Hypomanganate

Hypomanganate

Hypomanganate solution in 40% NaOH
Names
IUPAC name Hypomanganate
Other names Manganate(V)
Properties
Chemical formula	MnO43-
Molar mass	118.934 g/mol
Appearance	Turquoise solid
Solubility in water	Disproportionate in to hydrated manganese dioxide and permanganate
Solubility	Soluble in cold concentrated solutions of alkali metal hydroxides
Acidity (pKa)	13.7 ± 0.2
Related compounds
Related compounds	Manganese dioxide Manganate 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 anion have formula MnO₄^3-. 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 low temperatures (0-15°C). In less alkaline solutions or at higher temperatures they decompose in to manganate and hydrated manganese dioxide:

MnO₄^3- + H₂O → HMnO₄^2- + OH^-

2HMnO₄^2- → MnO₄^2- + MnO₂ + 2OH^-

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

2MnO₄^2- + 4H₃O⁺ → MnO₂ + MnO₄^- + 6H₂O

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

2MnO₄^3- + H₂O₂ + 2H₂O → 2MnO₂ + O₂ + 6OH^-

They are also reduced by sodium dithionite:

2MnO₄^3- + S₂O₄^2- + 2H₂O → 2MnO₂ + 2SO₃^2- + 4OH^-

They can oxidize certain organic compounds such sucrose, tartaric acid, ascorbic acid or sorbitol, while they are reduced in to hydrated manganese dioxide.

They can oxidize glucose and fructose as well, but with different course of reaction. Firstly is hypomanganate reduced in to hydrated manganese dioxide. If it’s sugar in excess then reduction goes further in to Mn²⁺. In this case are Mn²⁺ ions present as colourless complex with glucose or fructose which is in excess of reagent stable against aerial oxidation (sugar prevents oxidation of Mn(II) in to Mn(III)). At the surface of solution can be observe brown ring which indicates oxidation by atmospheric oxygen. After swirling of the solution brown ring disappear for a while. Reduction by glucose is much slower than reduction by fructose.

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 oxidizing properties:

MnO₄^2- + CH₃-CH₂-OH → MnO₂ + CH₃-CHO + 2OH^-

Oxidation of ethanol by hypomanganate in hot water bath. From left: 1. Beginning of the reaction; 2. Disproportionation; 3. Reduction of manganate in to MnO₂.nH₂O

Hypomanganates can be oxidized by permanganates in to manganates:

MnO₄^3- + MnO₄^- → 2MnO₄^2-

Solid hypomanganates are stable up to 900°C.

Preparation

Solutions of hypomanganates can be prepared by reduction of potassium permanganate by sodium thiosulfate, sodium sulfite, potassium iodide, sodium tartrate 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^- + S₂O₃^2- + 10OH^- → 8MnO₄^2- + 2SO₄^2- + 5H₂O

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

8MnO₄^2- + S₂O₃^2- + 10OH^- → 8MnO₄^3- + 2SO₄^2- + 5H₂O

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

Ba²⁺ + MnO₄^3- → 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(II) 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 (preferably under an inert atmosphere, but it isn't necessary). 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

• https://dk.upce.cz/bitstream/handle/10195/19372/D10892.pdf?sequence=1&isAllowed=y

• https://colourchem.wordpress.com/2019/07/13/manganese-redox-chemistry-in-alkaline-media/

• https://en.wikipedia.org/wiki/Hypomanganate

Relevant Sciencemadness threads

• Manganese redox chemistry in strongly alkaline media (hypomanganates, manganates)