Hypomanganate

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Hypomanganate
Hypomanganate solution in 40% NaOH
Names
IUPAC name
Hypomanganate
Other names
Manganate(V)
Properties
MnO43-
Molar mass 118,934 g/mol
Appearance Turquoise solid
Disproportionate in to hydrated manganese dioxide and permanganate
Solubility Soluble in cold concentrated solutions of alkali metal hydroxides
Acidity (pKa) 13,7
Related compounds
Related compounds
Manganese dioxide
Potassium manganate
Potassium permanganate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Hypomanganate is the name for the anion containing manganese in V oxidation state. This ion have formula MnO43-. It have intense turquoise colour.

Structure

Hypomanganate anion consist of one manganese atom surrounded with four oxygen atoms in a tetrahedral arrangement.

Properties

Solutions of hypomanganates are stable only in very strongly alkaline conditions at lower temperatures (0-15°C). In less alkaline solutions or at higher temperatures they decompose in to manganate and hydrated manganese dioxide:

MnO43- + H2O → HMnO42- + OH-
2HMnO42- → MnO42- + MnO2 + 2OH-

In neutral or acidic solutions disproportionation goes further in to hydrated manganese dioxide and permanganate:

2MnO42- + 4H3O+ → MnO2 + MnO4- + 6H2O

Hypomanganates are mild oxidizing agents. They can oxidize hydrogen peroxide in to oxygen:

2MnO43- + H2O2 + 2H2O → 2MnO2 + O2 + 6OH-

Cold solutions doesn’t react with ethanol. However hot solutions can oxidize it. This is caused by disproportionation of hypomanganate in to manganate which have better oxidation properties:

MnO42- + CH3-CH2-OH → MnO2 + CH3-CHO + 2OH-

Hypomanganates can be oxidized by permanganates in to manganates:

MnO43- + MnO4- → 2MnO42-

Solid hypomanganates are stable up to 900°C.

Preparation

Solutions of hypomanganates can be prepare by reduction of potassium permanganate by sodium thiosulfate, sodium sulfite, potassium iodide or acetone in 40% NaOH at 0-5°C. Reducing agent should be add as a solid or concentrated solution in 20% NaOH to avoid dilution of 40% NaOH medium.

Reaction have two steps. First step is quick reduction of permanganate in to green manganate:

8MnO4- + S2O32- + 10OH- → 8MnO42- + 2SO42- + 5H2O

Second step is slow reduction of manganate in to turquoise hypomanganate. This reaction lasts about half an hour:

8MnO42- + S2O32- + 10OH- → 8MnO43- + 2SO42- + 5H2O

From these solutions can be precipitate barium hypomanganate by addition of solution of barium salt:

Ba2+ + MnO43- → Ba3(MnO4)2

Solid hypomanganates can be prepare by heating manganese dioxide with sodium nitrite and sodium hydroxide:

MnO2 + NaNO2 + 2NaOH → Na3MnO4 + NO + H2O

Another method is oxidation of manganese dioxide by dry air in molten sodium or potassium hydroxide:

4MnO2 + O2 + 12KOH → 4K3MnO4 + 6H2O

Similar preparation is heating of manganese carbonate with potassium carbonate in the presence of dry air:

4MnCO3 + 3O2 + 6K2CO3 → 4K3MnO4 + 10CO2

Barium hypomanganate can be also make by heating of any manganese oxide with excess of barium oxide at 900°C:

4MnO2 + O2 + 6BaO → 2Ba3(MnO4)2

Projects

Making barium hypomanganate

Handling

Safety

Hypomanganates are oxidizing agents. They should be kept away from reducing agents and combustible materials.

Hypomanganates are harmful if ingested.

Solutions of hypomanganates are very corrosive because they contain high concentration of hydroxide. When working with it, gloves and goggles should be weared.

Hypomanganates can‘t be mixed with concentrated sulfuric acid because of formation of unstable and explosive manganese heptoxide.

Storage

Solutions should be stored in fridge with excess of reducing agent. In these conditions they are relatively stable.

Solids should be stored in desiccator because moisture and CO2 cause disproportionation.

Disposal

Small amount of dilute solution can be poured in to the drain with lot of water. Larger amounts can be dilute, neutralized with acid and reduced by reducing agent in to hydrated manganese dioxide, which can be filtred off and brought in to municipal waste processing facility.

References

  • Personal experience with handling and investigating hypomanganates by forum user Bedlasky
  • Heinrich Remy - Inorganic chemistry

Relevant Sciencemadness threads