Devarda's alloy (CAS # 8049-11-4), is an

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

(44% – 46%),

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

(49% – 51%) and

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

(4% – 6%). Devarda's alloy is used as

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth me ...

analytical chemistry Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...

for the determination of nitrates after their reduction to

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

under alkaline conditions. It is named for Italian chemist Arturo Devarda (1859–1944), who synthesised it at the end of the 19th century to develop a new method to analyze nitrate in

Chile saltpeter Nitratine or nitratite, also known as cubic niter (UK: nitre), soda niter or Chile saltpeter (UK: Chile saltpetre), is a mineral, the naturally occurring form of sodium nitrate, NaNO3. Chemically it is the sodium analogue of saltpeter. Nitratine ...

. It was often used in the quantitative or qualitative analysis of nitrates in agriculture and soil science before the development of

ion chromatography Ion chromatography (or ion-exchange chromatography) separates ions and polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including large proteins, small nucleotides, and amino acid ...

, the predominant analysis method largely adopted worldwide today.

General mechanism

When a solution of nitrate ions is mixed with

aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

sodium hydroxide, adding Devarda's alloy and heating the mixture gently, liberates

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

. After conversion under the form of ammonia, the total nitrogen is then determined by

Kjeldahl method The Kjeldahl method or Kjeldahl digestion () in analytical chemistry is a method for the quantitative determination of nitrogen contained in organic substances plus the nitrogen contained in the inorganic compounds ammonia and ammonium (NH3/NH4+) ...

. The reduction of nitrate by the Devarda's alloy is given by the following equation: :3 + 8 Al + 5 + 18 → 3 + 8

Distinction between NO₃⁻ and NO₂⁻ with spot tests

To distinguish between nitrate and nitrite, dilute HCl must be added to the nitrate. The brown ring test can also be used.

Similarity with the Marsh test

Devarda's alloy is a

that was commonly used in wet

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 metho ...

, used for arsenic determination, hydrogen is generated by contacting

powder with

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

. 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 Arsine (IUPAC name: arsane) is an inorganic compound with the formula As H3. This flammable, pyrophoric, and highly toxic pnictogen hydride gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in ...

gas (AsH₃), while in hyperalkaline solution, the degassing of the reduced

(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 chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

in the absence of a metal catalyst. Nascent hydrogen was supposed to be responsible for the reduction of arsenate or nitrate in

respectively. Nowadays, isotopic evidence has closed the nascent hydrogen debate, presently considered to be a '' Gedanken'' artifact of romanticism.

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

General mechanism

Distinction between NO₃⁻ and NO₂⁻ with spot tests

Similarity with the Marsh test

Long-debated question of the nascent hydrogen

See also

References

External links

Further reading

General mechanism

Distinction between NO3− and NO2− with spot tests

Similarity with the Marsh test

Long-debated question of the nascent hydrogen

See also

References

External links

Further reading

Distinction between NO₃⁻ and NO₂⁻ with spot tests