Chemical Treatment Biological
Septic Tank Aid
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
- 500-millilitre (ml) Erlenmeyer flask with
standard (24/40) tapered glass joints
- Friedrichs reflux condensers (12-inch) with
standard (24/40) tapered glass joints
- Electric hot plate or six-unit heating shelf
- Volumetric pipettes (10, 25, and 50ml
- Burette, 50 ml - 0.1 ml accuracy
- Burette stand and clamp
- Analytical balance, accuracy 0.001gram (g)
- Volumetric flasks (1,000ml capacity)
- Boiling beads, glass
- Magnetic stirrer and stirring bars
- Potassium dichromate (K2Cr2O7)
- Sulphuric acid (H2SO4) /
silver sulphate (Ag2SO4) solution
- Mercuric sulphate (HgSO4) crystals
- Ferrous ammonium sulphate (FAS) [Fe(NH4)2(SO4)2],
- 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.
- 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
- 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.
- Use 1 g of mercuric sulphate (HgSO4)
to complex 100 mg chloride (2,000 mg/l).
- 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.)
- 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.
- 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
- Add 5 to 7 glass boiling beads.
- 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
- Accurately add 25ml of 0.25 N potassium dichromate (K2Cr2O7)
- Add while mixing, an additional 70ml of
concentrated sulphuric acid-silver sulphate solution.
- 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.
- A reagent blank containing 50ml of distilled
water treated with the same reagent as the sample should be refluxed with
each set of samples.
- 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.
- Remove flask from the condenser and dilute to
a final volume of approximately 350ml with distilled water.
- Add 4 to 5 drops of Ferroin indicator and a
magnetic stirring bar.
- Place flask on a magnetic stirrer and rapidly
titrate with 0.1 N ferrous ammonium sulphate to the first red-brown
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 /
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
Sources of Error
- 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.
- Always use the largest sample practical and
use the largest glassware that is in keeping with good laboratory practice.
- Use volumetric flasks and volumetric pipettes
with a large bore.
- The K2Cr2O7
oxidizing agent must be precisely measured. Use a volumetric pipette and use
the same one each time if possible.
- When titrating, be certain that the burette is
clean and free of air bubbles.
- Always read the bottom of the meniscus and
position the meniscus at eye level.
COD Range and Sample Size
|COD Range (mg/l)
|Volume of Sample (ml)
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:
- 500ml of sample diluted to 1,000 ml = 0.5 ml
sample/ml of diluent, .: 50 ml of diluent = 25 ml of sample.
- 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.