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CHEMICAL OXYGEN DEMAND (COD)

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Chemical oxygen demand (COD) is used as a measure of oxygen requirement of a sample that is susceptible to oxidation by strong chemical oxidant. The dichromate reflux method is preferred over procedures using other oxidants (eg potassium permanganate) because of its superior oxidizing ability, applicability to a wide variety of samples and ease of manipulation. Oxidation of most organic compounds is 95-100% of the theoretical value. 

Dichromate Reflux Technique Standard Method.

Equipment Required

  1. 500-millilitre (ml) Erlenmeyer flask with standard (24/40) tapered glass joints
  2. Friedrichs reflux condensers (12-inch) with standard (24/40) tapered glass joints
  3. Electric hot plate or six-unit heating shelf
  4. Volumetric pipettes (10, 25, and 50ml capacity)
  5. Burette, 50 ml - 0.1 ml accuracy
  6. Burette stand and clamp
  7. Analytical balance, accuracy 0.001gram (g)
  8. Spatula
  9. Volumetric flasks (1,000ml capacity)
  10. Boiling beads, glass
  11. Magnetic stirrer and stirring bars

Chemicals Required

  1. Potassium dichromate (K2Cr2O7) 0.25N
  2. Sulphuric acid (H2SO4) / silver sulphate (Ag2SO4) solution
  3. Mercuric sulphate (HgSO4) crystals
  4. Ferrous ammonium sulphate (FAS) [Fe(NH4)2(SO4)2], approximately 0.01N
  5. Ferroin indicator (1, 10-phenanthroline and ferrous ammonium sulphate)

Caution: In carrying out the following procedures, use proper safety measures, including protective clothing, eye protection, and a fume hood. Reagents containing heavy metals (HgSO4 and Ag2SO4) should be disposed of as toxic wastes.

Chemical Preparation

  1. Dissolve 12.259g of oven-dried (primary standard grade dried at 103oC to a constant weight) potassium dichromate in distilled water and dilute to 1 litre volume in a volumetric flask.
  2. Add 22g of reagent grade silver sulphate to a 4kg bottle of concentrated sulphuric acid (H2SO4) and mix until the silver sulphate goes into solution.
  3. Use 1 g of mercuric sulphate (HgSO4) to complex 100 mg chloride (2,000 mg/l).
  4. Dissolve 1.485g of 1,10-phenanthroline monohydrate and 0.695g of ferrous ammonium sulphate heptahydrate in distilled water and dilute to approximately 100ml. (Alternatively, this indicator may be purchased as Ferroin Indicator from most scientific suppliers.)
  5. Dissolve 39g reagent grade ferrous ammonium sulphate hexahydrate in distilled water. Add 20ml of concentrated sulphuric acid (H2SO4). Cool and dilute to exactly 1 litre in a volumetric flask using distilled water. The ferrous ammonium sulfate (FAS) titrant must be standardized daily by the following procedure: 

    Dilute 10ml of standard potassium dichromate (K2Cr2O7) solution to 100ml with distilled water. Slowly add 30ml of concentrated sulphuric acid and cool to room temperature. Titrate with ferrous ammonium sulphate titrant, using 2 to 3 drops (0.10 to 0.15 ml) of Ferroin indicator.

    Normality of FAS = (ml K2Cr2O7)(0.25)
                                  ml FAS required

    The deterioration of FAS can be decreased if it is stored in a dark bottle.

Procedure

  1. Place a 50ml sample or an aliquot diluted to 50ml in a 500ml refluxing flask. The blank is prepared using 50ml of distilled water. This is a precise measurement and a 50ml volumetric pipette should be used. Refer to COD Range and Sample Size below for dilution.
  2. Add 5 to 7 glass boiling beads.
  3. Add 1g of mercuric sulphate (HgSO4), 5ml of concentrated sulphuric acid / silver sulphate solution, and mix until the HgSO4 is in solution. The function of the mercuric sulphate is to bind or complex chlorides. One gram may not be required if the chloride concentration is low. (Caution: Always add acid slowly down the side of the flask while mixing to avoid overheating. It may be necessary to use gloves because of the heat generated.)
  4. Accurately add 25ml of 0.25 N potassium dichromate (K2Cr2O7) and mix.
  5. Add while mixing, an additional 70ml of concentrated sulphuric acid-silver sulphate solution.
  6. After thorough mixing, attach the flask to the reflux condenser, apply heat, and reflux for 2 hours. Refluxing time can be decreased depending on the ease of oxidation of organic materials. This time may be determined by refluxing for periods from 15 minutes to 2 hours and comparing the results.
  7. A reagent blank containing 50ml of distilled water treated with the same reagent as the sample should be refluxed with each set of samples.
  8. Cool the apparatus to room temperature after the refluxing period. Wash down the interior of the condenser and flask twice with approximately 25ml portions of distilled water.
  9. Remove flask from the condenser and dilute to a final volume of approximately 350ml with distilled water.
  10. Add 4 to 5 drops of Ferroin indicator and a magnetic stirring bar.
  11. Place flask on a magnetic stirrer and rapidly titrate with 0.1 N ferrous ammonium sulphate to the first red-brown endpoint.

    Caution: Use care in titration. The endpoint is very sharp and may be reached rapidly.

     

Formula to determine COD:

COD (mg/l) = (a-b)(N) x 8,000 / sample size (ml) 

Where:

a = ml Fe(NH4)2(SO4)2 used for blank 

b = ml Fe(NH4)2(SO4)2 used for sample 

N = normality of FAS titrant (Fe(NH4)2(SO4)2

ml sample = the actual volume of sample used before dilution

 

Sources of Error

  1. The largest error is caused by using a nonhomogeneous sample. Every effort should be made to blend and mix the sample so that solids are never excluded from any aliquot.
  2. Always use the largest sample practical and use the largest glassware that is in keeping with good laboratory practice.
  3. Use volumetric flasks and volumetric pipettes with a large bore.
  4. The K2Cr2O7 oxidizing agent must be precisely measured. Use a volumetric pipette and use the same one each time if possible.
  5. When titrating, be certain that the burette is clean and free of air bubbles.
  6. Always read the bottom of the meniscus and position the meniscus at eye level.

 

 

COD Range and Sample Size

COD Range (mg/l) 50-800 100-1500 240-3700 480-7500 1200-18800 2400-3700 40000-375000
Volume of Sample (ml) 50 25 10 5 2 1 0.1

All samples high in solids should be blended for 2 minutes at high speed and stirred when an aliquot is taken. Sample volumes less than 25ml should not be pipetted directly, but serially diluted and then a portion of the diluent removed:

  1. 500ml of sample diluted to 1,000 ml = 0.5 ml sample/ml of diluent, .: 50 ml of diluent = 25 ml of sample.
  2. 100 ml of sample diluted to 1,000 ml = 0.1 ml sample/ml diluent, .: 50 ml of diluent = 5 ml of sample.

 

Elimination of Interference

One gram of mercuric sulphate (HgSO4) will complex 100mg of chloride in a 50ml sample (2,000 mg/l). For samples higher in chloride more HgSO4 should be used in the ratio of 10:1 HgSO4.

Interference from nitrites can be prevented by the addition of 10:1 ratio of sulfamic acid:nitrite. The addition of the silver sulphate (AgSO4) concentrated sulphuric acid (H2SO4) refluxing acid will aid in the oxidation of some organic nitrogen compounds, but aromatic hydrocarbons and pyridine are not oxidized to any appreciable amount.

2006 Oasis Environmental Ltd