br ...
(1859–1944), who synthesised it at the end of the 19th century to develop a new method to analyze
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
in Chile saltpeter. It was often used in the quantitative or qualitative analysis of
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
s in agriculture and soil science before the development of ion chromatography, the predominant analysis method largely adopted worldwide today.


General mechanism

When a solution of nitrate ions is mixed with aqueous
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
, adding Devarda's alloy and heating the mixture gently, liberates ammonia gas. After conversion under the form of ammonia, the total nitrogen is then determined by Kjeldahl method. The reduction of nitrate by the Devarda's alloy is given by the following
equation In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...
: :3 + 8 Al + 5 + 18 → 3 + 8


Distinction between NO3 and NO2 with spot tests

To distinguish between
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
and
nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...
, dilute HCl must be added to the nitrate. The
brown ring test A Nitrate Test is a chemical test used to determine the presence of nitrate ion in solution. Testing for the presence of nitrate via wet chemistry is generally difficult compared with testing for other anions, as almost all nitrates are soluble in ...
can also be used.


Similarity with the Marsh test

Devarda's alloy is a reducing agent that was commonly used in wet analytical chemistry to produce so-called nascent hydrogen under alkaline conditions ''in situ''. In the
Marsh test The Marsh test is a highly sensitive method in the detection of arsenic, especially useful in the field of forensic toxicology when arsenic was used as a poison. It was developed by the chemist James Marsh and first published in 1836. The method ...
, used for arsenic determination, hydrogen is generated by contacting zinc powder with hydrochloric acid. So, hydrogen can be conveniently produced at low or high pH, according to the volatility of the species to be detected. Acid conditions in the Marsh test promote the fast escape of the arsine gas (AsH3), while in hyperalkaline solution, the degassing of the reduced ammonia (NH3) is greatly facilitated.


Long-debated question of the nascent hydrogen

Since the mid-19th century the existence of true nascent hydrogen has repeatedly been challenged. It was assumed by the supporters of this theory that, before two hydrogen atoms can recombine into a more stable H2 molecule, the labile H· free radicals are more reactive than molecular H2, a relatively weak reductant in the absence of a metal catalyst. Nascent hydrogen was supposed to be responsible for the reduction of arsenate or nitrate in arsine or ammonia respectively. Nowadays, isotopic evidence has closed the nascent hydrogen debate, presently considered to be a '' Gedanken'' artifact of romanticism.


See also

* Kjeldahl method * Nitrate test * Nitrite test * Nascent hydrogen *
Marsh test The Marsh test is a highly sensitive method in the detection of arsenic, especially useful in the field of forensic toxicology when arsenic was used as a poison. It was developed by the chemist James Marsh and first published in 1836. The method ...
* Detection of: ** Arsine **
Stibine Stibine (IUPAC name: stibane) is a chemical compound with the formula SbH3. A pnictogen hydride, this colourless, highly toxic gas is the principal covalent hydride of antimony, and a heavy analogue of ammonia. The molecule is pyramidal with H–S ...
* Raney nickel, prepared from an Al, Ni alloy also dissolved in concentrated NaOH * Zinc-copper couple


References


External links


Devarda's alloy - Substance Summary (SID 24856330)
( PubChem Substance)
Safety data for Devarda's alloy

Sigma Aldrich: Devarda’s alloy purum, for preparative purposes, grit





Further reading

* * {{aluminium alloys Aluminium alloys Copper alloys Name reactions Hydrogen Reducing agents