Calculation of Chemical Oxygen Demand (C.O.D)
Apparatus:
Round bottom flask, water condenser, Burette, Pipette,Heating mantle.
Reagents:
* Mercuric sulphate (< 1gm)
* Sulfuric Acid (1 Kg H2SO4 + 5.5 gm Ag2SO4 ) dissolve for 24 hrs.
* 0.25 N Potassium Dichromate (12.259 gm K2Cr2O7 dried at 1030C + 1 Liter D.W.)
* 0.1 N Ferrous Ammonium Sulphate (39.2 gm Fe (NH4)2(SO4)2.6H2O + 20 ml Conc.H2SO4 + 1 Ltr D.W.)
* Glass beads.
Procedure
* Wash all the apparatus with distilled water
* Dry the round bottom flask by keeping in oven for 10-15 minute
* Follow the steps as per protocol mentioned below.
Protocol
Reagent Blank Sample
Hg2SO4 Pinch PinchK2Cr2O7 10 ml 10 ml
Distilled water 20 ml Sample + D.W = 20 ml
1 Kg H2SO4 + 5.5 gm Ag2SO4 30 ml 30 ml
Note: – for accuracy in results take raw effluent Sample of 0.2 ml to 0.5 ml.
* While adding acid keep the flask in ice bath to avoid loss of volatile Substances due to heat liberation.
* Add 1 – 2 glass beads to avoid the bumping during heating.
* Keep the flask for refluxing for 2 hours.
* Cool the flask thoroughly.
* Titrate with 0.1 N FAS (Ferrous Ammonium Sulphate) using Ferroin indicator
* Continue to titrate till colour turns wine red which is the end point
Calculations:
COD as mg/l of oxygen consumed =(A-B) x Normality of FAS x 8000 / ml. of sample taken
Where
A = ml. of FAS used for titration of blank
B = ml. of FAS used for titration of sample
Standardization of FAS
* 0.05 N Potassium Dichromate
(2.452 gm Anhydrous K2Cr2O7 in 1 Ltr of Distilled water)
* 0.25 N Potassium Dichromate
(12.259 gm Anhydrous K2Cr2O7 in 1 Ltr of Distilled water)
* Whenever a new solution of FAS or Potassium Dichromate is prepared it has to be standardized using Potassium Dichromate as primary Standard.
* Weigh 39.2 gms of FAS and dissolve it in 1000ml of distilled water by adding 20.0ml of concentrated sulphuric acid drop wise by cooling.
* Transfer to 1000ml volumetric flask and dilute it up to mark to get 0.1N FAS solution
* Take 10 ml of 0.25N Potassium Dichromate & 90 ml of distilled water
* Then add 30 ml of concentrated Sulphuric Acid (addition should be done by cooling
Simultaneously)
* Titrate this solution against freshly prepared FAS using Ferroin indicator
* Continue titrating till wine red colour appears which the end point is.
Calculations:–
FAS K2Cr2O7
N1V1 = N2V2
Where N1 = Normality of FAS
V1 = Volume of FAS used for titration of Dichromate
N2 = Normality of Potassium Dichromate
V2 = Volume of Potassium Dichromate taken for titration
By putting the above values the Normality of FAS can be calculated
Precision & Bias: For COD of 200mg/lit in absence of chloride the standard
Deviation was found to be + / – 13 mg /lit of O2 consumed.
Note: * The chief limitation of the COD test is its inability to differentiate between Biologically Oxidisable
And Biologically Inert Organic matter.
* It does not provide any evidence of the rate at which the Biologically active material would be
Stabilized under condition that exists in nature. ( Ref : Page 545 – Perry L. McCarty )
| %mixture | Range of BOD | ml. of sample | Range of BOD |
| 0.01 | 20,000-70,000 | 0.02 | 30,000-105,000 |
| 0.02 | 10,000-35,000 | 0.05 | 12,000-42,000 |
| 0.05 | 4,000-14,000 | 0.10 | 6,000-21,000 |
| 0.1 | 2,000-7000 | 0.20 | 3,000-10,500 |
| 0.2 | 1,000-35,000 | 0.50 | 1200-4200 |
| 0.5 | 400-1400 | 1.0 | 600-2100 |
| 1.0 | 200-700 | 2.0 | 300-1,050 |
| 2.0 | 100-350 | 5.0 | 120-420 |
| 10.0 | 20-70 | 20.0 | 30-105 |
| 5.0 | 40-140 | 10.0 | 60-210 |
| 20.0 | 10-35 | 50.0 | 12-42 |
| 50.0 | 4-14 | 100 | 6-21 |
| 100 | 0-17 | 300 | 0-7 |
Check Procedure for C.O.D.
- Lightly crush & Dry Potassium Hydrogen Phthalate at 1200C, Dissolve 425mgs in Distilled water and Dilute to 1000ml.
- This Solution has a theoretical C.O.D. of 500 ppm.
- This solution to be used as sample to cross check the procedure as well as the chemicals being added whether it gives the correct result.
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