Trioxonitrates(v)

Trioxonitrate(V) acid forms soluble salts that can be separated out of solution by crystallization. The salts are prepared by the neutralization of the acid with alkali, reaction of the acid with a metal , reaction with a base (metallic oxide) or with a trioxocarbonate(IV).

Action of tetraoxosulphate(V) acid

The action of tetraoxosulphate(VI) acid is important to the laboratory preparation of trioxonitrate(V) acid where gaseous trioxonitrate(V) acid is liberated when trioxonitrate(V) is heated with concentrated tetraoxosulphate(VI) acid.

NO₃^-(s) + H₂SO₄(aq) → HSO₄^-(aq) + HNO₃(g)

Trioxonitrate(V) are decomposed by heat to form products that are determined by the position of the metallic component in the electrochemical series. Apart from potassium, sodium and lead trioxonitrates(V), all others contain water of crystallization that dissolve the salts when heated. The salts are said to crackle when heated and will only decompose when the entire water content is driven off.

Now take a look at some examples of trioxonitrate(V).

White sodium trioxonitrate(V) will form pale yellow sodium dioxonitrate(III) when heated.

(white) 2NaNO₃(s) → O₂(g) + 2NaNO₂(s) (pale yellow)

While colourless Zinc(II) trioxonitrate(V) is heated to yellow zinc oxide when hot, that will soon turn white when cold. Brown fume of nitrogen(IV) oxide is also formed.

(colourless) 2Zn(NO₃)₂(s) → O₂(g) + 4NO₂(g) + 2ZnO(s)

Reddish-brown (when hot) lead oxide and brown fumes of nitrogen(IV) oxide are formed from heating white lead(II) trioxonitrate(V). The lead oxide soon turns yellow when cold, with the formation of brown nitrogen(IV) oxide gas.

(white) 2Pb(NO₃)₂(s) → O₂(g) + 2PbO(s) + 4NO₂(g) (brown)

Copper nitrate is greenish-blue in colour. When heated, it forms black copper(II) oxide with brown nitrogen(IV) oxide gas.

(greenish-blue) 2Cu(NO₃)₂(s) → O₂(g) + 2CuO(s) + 4NO₂(g) (brown)

Colourless silver trioxonitrate(V) will form lustrous white silver and the brown nitrogen(IV) oxide gas when heated.

(colourless) 2AgNO₃(s) → O₂(g) + 2Ag(s) + 2NO₂(g) (brown)

There is another important trioxonitrate(V) that is not a metallic compound. It is ammonium trioxonitrate(V). It is different from other trioxonitrate(V) as it decomposes on heating to form nitrogen(I) oxide and water.

NH₄NO₃(s) → N₂O(g) + 2H₂O(l)

Note: Oxygen gas is a compulsory product evolved with other products when trioxonitrate(V) is decomposed.

The addition of drops of concentrated tetraoxosulphate(VI) acid to the side of a slant test-tube containing trioxonitrate(V) and iron(II) tetraoxosulphate(VI) allows the acid to sink to the bottom of the test-tube so that two layers of liquid is formed in the test-tube.

The interaction between the upper layer [acidified trioxonitrate(V) and iron(II) tetraoxosulphate(VI)] and the lower bottom layer (conc. tetraoxosulphate(VI) acid) results to the formation of a brown ring at the junction of the two layer. This is the basis of the brown ring test!

The brown ring is actually the brown addition compound, FeSO₄.NO, formed by three essential stages.

•The tetraoxosulphate(VI) acid solution reacts with the trioxonitrate(V) ion to form trioxonitrate(V) acid.

H₂SO₄(aq) + NO₃^-(aq) → HSO₄^-(aq) + HNO₃(aq)

•Nitrogen(II) oxide is formed when iron(II) tetraoxosulphate(VI) reacts to reduce some of the trioxonitrate(V) acid.

2HNO₃(aq) + 3H₂SO₄(aq) + 6FeSO₄(aq) → 3Fe₂(SO₄)₃(aq) + 4H₂O(l) + 2NO(g)

•More of the iron(II) tetraoxosulphate(VI) acid then combines with the nitrogen(II) oxde to form the brown addition compound that is vicible as a brown ring.

FeSO₄(aq) + NO(g) → FeSO₄.NO(aq)

Note: The brown ring will disappear when the test-tube is shaken as the heat produced by the mixing the water content with the conc. tetraoxosulphate(VI) acid will decompose the brown addition compound.

Apart from the brown ring test, two other important tests are with tetraoxosulphate(VI) acid and copper turnings.

Test with copper turnings

Reddish-brown fumes of nitrogen(IV) oxide is affirmative for the presence of trioxonitrate(V) when a mixture of a sample and copper turnings is warmed with some concentrated tetraoxosulphate(VI) acid. The tetraoxosulphate(VI acid will initially react with the nitrate ion to form trioxonitrate(V) acid.

H₂SO₄(aq) + NO₃^-(s) → HNO₃(l) + HSO₄^-(aq)

The trioxonitrate(V) acid then reacts with the copper turnings to form the reddish-brown nitrogen(IV) oxide gas.

Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

Test with tetraoxosulphate(VI) acid

Here, solid samples containing trioxonitrate(V) ion are treated with little concentrated tetraoxosulphate(VI) acid to condense to some oily drops of trioxonitrate(VI) acid at the upper part of the test-tube.

H₂SO₄(aq) + NO₃^-(s) → HNO₃(l) + HSO₄^-(aq)

The trioxonitrate(V) acid will decompose to brown fumes of nitrogen(IV) oxide when heated.

4HNO₃(l) → 4NO₂(g) + O₂(g) + 2H₂O(l)