Devard ash alloy

The Devardash alloy , also called Devarda alloy or Devarda for short , is an alloy of aluminum , copper and zinc named after the Italian chemist Arturo Devarda (1859–1944) . The brittle, gray mixture of metals is used as a strong reducing agent , especially in analytical chemistry . The most important applications of the alloy include the detection of nitrate and the quantitative determination of nitrates, for example in fertilizers or in wastewater .

properties

safety instructions
Surname	Devard ash alloy Devard's alloy Devarda
CAS number	8049-11-4
GHS labeling of hazardous substances danger H and P phrases H: 228 P: 210

The typical composition of 45% Al, 50% Cu and 5% Zn (mass percent) results in a ratio formula of Al _21.8 Cu _10.3 Zn or approximately Al ₂₂ Cu ₁₀ Zn when rounded . This composition corresponds to an average molar mass of 39 , 5 g mol ⁻¹ .

The composition of commercially available alloys vary in the range 44-46% aluminum, 49-51% copper and 4-6% zinc.

The Devard ash alloy is a gray, odorless powder and so brittle that it can be pulverized in a mortar. The density is 5.79 g · cm ⁻³ , the melting point is 490–560 ° C , the boiling point is 906 ° C. It is almost insoluble in water.

Other features:

• It can be easily broken up, making the use of small amounts, e.g. B. for analyzes on the microscale and allows an adapted dosage, so that it allows a more flexible application compared to aluminum granules.
• Due to the large surface area of ​​the finely ground alloy, a faster reaction occurs.
• Freshly divided metal has the advantage over zinc or aluminum powder that it is not covered to the same extent with a passivating oxide layer. The high copper content of the alloy also contributes to the fact that fewer such layers are formed than with pure Al or Zn.
• The copper has a catalytic effect through the formation of a local element , i. That is, reductions with Devardash alloy are significantly faster than with the use of zinc or aluminum.
• Reductions with aluminum often require a strongly alkaline solution. In contrast to this, reductions with Devardash alloy can be carried out in a weakly alkaline solution or even in a neutral solution, even if the reaction takes much longer in neutral solution.

The alloy is often used in a strongly alkaline environment.

Reaction with acids or bases

The aluminum and zinc dissolve in acids or alkalis, and hydrogen is developed:

${\displaystyle \mathrm {2\ Al\ +\ 6\ H_{3}O^{+}\longrightarrow 2\ Al^{3+}\ +\ 3\ H_{2}\uparrow \ +\ 6\ H_{2}O} }$

or.

${\displaystyle \mathrm {2\ Al\ +\ 2\ OH^{-}\ +\ 6\ H_{2}O\longrightarrow \ 2\ [Al(OH)_{4}]^{-}\ +\ 3\ H_{2}\uparrow } }$

and

${\displaystyle \mathrm {Zn\ +\ 2\ H_{3}O^{+}\longrightarrow \ Zn^{2+}\ +\ H_{2}\uparrow \ +\ 2\ H_{2}O} }$

Most copper remains undissolved in non-oxidizing, oxygen-free acids. The reducing effect in the application of the alloy is based in many cases on the reactivity of the resulting hydrogen.

Detection of nitrates

Devarda is used for the detection of nitrates . If you add Devarda and sodium hydroxide solution to a sample containing nitrate and carefully heat the solution, the nitrate is reduced to gaseous ammonia , which can be easily identified by its characteristic odor:

${\displaystyle \mathrm {3\ NO_{3}^{-}+8\ Al+5\ OH^{-}+18\ H_{2}O} }$

${\displaystyle \mathrm {\longrightarrow 3\ NH_{3}+8\ [Al(OH)_{4}]^{-}} }$

the Na ions of the sodium hydroxide solution and the (any) cation of the nitrate have been omitted from the formula

Kjeldahl's nitrogen determination

In the Kjeldahl nitrogen determination , initially only nitrogen components present as amines , amides or in heterocycles are determined after reaction with concentrated sulfuric acid . For further detection of nitrogen from azo or nitro groups , these must first be reduced - similar to nitrates - which can be done with Devard's alloy.

Detection of chlorates

Chlorates are reduced to chlorides by the Devardash alloy in acidic, neutral or alkaline solutions . Although the reduction is also possible by boiling with zinc dust, the reaction proceeds very slowly, especially in neutral solution with zinc, which is why the Devardash alloy is preferred. To detect chlorate, any chloride present is removed by precipitation with silver nitrate and silver sulfate . After precipitated silver chloride has been removed by filtration, a little Devarda's alloy is added and the mixture is boiled.

${\displaystyle \mathrm {ClO_{3}^{-}+3\ Zn+3\ H_{2}O\longrightarrow 3\ Zn(OH)_{2}+Cl^{-}} }$

After the copper has been filtered off, the filtrate is acidified with nitric acid and silver nitrate is added. A precipitation of silver chloride then proves that the initial sample contained chlorates.

Perchlorates do not interfere with the detection, since they are not reduced by the Devardash alloy either.

Individual evidence

1. ↑ ^{a b} Data sheet Devarda's alloy from Sigma-Aldrich , accessed on May 15, 2017 ( PDF ).
2. ↑ ^{a b c d} Datasheet Devarda Alloy (PDF) from Merck , accessed on January 19, 2011.