BOD

OBJECTIVE
To determine the Biochemical Oxygen Demand of the given water samples.

PRINCIPLE
The iodometric titration is precise & reliable titrimetric procedure for dissolved oxygen (DO) analysis. The test is based on the addition of divalent manganese solution; followed by strong alkali, to the sample in a BOD glass stoppered bottle. DO present, rapidly oxidize an equivalent amount of the dispersed divalent manganous hydroxide of higher oxidation state (brownish orange in colour). This brownish orange colour indicates presence of oxygen. In the presence of iodide and on subsequent acidification, higher manganese hydroxide revert to the divalent state, & liberate iodine equivalent to the original DO content of the sample. The iodine liberated is titrated with a standard solution of sodium thiosulphate using starch as indicator.

REACTIONS INVOLVED

MnSO4 + 2 KOH  Mn(OH) 2 + K2 SO4
(white ppt.)
2Mn(OH) 2 + O2  2MnO(OH) 2
(brown ppt.)
MnO(OH) 2 + 2H2SO4  Mn(SO4) 2 + 3 H2 O
Mn(SO4) 2 + 2KI  MnSO4 + K2SO4 + I2
2Na2 S2 O3 .5 H2 O + I2  Na2 S4 O6 + 2 Na I + 10 H2 O
2NaN3 + H2SO4  2HN3 + 2Na2 SO4
HNO2 + HN3  N2 + N2O + H2 O

INTERFERENCE
It is caused by certain oxidizing agents which liberate iodine from iodides (such as ozone, chlorine, ferric compounds, higher oxidation state chromate, nitrate, persulphate, peroxides, etc), by certain reducing agents which may reduce iodine to iodide (such as ferrous compounds, thiosulphate, readily oxidizable organic matter, nitrite, etc). some organic compound hinder the setting of precipitate and colour of starch end point.

PROCEDURE
1) Fill the BOD bottle without turbulently exposing the sample to the air, & stopper it.
2) Add 2ml manganous sulphate followed by 2ml alkali-iodide-azide reagent, dipping the pipette little below the surface
3) Stopper carefully to exclude the air bubbles, & thoroughly mix the contents by inverting the bottle.
4) Allow the precipitate to settle to the 1/3rd of the bottle, repeat the mixing & allow the precipitate to settle completely leaving a clear supernatant liquid.
5) Add 2ml conc H2SO4 immediately after removing the stopper. Restopper and mix by gentle inversion until all the precipitate dissolve, if it doesn’t, allow to stand for a few minutes and repeat the mixing.

6) Distribute the iodine uniformly before taking a part of it for titration.
7) Pipette 100ml of sample for titration in 250ml conical flask and immediately titrate the liberated iodine with the standard sodium thiosulphate solution to a pale yellow straw colour. Add 2ml starch solution and continue the titration till the first disappearance of the blue colour.
8) Repeat the steps (2-7) for the bottles incubated at 27°C for 3 days to get the dissolved oxygen on the finale day.
NOTE: It must be remembered that iodine is volatile and therefore, the titration must be carried out as expediously as possible and with the minimum exposure to the air.


FORMULA USED

BOD (mg/l) = {(IDDOS- FDDOS) – (IDDOB-FDDOB)X100}/ Percent Of Solution .
Where, IDDOS- Initial day dissolved oxygen Sample
FDDOS- Finale Day Dissolved Oxygen Sample
IDDOB- Initial Day Dissolved Oxygen Blank
FDDOB-Finale Day Dissolved Oxygen Blank

OBSERVATIONS
The BOD of the groundwater samples is negligible, hence it is not determined. The BOD values of the polluted river stretch is only determined.

Total Observations= 6 (blank) + 3 x 2 (bottles per sample x no. of samples)
= 12
2 blanks are for initial day DO and 4 for final day DO.
1 sample bottle is for initial day DO and 2 for final day DO.

% Dilution = 10 %
S.No. Bottle No. Sample Description Volume (ml)
1. 4 Blank: Initial Day 6.10
2. 8 Blank: Initial Day 6.15
3. 22 Blank: Final Day 6.24
4. 45 Blank: Final Day 6.45
5. 60 Blank: Final Day 6.04
6. 70 Blank: Final Day 6.00
13. 198 Sample 3: Initial Day 4.92
14. 202 Sample 3: Final Day 2.19
15. 207 Sample 3: Final Day 1.96
16. 370 Sample 4: Initial Day 5.05
17. 465 Sample 4: Final Day 2.23
18.. 785 Sample 4: Final Day 2.81

RESULTS
Sample No. BOD (mg/l)
3. 29.025
4. 25.875
By-Riddhi singh