Dissolved Oxygen Kit Reactions

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

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