25.3.8 Manganese V
Manganese V is very hard to produce. There have been numerous attempts using reducing agents on permanganate or manganate starting points or oxidizing manganese II or IV. Here we oxidize manganese IV oxide using only air in alkaline conditions.
1. Add 350mg of manganese dioxide and 530mg of sodium hydroxide ( a 10% excess) to a pestle and mortar and carefully grind together to form a smooth powder.
2. Quickly transfer this powder to a TOTALLY DRY tube. Do not dither about as the powder will absorb moisture from the atmosphere quickly and water will destroy the experiment!
3. Heat by holding the tube over a Bunsen with the lid removed. Heat for 15s then remove the tube for 15s. Repeat this until 4 minutes have passed. Now allow the tube to cool for 1 minute. Repeat the whole of this step a further 3 times then remove and allow to cool for 4 extra minutes. At all times make sure any steam is allowed to escape the tube and not run down the sides. Manganese V is instantly destroyed by water.
4. Watch the tube as it cools. The colour will lighten and a bright blue crust of sodium hypomanganate will have formed above the black dioxide. Where any water has touched the tube green manganese VI will be formed instead.
12NaOH + 4MnO2 + O2 gives 4Na3MnO4 + 6H2O.
A photograph of the reaction product is shown below.
25.3.3 Oxidation of Manganese II with Persulfate
In these experiments I look at oxidation reactions involving Manganese II sulfate and sodium persulfate. Concentration, PH and temperature are varied to give a surprisingly complex set of results.
Sequence 1:Mn2+ to MnO2. Neutral PH
1a. Dissolve 200mg sodium persulfate and 50mg of manganese sulfate in 3ml water. There is no reaction even after 15minutes, the liquid remains clear.
1b. Heat in water bath to 75C. A feint orange-brown colour appears at about 55C and by 70C a strong brown colour and eventually clumps of MnO2 form. Heat is required to oxidise manganese 2+ to MnO2 in neutral PH.
Sequence 2: Mn2+ to MnO2 Alkali
2. Dissolve 200mg sodium persulfate and 50mg of manganese sulfate in 3ml water. Add 1ml of 6% sodium hydroxide solution. Instantly a dark brown mass of MnO2 is formed. Heating is not necessary to oxidize manganese 2+ to MnO2 when alkali is used.
Sequence 3:Control Sequence for Mn(OH)2
3a. Dissolve 50mg manganese sulfate in 3ml of 6% sodium hydroxide solution. A mid brown liquid with a paler brown precipitate underneath. This is not plain Mn(OH)2 which is white but probably an aerial oxidation product, perhaps Mn(OH)3 or even MnO(OH)2. It may be worth repeating this using less hydroxide.
Sequence 4: Mn2+ to MnO2 in Strong Alkali
4. Dissolve 200mg sodium persulfate and 50mg of manganese sulfate in 3ml sodium hydroxide solution. A fine black precipitate forms. This is the anhydrous form of MnO2. Heating produced no further change. When strong alkali is used then the brown hydrated form of MnO2 is not formed but only the black anhydrous form.
Sequence 5: Mn2+ to Mn7+ in intense Alkali
5a. Dissolve 200mg sodium persulfate, 150mg of solid sodium hydroxide and 50mg of manganese sulfate in 1ml 6% sodium hydroxide solution. The tube will heat up rapidly.
5b. Heat in water bath to 75C agitating the contents occasionally. A purple colour forms with a black precipitate. The purple colour is permanganate the black precipitate is anhydrous MnO2 like in the previous sequence.
5c. Carefully pour the contents of the tube onto a stack of paper towels. The purple colour will flash to green and then brown confirming that we have made some permanganate. Notes: Sequences I & II can be combined to show that persulfate and Mn2+ only react if one or more of heat and alkali is used.
25.4.1 The Aqueous Oxidation States of Manganese
1. Prepare three pots of potassium permanganate solution as follows:
Pot A: Add 25ml of water and then carefully add grains of sodium bisulfate until the PH drops to 2.0. Add about 70mg of potassium permanganate and dissolve.
Pot B: Add 25ml of water and 70mg of potassium permanganate and dissolve.
Pot C: Add 25ml of water and then carefully add sodium hydroxide solution until the PH rises to 11.5. Add about 70mg of potassium permanganate and dissolve.
