Hypomanganate solution in 40% NaOH
| IUPAC name
| Other names
|Molar mass||118.934 g/mol|
|Disproportionate in to hydrated manganese dioxide and permanganate|
|Solubility||Soluble in cold concentrated solutions of alkali metal hydroxides|
|Acidity (pKa)||13.7 ± 0.2|
| Manganese dioxide|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Hypomanganate is the name for the anion containing manganese in V oxidation state. This anion have formula MnO43-. It have intense turquoise colour.
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:
- MnO43- + H2O → HMnO42- + OH-
- 2HMnO42- → MnO42- + MnO2 + 2OH-
- 2MnO42- + 4H3O+ → MnO2 + MnO4- + 6H2O
- 2MnO43- + H2O2 + 2H2O → 2MnO2 + O2 + 6OH-
They are also reduced by sodium dithionite:
- 2MnO43- + S2O42- + 2H2O → 2MnO2 + 2SO32- + 4OH-
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.
- MnO42- + CH3-CH2-OH → MnO2 + CH3-CHO + 2OH-
- MnO43- + MnO4- → 2MnO42-
Solid hypomanganates are stable up to 900°C.
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.
- 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 ↓
- MnO2 + NaNO2 + 2NaOH → Na3MnO4 + NO + H2O
- 4MnO2 + O2 + 12KOH → 4K3MnO4 + 6H2O
- 4MnCO3 + 3O2 + 6K2CO3 → 4K3MnO4 + 10CO2
- 4MnO2 + O2 + 6BaO → 2Ba3(MnO4)2
- Making barium hypomanganate
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.
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.
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.
- Personal experience with handling and investigating hypomanganates by forum user Bedlasky
- Heinrich Remy - Inorganic chemistry