LaMotte Dissolved Oxygen Water Quality Test Kit - Manual (Page 6)

6
MEASURING BOD (BIOCHEMICAL OXYGEN DEMAND)
Biochemical oxygen demand is determined by measuring the dissolved oxygen
concentration in a freshly collected water sample and comparing it to the dissolved
oxygen level in a sample that was collected at the same time but incubated under
specifi c conditions for a specifi c length of time. The difference between the two
oxygen levels represents the amount of oxygen required for the decomposition of
organic material and the oxidation of chemicals in the water during the storage period,
a measurement known as the BOD.
Unpolluted, natural waters will have a BOD of 5 ppm or less. Raw sewage may have
levels of 150 to 300 ppm. Wastewater treatment plants must reduce BOD to levels
specifi ed in their discharge permits, usually between 8 and 150 ppm BOD.
TESTING DISSOLVED OXYGEN
The fi rst step in a DO titration is the addition of Manganous Sulfate Solution (4167) and
Alkaline Potassium Iodide Azide Solution (7166). These reagents react to form a white
precipitate, or fl oc, of manganous hydroxide, Mn(OH)
2
. Chemically, this reaction can
be written as:
MnSO
4
+ 2KOH Mn(OH)
2
+ K
2
SO
4
Manganous + Potassium Manganous + Potassium
Sulfate Hydroxide Hydroxide Sulfate
Immediately upon formation of the precipitate, the oxygen in the water oxidizes
an equivalent amount of the manganous hydroxide to brown-colored manganic
hydroxide. For every molecule of oxygen in the water, four molecules of manganous
hydroxide are converted to manganic hydroxide. Chemically, this reaction can be
written as:
4Mn(OH)
2
+ O
2
+ 2H
2
O 4Mn(OH)
3
Manganous + Oxygen + Water Manganic Hydroxide
Hydroxide
After the brown precipitate is formed, a strong acid, such as Sulfamic Acid Powder
(6286) or Sulfuric Acid, 1:1 (6141) is added to the sample. The acid converts the
manganic hydroxide to manganic sulfate. At this point the sample is considered
“fi xed” and concern for additional oxygen being introduced into the sample is reduced.
Chemically, this reaction can be written as:
2Mn(OH)
3
+ 3H
2
SO
4
Mn
2
(SO
4
)
3
+ 6H
2
O
Manganic + Sulfuric Manganic + Water
Hydroxide Acid Sulfate