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Dichromate 0,25 N ml
10
bringing to 100 ml volume using distilled water. After cooling the titration is performed with ammonium ferrous sulfate
solution till turning to orange. The used volume in ml is used to compute the correction factor. If the titer of solution is exact,
20 ml are necessary to titre the used amount of dichromate while a larger volume is necessary with lower normality.
Accordingly the correction factor is given by F = 20/n where "n" is the number of ml actually used.
Evaluation of blank
The same procedure used at 2.3 is used to measure the amount of ammonium ferrous sulfate standard solution necessary
to turn the colour in a volume of distilled water.
Calculation of COD
The C.O.D. value of the examined sample is obtained by:
8000.N
=
−
C.O.D.mg/l
(b
a).
where:
b = ml of ammonium ferrous sulfate solution used to titer blank.
a = ml of ammonium ferrous sulfate solution used to titer the sample.
N = normality of ammonium ferrous sulfate.
The reactions interested in the analysis are the following:
K
Cr
O
+ 6 Fe (NH
)
(SO
2
2
7
4
2
--
++
+
Cr
O
+ 6 Fe
+ 14 H
2
7
Bibliography
- Water Research Institute - National Research Council of Italy (1993) Method n° 5110.
Chemical Oxygen Demand (C.O.D.) pages 285-289 (in Italian) Graphical Institute and Mint of the State. Rome, Italy.
- APHA-AWWA-WPCF (1985) Standard Methods for the examination of water and wastewater.
Method n° 508 - Oxygen demand (chemical). pages 532-538. 16th Ed. Washington DC USA.
Concentrated sulfuric acid ml
30
.Fc
X
X = ml of sample under examination.
→ 2 KHSO
)
+ 8 H
SO
4
2
2
4
+++
+++
= 2 Cr
+ 6 Fe
+ 7 H
+ 6 (NH
)
SO
+ Cr
4
4
2
4
2
O.
2
4
Fc = correction factor.
(SO
)
+ 3 Fe
(SO
)
+ 7 H
4
3
2
4
3
O
or
2
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