COD

Chemical

Chemical Oxygen Demand (COD)

COD is the amount of oxygen necessary to oxidise the organic and oxidizable inorganic compounds in the sample completely to CO2 and H2O.

Reagents

  1. Dichromate digestion solution-

Mass of K2Cr2O7

Method

Range

4.903g

Titrimetric

Total

10.216g

Colourimetric

High Range (100-900mg/L)

1.022g

Colourimetric

Low range (<90mg/L)

     

  1. Sulfuric Acid Reagent- (This reagent needs to be made at least 2 days in advance to allow for proper dissolving of the silver sulfate solution)
  2. Add 5.5g AgSo4/ kg H2SO4­
  3. Ferroin Indicator Solution-
    1. Add 1.485g 1,10-phenanthroline monohydrate & 695mg FeSO4.7H2O to 100ml ddH2O
    2. Dilution by a factor of 5 before use (i.e.: 1 part sample : 4 parts ddH2O)
  4. Standard Ferrous Ammonium Sulfate (This is your titrant)-
    1. Add 39.2g Fe(NH4)2(SO4)2.6H2O in 500ml ddH2O
    2. Add 20ml conc H2SO4 and dilute to 1L.
  5. Sulfamic Acid (This reagent is optional and is used to eliminate interference by high NO2- concentrations)-
    1. Add 10mg sulfamic acid/mg NO2- -N in the sample.
  6. Potassium hydrogen phthalate (This is a Standard):
    1. Crush and dry KHP
    2. Dissolve 425mg in 1L (this solution has a COD of 500µg O2/ml)

Notes:  

For titrimetric analysis, the Standard Ferrous Ammonium Sulphate solution must be standardised before use by doing a dry-run titration using water instead of sample to calculate the exact concentration.

Mol of the Ferrous Ammonium Sulphate solution=  (Volume of Digestion solution titrated)/(Volume of Titrant used)  ×0.1

Protocol (Samples should be done in duplicate):

Digestion Vessel

Sample (ml)

Digestion Solution (ml)

Sulfuric Acid Reagent (ml)

Final Volume (ml)

16 x 100mm

2.50

1.50

3.50

7.5

20 x 150mm

5.00

3.00

7.0

15.0

25 x 150mm

10.00

6.00

14.0

30.00

 

COD (mg Oxygen/L)=  ((Blank Vol-Sample vol)  ×Conc of Titrant ×8000)/(mL sample)

  1. Prep a standard curve:
  2. 5 standards of KHP solution at COD equivalent concs to cover the target range
  3. These samples need to be used in the same digestion procedure with the same reagents and volumes.
  4. The curve should be linear and needs to be remade if there is >5% deviation.
  5. Add the sample to clean cuvettes and measure at 600nm.

Notes:

  1. For the tirimetric method, the Blue-green colour will return due to atmospheric oxidation and the volume should be recorded at the first instance of a Red-Brown colour.
  2. For the colourimetric method- keep an undigested blank as a reference solution and the digested solution as a blank.