Definition:
Chemical oxygen demand (COD) - The mass
concentration of oxygen equivalent to the amount of dichromate consumed by dissolved and suspended matter
when a water sample is treated with that oxidant under defined conditions.
Preparing Reagents:
Water:
All blank and reagent water
must have a conductivity of less than 2 microsiemens per centimeter, a Total
Organic Content of less than 1 mg/l C and be free from reducing agents.
Distilled or deionised water
can be used provided the blank remains acceptable.
Sulphuric acid (d20 1.84):
Unacceptable
blank values can be caused by minute amounts of contaminants in the reagents.
The major cause is the
sulphuric acid used, analytical reagent grade is necessarily better than
ordinary acid, and when satisfactory
batch has been found it should be reserved for COD use only.
1% m/v Silver sulphate in sulphuric acid:
Dissolve 5.0±0.05g of silver sulphate in 500 ml of sulphuric acid (d20
1.84). To obtain a satisfactory solution the initial mixture should be shaken
allowed to stand overnight and then shaken again in order to dissolve all the
silver sulphate. Store in a dark brown glass bottle out of direct sunlight.
Mercuric sulphate solution:
Dissolve 100±1g mercuric sulphate in a mixture of 500 ml water and 50±2ml sulphuric acid (d20 1.84). The microanalytical
reagent grade of mercuric sulphate is advised when measuring very low COD
values.
Standard reference solution of potassium
dichromate 0.02083M (M/48)
Dissolve 6.129±0.001g of Analar potassium dichromate, previously dried at 140 -
150 oC in water and make up to one liter in calibrated flasks.
1:10 phenanthroline ferrous complex (Ferroin)
This reagent is commercially
available in either plastic or glass bottles: both are acceptable. If necessary
a suitable indicator can be prepared as follows. Dissolve 3.5±0.1g ferrous sulphate heptahydrate in 500±1ml of water. Add 7.4±0.1g of 1:10 phenanthroline
monohydrate and shake until dissolved.
Standard solution of ferrous ammonium sulphate
0.0125M
Add 40 ml of sulphuric acid
(d20 1.84) to 400 ml water, mix and cool. Dissolve 9.803 g of
ferrous ammonium sulphate hexahydrate in the cooled dilute acid and make up to
2 litres in a calibrated flask.
Preparation from solid
Standardise the solution,
each day before use, against the standard potassium dichromate as follows:
Prepare a stock of diluted
sulphuric acid by adding 375±5ml of sulphuric acid (d20 1.84) to 600 ±5ml of water and cool. To a 50 ml flask add 19.5 ml of diluted sulphuric
acid, followed by 2.0 ml of 0.02604M potassium dichromate solution and titrate
with ferrous ammonium sulphate using not more than two drops of Ferroin as
indicator. The color change is from blue-green to pink. Alternatively the
diluted sulphuric can be made up in
Situ by adding 7.5-ml
sulphuric acid (d20 1.84) to 12 ml of water in a 50 ml flask and
cooled. It is important to add the dichromate after the diluted sulphuric acid
has cooled to avoid the oxidation of any organic contaminants by the dichromate
before it can be titrated.
Standard reference solution of potassium hydrogen
phthalate, COD of 200 mg/l
Dissolve
0.1701±0.001g of potassium hydrogen phthalate, previously dried at 120 oC
for 2 hours, in water and dilute
to one liter in a calibrated flask. The standard must be stored, without
freezing, in a refrigerator and be renewed
monthly.
Standard reference solution of potassium hydrogen
phthalate, COD of 400 mg/l
Use
0.340±0.001g of potassium hydrogen phthalate.
Standard reference solution of potassium hydrogen
phthalate, COD of 800 mg/l
Use
0.680±0.001g of potassium hydrogen phthalate.
PROCEDURE:
All operations must be carried out with care. If
any difficulties arise during the course of the analysis, then these must
reported immediately to the operator's supervisor.
Digestion:
1. The boiling point of the
reaction mixture is defined by the ratio of sulphuric acid to
water present;
therefore the reagent and sample volumes used must be kept
constant.
2. Insert the anti-bumping aid or granules into the boiling flask.
Measure 5 ml±0.03 ml of sample into the flask.
3. Add 0.5 ml of mercuric sulphate solution and swirl to mix. Add 2.0±0.01 ml 0.0283M
potassium dichromate and 7.5±0.1ml m/v silver sulphate in concentrated sulphuric
acid.
4. Fit the condenser and swirl the flask and its contents, and boil
gently under reflux for 2
hours ± 10 min.
5. Remove the flask from the source of heat and allow to cool for
approximately 10 min
then add 10±5 ml of water via condenser.
Disconnect the flask from the condenser
and cool the flask to room temperature
in running water. Determine the residual
dichromate by titration.
Determination of
residual dichromate:
1. Add not more than two drops of Ferroin indicator to the flask
and titrate the residual dichromate with standardized
ferrous ammonium sulphate. Empty residual solutions into special container for
disposal by chemical stores, then
rinse flask with tap water and then de-ionized water.
2. Blank Determination
Ideally the blank value
should be the mean of at least three determinations, but if any value differs
by more than ±0.5 ml from the mean value it must be rejected and a new mean
recalculated from the acceptable blank values, additional determinations being
made if necessary.
STANDARD DETERMINATION:
With each of the samples run
potassium hydrogen phthalate standards. Take a 5.0 ml aliquot of the standard
potassium hydrogen phthalate solutions and carry out the determinations.
1. Once in the position they need not be removed between
determinations.
2. If necessary, the sample should be diluted accurately and a 5 ml
aliquot taken for
oxidation, which should consume dichromate equivalent to
between 5.0 and 20 ml of
0.0125M ferrous ammonium sulphate.
3 . Use a grade A pipette or micro burette to add the potassium
dichromate.
4 . Run the acid down the side of the flask whilst gently swirling.
Carry out this operation
in a fume cupboard. When samples are known to contain
volatiles, cool the flask
under running cold water to minimize loss of
volatiles.
5. The amount of mercuric sulphate in this mixture will suppress, but
not entirely
eliminate, the effect of chloride ion.
6. Excessive reflux times may result in high blank values.
7. After the first addition of ferrous ammonium sulphate solution the
indicator is
blue-green in color and the endpoint occurs when the color changes
sharply to
through deep blue to pink. The blue colour may reappear a few
minutes later but his
phenomenon should be ignored.
8. An acceptable blank test should require at least 23.5 ml of 0.0125M
ferrous
ammonium sulphate or its equivalent, in the titration.
9. This result should be plotted on a control chart. If the standard
determination is
outside the ±3 times the standard deviation limits, some action should be taken
to bring
the method back in control.
CALCULATION OF RESULTS:
The
COD of the samples can be calculated from the following expression:
COD
mg/l = (T1 - T2) x F x D
T1
= average number of ml ferrous ammonium sulphate used to titrate
appropriate
blank
T2
= number of ml ferrous ammonium sulphate used to titrate sample
F
= standardization factor for ferrous ammonium sulphate
25
=
----------------------------------------------------- x 20*
standardization titration, vol. In ml FAS
D
= dilution factor
total volume of the diluted sample
=
-----------------------------------------------
sample volume taken for dilution
*1.0 ml 0.0125M ferrous
ammonium Sulphate is equivalent to 0.10 mg oxygen and is equivalent to 20 mg/l
COD with 5.0 ml sample
