Copper in Nitric Acid

Copper Meets Strong Acid

Concentrated nitric acid in a laboratory is 68 to 70% acid, about 15 mol in a litre. Copper turnings dropped in react almost at once: bubbles rise off the metal, the liquid quickly turns green, and a red-⁠brown gas wells up in the flask.

The gas is nitrogen dioxide: Cu + 4 HNO₃ → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O. The copper gives up electrons and goes into solution as copper ions; the nitrogen in the acid takes them and becomes nitrogen dioxide. No hydrogen is given off.

The Royal Society of Chemistry’s demonstration uses 5 g of copper turnings and 40 cm³ of concentrated acid, done by a teacher in a fume cupboard with the sash pulled down in front. It is fast, and slows only after about 80 s.

Cu + 4 HNO₃ → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O · concentrated acid about 68⁠–⁠70%, about 15 mol⁠/⁠L

More Than the Flask Holds

By the equation, 5 g of copper makes about 0.16 mol of nitrogen dioxide: about 3.8 L at 20 °C and one atmosphere, fifteen times this 250 mL conical flask. The flask is soon full, and the gas wells out of its mouth.

Nitrogen dioxide’s vapour density is 1.58 times air’s. It does not drift up: it spills over the lip, sinks and gathers low down.

A fume cupboard draws air in at the open front and takes it away through slots in the back. For heavy vapours the slot nearest the worktop should draw the most.

5 g of copper → about 3.8 L of nitrogen dioxide (20 °C) · vapour density 1.58 (air = 1)

Red-⁠Brown and Colourless

Nitrogen and oxygen join into several molecules. Nitrogen dioxide, NO₂, is red-⁠brown; nitric oxide, NO, has no colour at all.

Colour is light taken out. By its measured cross sections, NO₂ absorbs most strongly at 400 nm, in the violet; blue light at 450 nm is still taken at three-⁠quarters of that, and red at 650 nm at only about a fiftieth. With the violet and blue gone, what comes through leans to yellow and red: red-⁠brown. NO absorbs outside the visible, so it cannot be seen.

The textbook puts NO₂’s colour down to an unpaired electron. Two NO₂ molecules can also join into colourless N₂O₄, and the colour fades in the cold; that reversible change is left to a later page on equilibrium.

Cross section: strongest at 400 nm · about a fiftieth at 650 nm · NO absorbs no visible light

Green, Then Blue

Copper going into nitric acid becomes copper(II) ions, and copper ions dissolved in water are blue. Yet in concentrated acid the solution is green at first.

The Royal Society of Chemistry’s explanation: in strong acid there are so many nitrate ions that they gather round the copper ion, and copper held that way is green; add water and the water takes their place, giving the familiar blue. When water is let into the flask in the demonstration, the green solution turns blue.

The blue belongs to the copper ions in solution: it is not the colour of copper metal, and not the colour of nitrogen dioxide.

Green while strong · blue once diluted · the blue is the copper ions in solution

A Colourless Gas from Dilute Acid

In the boiling tube beside the flask, a coil of copper wire sits in dilute nitric acid. Bubbles come off the wire too, and the liquid slowly turns blue, but the gas in the tube has no colour you can see.

Copper and dilute nitric acid give mainly nitric oxide: 3 Cu + 8 HNO₃ → 3 Cu(NO₃)₂ + 2 NO + 4 H₂O. The same 1 g of copper gives about 757 mL of nitrogen dioxide in concentrated acid, and only about 252 mL of nitric oxide in dilute.

Nitric oxide meeting the oxygen of the air becomes nitrogen dioxide: 2 NO + O₂ → 2 NO₂. So the brown shows at the mouth of the tube, where the air gets in. Real reactions always give a mixture: the stronger the acid, the more NO₂; in a 2024 study, copper dissolving in 8 to 11 mol⁠/⁠L acid at 80 °C still gave only about nine parts in ten NO₂.

3 Cu + 8 HNO₃ → 3 Cu(NO₃)₂ + 2 NO + 4 H₂O · 2 NO + O₂ → 2 NO₂

Hours Later

Nitrogen dioxide is toxic, and the harm can come late. The US Agency for Toxic Substances and Disease Registry writes that after breathing it in, a symptom-⁠free period of 3 to 30 hours may be followed by pulmonary oedema.

Britain’s workplace limits are 0.5 ppm over 8 hours and 1 ppm over 15 minutes. NIOSH in the US counts 13 ppm as immediately dangerous to life or health.

So only a teacher does this demonstration, in a fume cupboard, with small amounts and the sash pulled down in front. This page only draws it; it does not say how.

