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

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

 T.P.Sivanandan 12 BOD, Calculation of COD, COD, Waste Water Calculation, Water analysis

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.)

Technorati Tags: Calculation of Chemical Oxygen Demand