http://www.lamotte.com/images/pdf s/instructions/7414.pdf Step 1: Manganous (Manganese II) Sulfate (Manganous Sulfate Solution) reacts with Potassium Hydroxide (Alkaline Potassium Iodide Azide Solution [KOH & KI]) to form Manganous (Manganese II) Hydroxide and Potassium Sulfate. MnSO4 + 2KOH ⎯⎯→ Mn(OH)2 + K2SO4 A precipitate is formed… Mn(OH)2 (s) Step 2: Manganous (Manganese II) Hydroxide reacts with Oxygen & Water to produce Manganic (Manganese III) Hydroxide, (a redox reaction). The ratio of oxygen to Manganese II Hydroxide is 1:4. 4Mn(OH)2 (s) + O2 (aq) + 2H2O (l) ⎯⎯→ 4Mn(OH)3 (s) Product is brown precipitate. Step 3: Manganic (Manganese III) Hydroxide reacts with Sulfuric Acid to produce Manganic (Manganese III) Sulfate and Water. 2Mn(OH)3 (aq) + 3H2SO4 (aq) ⎯⎯→ Mn2(SO4)3 (s) + 6H2O (l) Oxygen is considered “fixed” at this point. The previous reaction is immediately followed by Manganic (Manganese III) Sulfate reacting with Potassium Iodide (from the step 1 addition of Alkaline Potassium Iodide Azide Solution) to form Manganous (Manganese II) Sulfate and Potassium Sulfate and Iodine. (another redox reaction) Mn2(SO4)3 (s) + 2KI (aq) ⎯⎯→ 2MnSO4 (aq) + K2SO4 (aq) + I2 (aq) The iodine causes the solution to appear yellow-brown. The amount of iodine is proportional to the amount of oxygen in Step 2. Step 4: Sodium Thiosulfate (S2O3-2) reacts with the Iodine to form Sodium Tetrathionate (S4O6-2) and Sodium Iodide. 2Na2S2O3 (aq) + I2 (aq) ⎯⎯→ Na2S4O6 (aq) + 2NaI (aq) When all of the iodine has finished reacting, the solution changes color from yellow-brown to colorless. Starch indicator may be added to enhance this endpoint. It will form a blue-black complex with iodine (not iodide ion) when present.