2. In a separate pot dissolve 500mg of sodium sulfite in 30ml water.
3. Add a few drops of solution B onto a white tile of surface. Observe the intense purple colour of the permanganate ion. Now add a drop of sulfite to the drops. A brown/yellow colour will be seen with some brown precipitate. The sulfite has reduced the permanganate to manganese (IV) oxide. Add more sulfite. The only oxidation state that is produced in neutral conditions is IV as MnO2.xH2O.
4. Take a clean white surface and now add a few drops of solution C. Add one drop of sulfite and observe the sea green colour that is formed. This is the manganate ion MnO4 2- . Slowly add more sulfite. The green colour turns yellow/brown and flecks of precipitate are seen. This is now MnO2 as seen in step 3. In alkaline conditions manganese is reduced to oxidation state VI then to IV.
5. Take another clean white surface and add a few drops of solution A. Add one drop of sulfite. No change is seen. Slowly add more drops of sulfite until the colour is a ruby red. This may be manganese III with some manganese VII still present. It may also be a complex of manganese III and sulfite. Continue to slowly add sulfite until quite suddenly the colour fades totally and a clear liquid remains. This is manganese II. In acidic conditions manganese is reduced to oxidation state III and then to II. No other states are stable. It may be possible to produce better results by using PH 1 for acid and 13 for alkaline.
Oxidation State Stable in Colour
+7 Acid, Neutral and Base Purple
+6 Base Green
+5 Not Stable – Not observed Blue
+4 Neutral, Base Yellow/orange/brown
+3 Acid Pink/red/flesh
+2 Acid, Neutral and Base* colourless *
*This is not observed in neutral and base when reducing permanganate due to the stability of the manganese dioxide produced.
25.3.9 Disproportionation of Manganese VI
1. Add a solution of potassium manganate to a large quantity of water. The green colour soon changes to purple and a sediment settles on the bottom.
The manganate has undergone disproportionation to manganese dioxide and permanganate:
3K2MnO4 + 2H2O gives 2KMnO4 + MnO2 + 4KOH
A video of the reaction is shown below
25.3.5 The Dry Oxidation of Manganese (IV) to Manganese (VI)
1. Add 160mg of manganese dioxide, 280mg of potassium hydroxide and 260mg of potassium nitrate to a clean, dry tube. Shake to mix together.
2. Heat over a flame until just molten (about 3 minutes). Some steam is expelled.
3. Allow the tube to cool and then add 2ml of 6% sodium hydroxide solution. Shake. A beautiful deep green solution of potassium manganate (VI) is formed.
MnO2 + 2KOH + KNO3 + Heat gives K2MnO4 + KNO2 + H2O
A video of the reaction product being dissolved in water is shown below
25.3.4 Oxidation of Manganese (II) in Acid
Here I investigate the oxidation of manganese in bisulfate and then hydrochloric acid. If sulfuric acid is available then that may produce better results than bisulfate.
1. Dissolve 70mg of manganese sulfate, 70mg sodium bisulfate and 100mg of sodium persulfate in 3ml water.
2. Heat in a water bath for 30 minutes. After 1-2 minutes a flesh-peach colour of manganese 3+ ions can be seen. The colour then shifts towards brown/lavender (brown when lit from behind, lavender when viewed through the solution) then a black precipitate forms, probably of MnO2.
3. In a new tube dissolve 70mg of manganese sulfate and 100mg of sodium persulfate to 2ml of 6% hydrochloric acid.
4. Heat for 10 minutes or more in a water bath. Orange-pink manganese 3+ ions form but at a slower speed than with bisulfate. There is no further change.
It would appear that using sulfate the formation of manganese 3+ is quicker and that oxidation to +4 is possible. With chloride the oxidation is slower and only by 1 electron.
A photo of step 4 is shown below.
1. Dissolve 40mg of potassium permanganate in 8ml of water.
2. Dissolve 100mg of sodium metabisulfate in 4ml water.
3. Add 2. to 1. Almost immediately the intense purple becomes clear with a brown top. Shake. The liquid becomes totally clear. Is this Mn2+ or a complex between MnO2 and metabisulfate?
4. Repeat steps 1 and 2 but now add 2. to 1. a few drops at a time. First a dirty red colour is observed then heavy MnO2 precipitates out. Eventually this redissolves and the liquid is clear and pale yellow brown, then totally colourless.
5. Add 2ml of 6% sodium hydroxide to 4. A dirty orange jelly-like mass forms which is identical to the reaction product of Mn2+ and hydroxide. The clear liquid is Mn2+ after all.
Manganese - Reduction/Oxidation
A photo showing the MnO2 sediment is shown below