Devarda's alloy


Devarda's alloy, is an alloy of aluminium, copper and zinc.
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 owes its name to the Italian chemist Arturo Devarda, who synthesised it at the end of the 19th century to develop a new method to analyze nitrate in Chile saltpeter.
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.

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

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, while in hyperalkaline solution, the degassing of the reduced ammonia 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.