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Trioxosulphate(IV) Acid, H₂SO₃

Trioxosulphate(IV) acid - H₂SO₃ is formed by the dissolution of SO₂ in water.

SO_2(g) + H₂O_(l) → H₂SO_3(aq)

Properties of H₂SO₃

Trioxosulphate(IV) acid is a colourless liquid, which smells strongly of SO₂. It has all the reducing properties of SO₂ in the presence of water.

The reaction of trioxosulphate(IV) acid, H₂SO₃ with alkali, such as sodium hydroxide, NaOH or potassium hydroxide KOH produces two salts: a normal salt and an acid salt.

2NaOH_(aq) + H₂SO_3(aq) → Na₂SO_3(aq) + 2H₂O_(l) and

NaOH_(aq) + H₂SO_3(aq) → NaHSO_3(aq), + H₂O_(l)

The effect of acids on salts of trioxosulphate (IV):

Sulphur(IV) oxide, SO₂ , is always given off when salts of trioxosulphate(IV) are warmed with dilute acids (such as hydrochloric acid and H₂SO₄.

This occurrence is sometimes used as a method to prepare SO₂.

Na₂SO_3(aq) + H₂SO_4(aq) → Na₂SO_4(aq) + H₂O_(l) + SO_2(g)

Note: Salts of trioxosulphate(IV) used in dilute acidified solutions show all the reducing properties of trioxosulphate(IV), or of sulphur(IV) oxide in water.