Standard Operating Prucedure for the Measurement of Chemicsl Oxygen Demand

 PURPOSE:

To describe the Laboratory standard operating procedure for measurement of Chemical Oxygen Demand of Water and Waste Water samples.

SCOPE:

Applicable to the Laboratory for analysis of Water and Waste Water samples, where measurement of Chemical Oxygen Demand is required.

 

RESPONSIBILITY: 

Lab chemist:

Analysis of sample for measurement of Chemical Oxygen Demand.

Technical Manager:

Review of activity 

Quality Manager:

Implementation and compliance of SOP.

PROCEDURE: 

Principle:

A sample is refluxed in strong acidic solution with a known excess of potassium dichromate (K2Cr2O7).After digestion the remaining unreduced K2Cr2O7 is titrated with ferrous ammonium sulphate to determine the amount of potassium dichromate (K2Cr2O7) consumed and the oxidizable matter is calculated in terms of oxygen equivalent.

 Instrument and Equipment:

Heating Mantle, Burette 50 ml.

Round Bottom Flask 500 ml

Pipettes 50ml.

Measuring Cylinders

Reagents: 

Preparation of Potassium Dichromate Solution, 0.25N: Dissolve 12.259g K2Cr2O7 in distilled water and dilute it to 1000ml.

Ferrous Ammonium Sulphate, 0.25 N: Dissolve 98 g Fe(NH4)2(SO4)2.6H2O in distilled water. Add 20ml conc. H2SO4, cool, and dilute to 1000ml.Standardize this solution against K2Cr2O7 daily.

Ferroin indicator : you can purchase of Ferroin indicator from the market or prepare in your laboratory like that Dissolve 1.485g 1, 10-phenanthroline monohydrate and 695mg FeSO4.7H2O in distilled water and dilute to 100ml. 

Sulphuric Acid Conc.

Silver Sulphate

Mercuric Sulphate

Test Method:

Pipette out 20ml. Sample into a 500 ml. round bottom refluxing flask 

Note: For higher COD value, use smaller quantity of sample diluted to 20ml with distilled water, so that the color of the reaction mixture will remain yellow to greenish yellow after adding reagents and chemicals mentioned.

Add app 0.5 gm of HgSO4 and a pinch of Ag2SO4 to it.

Add 10ml of 0.25 N K2Cr2O7 solutions to it.

Add 30 ml of conc. H2SO4 with gentle swirling of reaction mixture.

Reflux the reaction mixture on a heating mantle for minimum two hours with a             water condenser attached to it.

Cool and wash down condenser with about 80ml of distilled water.

 Cool to room temperature and titrate the excess K2Cr2O7 with 0.25 N Ferrous Ammonium Sulphate solution, using 4 to 5 drops of Ferroin indicator.

End Point of titration is judged by color change from Bluish green to Reddish Brown.

Note down the Burette Reading (Sample Reading).

Carry out Blank determination under similar conditions with 20ml-distilled water, using same quantity of reagents and chemicals. (Blank Reading)

5. Calculation:

COD (mg/L)     =    (B-A) X Nor of FAS X 8000   

                                                   Sample taken in ml

Where,   B = Volume of FAS required for blank

                A = Volume of FAS required for sample.