Symptoms may come 3⁠–⁠30 hours later · UK limits 0.5 ppm (8 hours), 1 ppm (15 minutes) · NIOSH IDLH 13 ppm

Ammonia Burned into Acid

Industry makes nitric acid from ammonia, which is made as on the page “Nitrogen from the Air”. Ammonia and air pass through a pad of platinum⁠–⁠rhodium gauzes and burn at 810 to 940 °C to nitric oxide: 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O. The gauze is about nine parts platinum to one of rhodium, and 95 to 97% of the ammonia becomes nitric oxide.

As the gas cools, the nitric oxide joins oxygen as nitrogen dioxide: the very step at the mouth of the tube. In absorption towers the nitrogen dioxide meets water and becomes nitric acid: 3 NO₂ + H₂O → 2 HNO₃ + NO, and the nitric oxide set free goes round to join oxygen again. What comes out is 50 to 65% acid.

Taken together: NH₃ + 2 O₂ → HNO₃ + H₂O. Most of the acid goes into ammonium nitrate fertiliser. The method is called the Ostwald process; Wilhelm Ostwald’s Nobel Prize in Chemistry, in 1909, was for his work on catalysis, chemical equilibria and reaction rates.

4 NH₃ + 5 O₂ → 4 NO + 6 H₂O · platinum⁠–⁠rhodium gauze at 810⁠–⁠940 °C · NH₃ + 2 O₂ → HNO₃ + H₂O

Air Made Very Hot

Air holds nitrogen and oxygen, and normally they do not join: N₂ + O₂ → 2 NO takes in a great deal of heat. From the thermodynamic tables, air at room temperature holds next to no nitric oxide at equilibrium; heated to 1500 K, about 0.13%; at 2000 K, about 0.8%; at 2500 K, about 2.3%.

The textbook says that heating nitrogen and oxygen together makes a little nitric oxide. The US Environmental Protection Agency says nitrogen dioxide gets into the air mainly from burning fuel; 90% or more of a car’s tail-⁠pipe nitrogen oxides are nitric oxide, which in the air turns into nitrogen dioxide mainly by reacting with ozone.

Lightning does it too: the world’s lightning makes nitrogen oxides holding about 5 million tonnes of nitrogen a year (2 to 8 million), about a tenth of all sources; burning fossil fuels makes about six times as much. See the page “See the Flash, Then Hear the Thunder”. In 1785 Henry Cavendish passed spark after spark through air shut in a tube, and the alkali in the tube became a solution of nitre.

N₂ + O₂ → 2 NO · equilibrium NO in air: 1500 K about 0.13% · 2000 K about 0.8% · 2500 K about 2.3% (estimated)

In City Air

The US Environmental Protection Agency lists cars, trucks, buses, power plants and off-⁠road equipment as the sources of nitrogen dioxide. Of US nitrogen oxide emissions in 2023, highway vehicles made about 22% and non-⁠road mobile sources about 24%; in the 27 countries of the EU, transport made about 57% that year.

Nitrogen dioxide irritates the airways and can make asthma worse. The US standard is 100 ppb over an hour and 53 ppb over a year; the World Health Organization’s 2021 guideline is 10 µg in a cubic metre over a year, about 5.3 ppb.

A petrol car’s three-⁠way catalyst reduces nitrogen oxides to nitrogen, provided the engine runs close to the chemically exact mixture of fuel and air; diesel exhaust is rich in oxygen, so a three-⁠way catalyst will not do; urea is sprayed in, and selective catalytic reduction turns the nitrogen oxides into nitrogen and water.

US 2023: highway vehicles 22% · non-⁠road mobile 24% · standard 100 ppb (1 hour) · WHO 10 µg⁠/⁠m³ (annual mean)

A Copper Cent

The American chemist Ira Remsen told in an address of something from his youth: reading in a book that “nitric acid acts upon copper”, he wanted to see what “acts upon” meant, so he put a copper cent on the table and poured nitric acid on it.

In his words, a greenish blue liquid foamed and fumed over the cent and over the table; the air nearby became coloured dark red, and a great coloured cloud arose, disagreeable and suffocating. Trying to throw the mess out of the window, he learned that nitric acid acts upon fingers too; he drew them across his trousers, and found that nitric acid acts upon trousers.

The account was printed in 1920, in Benjamin Harrow’s book. The green liquid and the dark red gas he saw are what the fume cupboard demonstration shows today.

Ira Remsen’s account · B. Harrow, Eminent Chemists of Our Time, 1920

Part by Part

Seven things: the conical flask, the spill tray, the retort stand, the clamp, the test-⁠tube rack, the boiling tube and the stock bottle. What matters most: small amounts, a fume cupboard, and the sash in front.

Conical flask · spill tray · retort stand · clamp · test-⁠tube rack · boiling tube · stock bottle