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 (AsH
3), while in hyperalkaline solution, the degassing of the reduced
ammonia (NH
3) 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 H
2 molecule, the labile H· free radicals are more reactive than molecular H
2, 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 alloySigma Aldrich: Devarda’s alloy purum, for preparative purposes, grit
Further reading
*
*
{{aluminium alloys
Aluminium alloys
Copper alloys
Name reactions
Hydrogen
Reducing agents