Devarda's alloy

Devarda's alloy
Identifiers
CAS Number	8049-11-4;
ChemSpider	none;
ECHA InfoCard	100.209.703
PubChem CID	16211762;
UNII	GSX651880X ;
CompTox Dashboard (EPA)	DTXSID50583479 ;
Properties
Density	5.79 g/cm3
Melting point	490 to 560 °C (914 to 1,040 °F; 763 to 833 K)
Boiling point	906 °C (1,663 °F; 1,179 K)
Solubility in water	insoluble
Hazards
	GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H228
Precautionary statements	P210, P240, P241, P280, P378
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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 nitrates after their reduction to ammonia 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.^[2]^[3]^[4]

It was often used in the quantitative or qualitative analysis of nitrates in agriculture and soil science before the development of ion chromatography, the predominant analysis method largely adopted worldwide today.^[5]^[6]

General mechanism

When a solution of nitrate ions is mixed with aqueous sodium hydroxide, 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.^[7]

The reduction of nitrate by the Devarda's alloy is given by the following equation:

3 NO⁻
₃ + 8 Al + 5 OH⁻
+ 18 H
₂O → 3 NH
₃ + 8 [Al(OH)
₄]⁻

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 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, 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^[8] has closed the nascent hydrogen debate, presently considered to be a Gedanken artifact of romanticism.^[9]^[10]^[11]

References

^ "SICHERHEITSDATENBLATT". Merck.
^ Devarda, A. (1892). "Ueber die direkte bestimmung des stickstoffs im salpeter" [On the direct determination of nitrogen in saltpetre]. Chemiker Zeitung (in German). 16: 1952.
^ Devarda, A. (1894). "Eine neue methode zur bestimmung des stickstoffs im chilisalpeter" [A new method for the determination of nitrogen in Chile saltpetre]. Zeitschrift für analytische Chemie (in German). 33 (1): 113–114. doi:10.1007/bf01335775. S2CID 97552792.
^ Devarda, A.; Fields, J. (1899). "Ueber stickstoffbestimmung" [On nitrogen determination]. Zeitschrift für analytische Chemie (in German). 38 (1): 55–57. doi:10.1007/bf01386922. S2CID 197597366.
^ Feigl, Fritz (1961). "Spot tests based on redox reactions with Devarda's alloy and Raney alloy". Analytical Chemistry. 33 (8): 1118–1121. doi:10.1021/ac60176a018.
^ O'Deen, William A.; Lynn K. Porter (1980). "Devarda's alloy reduction of nitrate and tube diffusion of the reduced nitrogen for indophenol ammonium and nitrogen-15 determinations". Analytical Chemistry. 52 (7): 1164–1166. doi:10.1021/ac50057a044.
^ Liao, Christina F.H. (1981). "Devarda's alloy method for total nitrogen determination". Soil Science Society of America Journal. 45 (5): 852–855. Bibcode:1981SSASJ..45..852L. doi:10.2136/sssaj1981.03615995004500050005x.
^ Laborda, F.; Bolea, E.; Baranguan, M. T.; Castillo, J. R. (2002). "Hydride generation in analytical chemistry and nascent hydrogen: when is it going to be over?". Spectrochimica Acta Part B: Atomic Spectroscopy. 57 (4): 797–802. Bibcode:2002AcSpe..57..797L. doi:10.1016/S0584-8547(02)00010-1. ISSN 0584-8547.
^ Tommasi, D. (1897). "Comment on the Note of R. Franchot entitled "Nascent Hydrogen"". The Journal of Physical Chemistry. 1 (9): 555. doi:10.1021/j150591a004. ISSN 1618-2642.
^ Meija, Juris; D’Ulivo, Alessandro (2008). "Nascent hydrogen challenge". Analytical and Bioanalytical Chemistry. 391 (5): 1475–6. doi:10.1007/s00216-008-2143-4. ISSN 1618-2642. PMID 18488209. S2CID 19542514.
^ Meija, Juris; D’Ulivo, Alessandro (2008). "Solution to nascent hydrogen challenge". Analytical and Bioanalytical Chemistry. 392 (5): 771–772. doi:10.1007/s00216-008-2356-6. ISSN 1618-2642. PMID 18795271. S2CID 206900604.