LaMotte Dissolved Oxygen Water Quality Test Kit - Manual (Page 7)

7
Simultaneously, iodine from the potassium iodide in the Alkaline Potassium Iodide
Azide Solution is oxidized by manganic sulfate, releasing free iodine into the
water. Since the manganic sulfate for this reaction comes from the reaction between
the manganous hydroxide and oxygen, the amount of iodine released is directly
proportional to the amount of oxygen present in the original sample. The release of
free iodine is indicated by the sample turning a yellow-brown color. Chemically, this
reaction can be written as:
Mn
2
(SO
4
)
3
+ 2KI 2MnSO
4
+ K
2
SO
4
+ I
2
Manganic + Potassium Manganous + Potassium + Iodine
Sulfate Iodide Sulfate Sulfate
The fi nal stage in the Winkler titration is the addition of sodium thiosulfate. The
sodium thiosulfate reacts with the free iodine to produce sodium iodide. When all of the
iodine has been converted the sample changes from yellow-brown to colorless. Often a
starch indicator is added to enhance the fi nal endpoint. Chemically, this reaction can be
written as:
2Na
2
S
2
O
3
+ I
2
Na
2
S
4
O
6
+ 2NaI
Sodium + Iodine Sodium + Sodium
Thiosulfate Tetrathionate Iodide