Apparatus:
* Incubator
* Stoppard Bottles
* Burette
* Pipette
* Conical flask
Chemicals: Reagents
* MnSO4 soln (364 gm MnSO4.H2O + 1 ltr boiling D.W.)
* CaCl2 soln. (27.5gm anhydrous CaCl2 + 1 Ltr D.W.)
* FeCl3 soln. (0.25gm FeCl3.6H2O + 1 Ltr D.W.)
* Phosphate Buffer (8.5gm KH2PO4 +21.75 gm K2HPO4 +33.4gm Na2HPO4.7H2O +1.7gm NH4Cl + 1 Ltr D.W)
* MgSO4 soln. (22.5 gm MgSO4.7H2O + 1 Ltr D.W.)
* Alkali Azide (a + b)
a. (500 gm NaOH + 135 gm NaI + 1 Ltr D.W)
b. (10gm NaN3 + 40 ml D.W)
* Conc. H2SO4
* Starch indicator soln (2 gm starch +0.2 gm salicyclic acid +100 ml hot D.W.)
* 0.025 N Sodium Thiosulphate(6.205 gm Na2S2O3.5H2O + 1 Ltr D.W+ 0.4 gm NaOH )
Procedure:
1. Preparation of Dilution Water
* Wash 1 litre beaker in distilled water and take 1 litre of Distilled water in it.
*Add 1mL of MgSO4, CaCl2, FeCl3 & Phosphate buffer.
* Aerate the sample for ½ hr to 2 hr.
2. Determination of Initial D.O.
* Take dilution water in 300 ml stoppered bottle
* Add 1mL of MnSO4 Soln
* Add 1ml of “Alkali Azide” soln
* There is a formation of precipitate of Mn(OH)2
* Add 1ml of concentrated sulphuric acid
* Shake the bottle till precipitate dissolves completely
* Transfer 201 ml of solution to a 500ml conical flask
* Titrate this solution against 0.025 N Sodium Thiosulphate (6.205 gm Na2S2O3.5H2O + 1 Ltr D.W. + 0.4 gm NaOH ) till colour becomes pale yellow.
* Add starch indicator till solution becomes blue
* Continue titrating with Thiosulphate till the solution becomes colourless
* The reading of the burette corresponds to the initial dissolved Oxygen in ppm
* One ml of 0.025 N- thiosulphate = 1 ppm of Dissolved Oxygen.Determination of final dissolved oxygen
* Take dilution water in 300 ml of stoppered bottle
* Add sample quantity according to the chart attached.
* Keep the bottles in incubator for 3 days at 270C.
* Incubate a blank sample with each batch of samples
* After 3 days remove sample from the incubator
* Add 1ml of MnSO4 followed by 1ml of Alkali azide reagent
* The precipitate of Mn (OH)2 is dissolved by adding 1ml of concentrated sulphuric acid
* Transfer 201 ml of solution to a 500 ml conical flask and titrate with 0.025N
Thiosulphate
* The pale yellow colour is titrated further by adding starch till the blue colour is completely discharged & becomes Colourless.
* The burette reading obtained corresponds to the final dissolved oxygen.
Calculations: BOD ppm = (A – B) / P
Where
A = Initial Dissolved Oxygen
B = Final Dissolved Oxygen
P = Volumetric fraction of sample taken
Example: if 10 ml of sample is taken then P = (10 / 300)
Precision & Bias:There is no precision & bias for Biochemical Oxygen Demand.
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