COD (Open Reflux)

Methode : Open Reflux
Apparatus
a. Reflux flasks, consisting of 250 mL flask with flat bottom and with 24/29 ground
glass neck
b. Condensers, 24/29 and 30 cm jacket Leibig or equivalent with 24/29 ground glass
joint,or air cooled condensers, 60 cm long, 18 mm diameter, 24/29 ground glass
joint.
c. Hot plate or gas burner having sufficient heating surface.

Reagent
a. Standard potassium dichromate solution, 0.0417M (0.25N): Dissolve 12.259 g
K2Cr2O7,primary standard grade, previously dried at 103oC for 2 hours, in
distilled water and dilute to 1L.
b. Sulphuric acid reagent: Add 5.5g Ag2SO4 technical or reagent grade, per kg of
conc.H2SO4, keep for a day or two to dissolve.
c. Ferroin indicator solution: Dissolve 1.485g 1, 10-phenanthroline monohydrate and
695 mg FeSO4.7H2O in distilled water and dilute to 100 mL. Commercial preparation
may also be available.
d. Standard ferrous ammonium sulphate (FAS), titrant, 0.25M: Dissolve 98g Fe (NH4)2
(SO4)2.6H2O in distilled water, add 20 mL conc. H2SO4, cool and dilute to 1L,
standardise daily as follows.
e. Standardisation: Dilute 10 mL standard K2Cr2O7 to about 100 mL, add 30 mL conc
H2SO4, cool. Add 2 drops of ferroin indicator and titrate with FAS.



g. Mercuric Sulphate, HgSO4, powder
h. Potassium hydrogen phthalate (KHP) standard: Lightly crush and dry potassium
hydrogen phthalate (HOOCC6H4COOK), at 120oC, cool in desiccator, weigh 425 mg in
distilled water and dilute to 1L. This solution has a theoretical COD of 500
μgO2/mL, stable for 3 months in refrigerator.

Procedure
a. Add 50 mL of sample or an aliquot diluted to 50 mL with distilled water in a 500
mL refluxing flask. Add 1g HgSO4, few glass beads, and 5 mL sulphuric acid
reagent, mix,cool. Add 25 mL of 0.0417M K2Cr2O7 solution, mix. Connect the flask
to the condenser Laboratory Manual ID: 1.4 Version: 2 Page: 2/2 and turn on
cooling water, add additional 70 mL of sulphuric acid reagent through open
end of condenser, with swirling and mixing.
b. Reflux for 2 hours; cool, wash down condenser with distilled water to double the
volume of contents, cool.
c. Add 2 drops of Ferroin indicator, titrate with FAS the remaining potassium
dichromate,until a colour change from bluish green to reddish brown. Also reflux
and titrate a distilled water blank with reagents.
d. Use standard 0.00417M K2Cr2O7, and 0.025M FAS, when analysing very low COD
samples.
e. Evaluate the technique and reagents by conducting the test on potassium hydrogen
phthalate solution.
f. Do not add grease at the Leibig jacket to prevent jamming, use water instead.

Calculation

where:
A = FAS used for blank, mL
B = FAS used for sample, mL
M = Molarity of FAS
Note
• Theoretically the method is suitable for analysing samples containing 1000 mg/L COD
without dilution
• In order to economise on quantities of chemicals used in the test, use smaller
sample volumes and proportionally reduce quantities of chemicals as given in the
following table.