Q8(a)(ii). Peroxomonosulfuric acid (Caro's acid)

Preparation

1. Hydrolysis of peroxodisulfuric acid (or its salts in H₂SO₄): one S–O–O–S link is cut: H₂S₂O₈ + H₂O → H₂SO₅ + H₂SO₄. It is also a by-product of the electrolytic preparation of H₂S₂O₈ from H₂SO₄.
2. From chlorosulfuric acid and anhydrous hydrogen peroxide: ClSO₃H + H₂O₂ → H₂SO₅ + HCl
3. From concentrated sulfuric acid and H₂O₂ (an equilibrium): H₂SO₄ + H₂O₂ ⇌ H₂SO₅ + H₂O

Properties

1. A colourless crystalline solid; unstable, and may decompose explosively with organic matter.
2. With water it slowly gives hydrogen peroxide: H₂SO₅ + H₂O → H₂SO₄ + H₂O₂; its salts are also unstable and decompose, producing H₂O₂.
3. A very strong oxidising agent: it liberates iodine from KI at once (H₂O₂ and peroxodisulfate react only slowly), oxidises Fe²+ to Fe³+, aniline to nitrosobenzene, and ketones to esters (Baeyer–Villiger oxidation, first done with Caro's acid).
4. Only the OH proton is strongly acidic; the OOH proton is weak. Its salts are hydrogenperoxosulfates, KHSO₅.

Structure

1. S is tetrahedral: two S=O, one S–OH and one S–O–OH group. The O–O (peroxo) link carries the oxidising power.
2. Oxidation state of S is +6, not +8: the two peroxo O are -1 each: 2(+1)+x+3(-2)+2(-1)=0, x=+6

Application. the stable triple salt 2KHSO₅.KHSO₄.K₂SO₄ (“Oxone”) is used as a non-chlorine oxidant in swimming-pool shock treatment, denture cleaners and organic synthesis; the H₂SO₄/H₂O₂ mixture cleans glassware and silicon wafers.