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COD Analysis
It is the amount of oxygen from chemical oxidants required for the complete oxidation of substances dissolved or suspended
in water. By COD. determination it is possible to evaluate quantitatively substances which can be oxidized by strong
oxidants like potassium dichromate in highly acidic solutions by concentrated sulfuric acid. The method allows the
determination of substances which are not oxidized by the corresponding biological method (BOD, Biological Oxygen
Demand), e.g. cellulose. Piridine, benzene and toluene are not oxidized. Chemical oxidation is more complete than
biological oxidation and therefore COD values for a given sample are generally higher than BOD values. According to the
oxidant used and to its concentration, to the concentration of acid, to the catalyst, to the temperature and time, different
substances are more or less completely oxidized and analytical conditions must be strictly standardized. The determination
of COD is performed by "return titration" of excess dichromate, added in known amount, after the completion of the oxidation
of the solution containing chemically oxidable substances. The amount of oxidable substances which are present in a given
sample is directly proportional to the amount of dichromate consumed. The addition of silver sulfate as a catalyst increases
the oxidation speed of alcohols and acids but not that of aromatic hydrocarbons. The presence of high concentrations of
iodides, bromides or chlorides can result in results higher than real. This inconvenient is overcome by adding mercury
sulfate which forms insoluble complexes with halogens.
Reagents
Dichromate standard solution 0.250 N - 15-20 g of potassium dichromate analytical grade (K
two hours at 150 °C and then left to cool in a desiccator. 12.259 g are weighted and dissolved in 400-500 ml distilled water
in a volumetric flask of 1000 ml volume. The flask is then filled to the mark with distilled water.
Sulphuric acid reagent - 5.4 g of silver sulphate analytical grade (Ag
acid 96% analytical grade (H
preserved indefinitely in a well stoppered and dark glass bottle.
Ammonium ferrous sulfate standard solution 0.125 N - 49.02 g of ammonium ferrous sulfate exahydrate analytical grade
• 6 H
(Fe(NH
)
(SO
)
O), are weighed and dissolved in half a liter about of distilled water. 20 ml of concentrated sulphuric
4
2
4
2
2
acid analytical grade (H
2
Do not add the solution to the concentrated acid in order to avoid dangerous splashes. Fill to 1000 ml in a graduated flask
with distilled water. Keep in a dark bottle. The titre decreases with time and is to be controlled every time an analysis is
performed by comparison with a standard solution of potassium dichromate.
Solution of Ferroin indicator - 1.48 g of O-phenanthroline analytical grade (1.60 g of monohydrate) and 0.695 g of ferrous
sulfate heptahydrate analytical grade (Fe SO
The final volume of 100 ml is obtained by adding distilled water.
Mercury (II) sulphate - Small crystals of mercury (II) sulphate analytical grade (Hg SO
Potassium hydrogen phtalate standard solution - Using potassium hydrogen phtalate RPE (H
constant weight at 120 °C, a solution containing 425 mg in 1000 ml is prepared. The COD of this solution is 500 mg O
If stored in a refrigerator at + 4 °C its shelf life is 3-4 months.
Procedure
Sample digestion
The reagents and the sample to be tested are introduced by calibrated pipettes in perfectly clean digestion vials in the order
and in the amounts shown in the following table.
Reactor
Mercuric sulphate mg
ECO 6
400
The amounts of mercury sulphate, the only solid reagent, can be measured by a small spoon purposely calibrated, taking
into account that 100 mg of mercury sulphate are able to complex 10 mg of chloride ions. If the water sample contains
chloride ions in higher concentration, the amount of mercury sulphate added is to be increased on the basis of mercury
sulphate and chloride ions weight ratio of 10 to 1. The appearance of a precipitate after the addition of reagents does not
influence the results. Place the air condensers with antisplash bells upon the vials. Blank prepared by heating tend to
overheat and then to start boiling in a violent manner. To avoid this inconvenient it is advisable to add to the vials with blank
solutions some bumping stones (small spheres or capillaries or chips of glass; small pieces of pumice). The vials are
introduced into the VELP reactor. After the end of the heating period of 120 min. at 150 °C (the boiling point of the used 50%
sulphuric acid solution), which is shown by a buzzer, the vials are extracted and left to cool.
Sample titration
The content of each vial is poured in a wide mouth Erlenmeyer flask together with the distilled water used for rinsing 3-4
times. 5-6 drops of ferroin solution are then added. After cooling, the flask content is titrated with ammonium iron sulfate
standard solution until the color changes from blue-green to orange.
Control of the titer of ammonium ferrous sulfate standard solution 0.125 N
The titer of ammonium ferrous sulfate solution is to be controlled during each day because it decreases with time.
The variation of titer must be considered when computing COD.
This is obtained by introducing in an Erlenmeyer flask of suitable capacity.
SO
d = 1,835) corresponding to 545 ml. Complete solution requires two days. The reagent is
2
4
SO
96% d = 1.835) are slowly added to the ammonium and iron solution while shaking.
4
• 7 H
4
Dichromate 0,25 N ml
10
SO
2
O) are weighed and dissolved together in 50 ml about of distilled water.
2
3
Cr
2
2
) are added to 1 kg of concentrated sulphuric
4
).
4
Sulphuric acid ml
30
O
) are oven dried for
7
C
O
K) dried to
5
8
4
/l.
2
Sample ml
20
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