Calculation of Chemical Oxygen Demand (C.O.D)

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 Pinch

K2Cr2O7 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

(Reference – Table 3-10, Page – 72, Metcalf & Eddy)

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.

(This solution could be preserved for 3 months under refrigerated condition.)

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Article by Sivanandan

T.P.Sivanandan

2 Comments

  1. Santhosh says:

    Great Work sir.. its really useful sir.. but i found Chemical Oxygen Demand (COD) Calculator from http://calcverter.blogspot.in/2013/03/chemical-oxygen-demand-cod-calculation-28.html this link. its also useful to some students..

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