Peroxo Compounds

The peroxo acids, also called peracids for short, and their salts are of great importance for the chemical industry. They all carry the group -O-O-H or -O-O-. The perborates and percarboxylic acids are known as bleaching agents in detergents.

Sulfur also forms peroxy acids: peroxomonosulfuric acid H2SO5 (Caro’s acid) and peroxydisulfuric acid H2S2O8. They have the following structures:

The structure of peroxydisulfuric acid (left) and Caro's acid (right)
The structure of peroxydisulfuric acid (left) and Caro’s acid (right)

It can be seen that peroxomonosulfuric acid is a monoprotonic acid.

Both persulfuric acids can only be produced efficiently from sulfate solutions, using anodic oxidation on platinum electrodes. The problem is that they quickly start to react with water, to reform sulfate ions.

SO42- + H2O ⇌ SO52- + 2 H+ + 2 e

2 SO42- ⇌ S2O82- + 2 e

Since the potassium persulfates are relatively poorly soluble and can thus be removed from the equilibrium, potassium sulfate solutions are used for electrolysis. The same applies to the ammonium salts, since the potassium and ammonium ions have the same ionic radii.

You also have to work with highly concentrated solutions, since oxygen is formed in dilute solutions due to a competitive reaction:

2 SO42- + H2O —> 2 HSO4 + ½ O2

Persulfates are strong oxidizing agents

Their oxidation potential E0 is +2.1 volts and roughly corresponds to that of ozone.

Iodide is oxidized to iodine.

S2O82- + 2 I —> 2 SO42- + I2

Persulfates oxidize Mn(II) salts to manganese dioxide MnO2 · H2O or MnO(OH)2.

Mn2+ + S2O82- + 3 H2O —> MnO(OH)2 + 2 SO42- + 4 H+

When heated for a long time, persulfates release hydrogen peroxide in a slow reaction.

H2S2O8 + 2 H2O —> 2 H2SO4 + H2O2

The latter has a reducing effect on strong oxidizing agents. Therefore, a potassium permanganate solution is decolorized.

2 MnO4 + 5 H2O2 + 6 H+ —> 2 Mn2+ + 5 O2 + 8 H2O

In practice, persulfates are used similarly to hydrogen peroxide, for example as a bleaching agent. However, the oxidation via persulfates is much easier to control by adjusting the temperature. In polymer chemistry, peroxosulfates serve as polymerization starters.