Oxidation States of Manganese

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves

Equipment	Chemicals
Four 50 mL beakers	60 mL 0.01M KMnO₄
White sheet of paper	4 mL 3M H₂SO₄
	60 mL 0.02M NaHSO₃
	6 mL 2M NaOH

Procedure:

Place 15 mL of 0.01M KMnO₄ solution onto each of the four 50 mL beakers labeled A, B, C, and D. Set the beakers on a white sheet of paper to enhance visibility.
Set beaker D aside as a reference; it is the +7 oxidation state of Mn (purple)
In beaker A, add 4 mL of 3M H₂SO₄ (acidic condition). With stirring, slowly add ~32 mL of 0.02M NaHSO₃. Note the change from purple to colorless. This is the +2 state of Mn

2MnO4^- _(aq)+ H⁺ _(aq) + 5HSO₃^- _(aq) ↔ 2Mn²⁺ _(aq) + 5SO₄^2- _(aq) + 3H₂O _(l)

4) In beaker B, add 6 mL of 2M NaOH (basic condition). With stirring, slowly add ~8 mL 0.02M NaHSO₃. Note the color change from purple to green. This is the +6 state of Mn.

2MnO₄^-_(aq) + 3OH^- _(aq) + HSO₃^- _(aq) ↔ 2MnO₄^2-_(aq) + SO₄^2- _(aq) + 2H₂O _(l)

5) With stirring, slowly add ~20 mL 0.02M NaHSO₃ to beaker C (neutral condition). Note the formation of a brown precipitate, indicating the +4 state of Mn.

OH^- + 2MnO₄^-_(aq) + 3HSO₃^-_(aq) ↔ 2MnO_{2 (s)} + 3SO₄^2-_(aq)+ 2H₂O_(l)

Hazards:

Potassium permanganate is a strong oxidizer, so keep it away from any reducing agent. KMnO₄ will also stain skin. Sulfuric acid and sodium hydroxide can cause severe burns. In case of skin contact, wash with copious water.

SOP:

Corrosive – Sodium Hydroxide, Sulfuric Acid,

Oxidizer – Potassium Permanganate

Disposal (by Storeroom)

The objective is to convert all the manganese compounds to Mn(II) and discard it down the sink. Fill a 1L plastic container with 200 mL tap water. Add the contents of each beaker to this container one at a time. Acidify the solution with 1M HCl if needed. Add 2% NaHSO3 in small portions, with stirring, until all the brown manganese dioxide is reduced (solution turns clear). Neutralize and flush down the sink with plenty of water (at least 10X the total volume of solution).