A Schematic diagram of the procedure in BOD analysis (Note that some steps may not be presented explicitly)

1. BOD ANALYSIS
SCHEMATIC DIAGRAM
Janine V. Samelo
BSChem4
Cental Luzon State University

2. sample
s Pre-treatment
Satisfy
requirements?
adjustmentsadjustments
Proceed to the
analysis
yes
Check Laboratory
DO to determine
the % dilution
needed for each
sample
Check pH
Check if
chlorinated

3. sample
s
Check Laboratory DO
to determine the %
dilution needed for
each sample
Shake at least 20
times
Fill BOD bottles with
sample up to the neck
Add 1 ml manganese solution
1 mL alkali potassium iodide azide
Discard 100 mL
Add starch and titrate
Remarks: the
higher DO, make
more dilutions
v
v
Add 1 mL sulfuric
acid
shake

4. sample
sCheck the pH
Add 1 N NaOH
until pH
becomes 7.0
or near
Add 1 N H2SO4
until pH becomes
7.0 or near
6.5-7.2?
No change in color
no
yes
Proceed to
the analysis
Transfer at least 50 mL sample
in a small beaker Check pH at 20 C
< 6.5
> 7.2
7.0
?
no
yes
Note the volume needed to
neutralize the aliquot and using
ratio, compute the volume
needed for the whole sample

5. sample
s
Check if
chlorinated
Pipet out 100 mL
sample
+ 10 mL 1-> 50 H2SO4
10 mL KI and
4 drops starchBlue/black
color
chlorinated
not chlorinated
Titrate until the
blue color
disappears
Note the volume needed to
neutralize the 100 mL aliquot and
using ratio, compute the volume
needed for the whole sample
Neutralized
?
No change in color
no
yes Proceed to the
analysis

6. ANALYSIS PROPER

7. Add nutrientsPrepare dilution water
Aerate for 30 minutes
using vacuum air
pump
Prepare 2 sets of BOD bottles, usually 3 for each sample
(depending upon the number of dilution). Include some for
blank ,seed correction and Quality control. One is for the initial
measure of Do and the other for the final reading of DO
Fill the ¾ of the bottles
with the dilution water

8. Add volume of sample in
each dilution , 3 mL seed
and dilute to the mark.
Stopper the bottles
Add 1 ml manganese
solution
1 mL alkali potassium
iodide azide
Discard 100 mL of
the liquid Shake at least 20 times
Add 1 mL sulfuric
acid
shake

9. Add starch and titrate with
sodium thiosulfate
Incubate the 2nd set of BOD
Bottles covered with stopper and
aluminum foil at 20 C
Calculations* *via worksheet program of
computer
The endpoint is marked with
color change from blue to clear