Potassium ferrate

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Potassium ferrate
Ferrat1.jpg
Dry potassium ferrate in a jar
Ferrat2.jpg
Potassium ferrate in solution
Names
IUPAC name
Potassium ferrate
Systematic IUPAC name
Potassium ferrate (VI)
Properties
K2FeO4
Molar mass 198.04
Appearance Dark purple crystals
Odor None
Density 2.829
Melting point 198 °C (388 °F; 471 K) decomposes
Soluble (slowly decomposes unless pH is high)
Solubility in ethanol Insoluble (0 °C and below)
Reacts with solvent (20 °C)
Related compounds
Related ferrates
barium ferrate
sodium ferrate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium ferrate is a compound with a formula of K2FeO4. This is an unstable potassium salt of ferric acid, which itself is too unstable to exist in aqueous solution. It is a dark purple crystalline solid that dissolves to form a reddish-purple solution. This solution is stable at high pH, but at neutral or lower pH, the ferrate decomposes, liberating oxygen.

Potassium ferrate is a very strong, but relatively mild oxidizer.

Properties

Physical

It is a dark purple crystalline solid that dissolves to form a reddish-purple solution. Its solubility in water is limited, which makes potassium ferrate the most practical ferrate: it is soluble in water, unlike the insoluble barium ferrate, but relatively easy to crystallize out of the solution, unlike the very soluble sodium ferrate which is particularly hard to isolate as a solid salt.

Chemical

It is a very potent oxidizer. It oxidizes primary alcohols to carboxylic acids, carboxylic acids to carbon dioxide and water, chlorates to perchlorates, ammonia to nitrogen, nitrites and nitrates. It is not known whether it is capable of oxidizing bromate to perbromate.

At high pH, the oxidizing strength of ferrate is relatively low, and the ferrate itself can be prepared with weaker oxidizers such as hypochlorites and bromine. At low pH, the full oxidizing power of ferrate is revealed.

Ferric acid, of which it is a salt, is a mid-strength acid, between hydrofluoric and formic in pKa. However, since it immediately decomposes on formation, any acidic compound, including carbon dioxide in the air, displaces ferric acid from its salt according to Le Chatelier's principle. Thus, potassium ferrate, itself a slightly basic compound, reacts with all acids, evolving oxygen and precipitating iron (III) oxide or hydroxide.

Availability

There is a growing market for potassium ferrate as a practical and ecologically friendly water purificant. As such, it can be bought from certain companies.

Preparation

There are several methods of ferrate synthesis. The most practical are the electrolytic method and the hypochlorite method. In the electrolytic method, a solution of potassium hydroxide is electrolyzed with iron electrodes. In the hypochlorite method, salts of trivalent iron are oxidized with hypochlorites in solution.

In any case, preparing a solution of potassium ferrate isn't very hard, but crystallization of the stable pure salt is. It cannot be crystallized by evaporating the solution, because this provokes it to decompose, but it can be displaced from the solution by adding excess KOH. The trick is to quickly crystallize the ferrate, filter, dissolve in a weak solution of potassium hydroxide, recrystallize by adding a lot more hydroxide, filter and dry very quickly, because the impure compound decomposes on the fly. Vacuum filtering through sintered glass is mandatory, because ferrate oxidizes filter paper. Ice-cold anhydrous ethanol or diethyl ether is recommended for washing the crystals.

Handling

Safety

Potassium ferrate is non-toxic. The products of its decomposition aren't toxic either. However, dry potassium ferrate should not come in contact with flammable organic compounds. There were reports of warm ethanol catching fire on contact with potassium ferrate.

Storage

Potassium ferrate should be stored in a dark place, without access to air (it reacts with carbon dioxide in the air). Ideally, it should be kept under vacuum or inert gas.

Disposal

Potassium ferrate solutions can just be poured into the ground or drain. Contact with any organics causes the ferrate to be quickly reduced and decomposed.