Environmental Engineering LaboratoryBy: Ngoc Thuan Le

Chemical Oxygen Demand (COD) - Mn

Apparatus: COD-300ml refluxing flask, condenser with cooling system.

Before doing experiments, turn on water bath

Procedure:

1. Place 100ml of sample in a 300ml refluxing flask.

2. Very slowly add 10ml of H2SO4 : water (1:2),

3. Add 1g Ag2SO4 (less than 1g is possible) -- mixing well to dissolve. Wait for 5 mins,

solution will become milky color.

4. Add 10ml of KMnO4 0.025N and mix.

5. Attach flask to condenser and turn on cooling water system, boiling for 30 minutes

Caution: Mix flux mixture thoroughly before applying heat to prevent local heating of flask

bottom and a possible blowout of flask contents

6. Disconnect reflux condenser, add 10ml sodium oxalate Na2C2O4 0.025N (수산나트 륨).

7. Titrate with solution of KMnO4 0.025N from a burette, with a magnetic bar. During the

titration, keep temperature of solution around 60-80oC.

Maximum COD can be measured around 10mg/l.

Prepare 1 blank sample with distilled water.

Caculation:

a. volume of KMnO4 0.025N used for blank.

b. volume of KMnO4 0.025N used for sample.

f. factor.

V. Volume of sample (ml).

Factor:

- Place 100ml in refluxing flask, add 10 ml H2SO4 (1+2), 10ml Na2C2O4 0.025N, warming at

60-80oC. Titrate the mixture with KMnO4 0.025N, record a ml.

- Place 100ml in refluxing flask, add 10 ml H2SO4 (1+2), warming at 60-80oC. Titrate the

mixture with KMnO4 0.025N, record b ml.

Reagents:

- KMnO4 0.025N: 158.3 g/mol. Standard solution 0.02M (0.1N) ~ 3.161g, dilute 4 times before

using.

- H2SO4 1:2, 1V H2SO4: 2V dW.

- Ag2SO4, crystals or powder.

- Na2C2O4 0.025N (0.0125M): Dissolve 1.675g Na2C2O4 primary standard grade, in water and dilute

to 1000ml.

(Stock solution: Na2C2O4 0.25N (0.125M): Dissolve 16.75g Na2C2O4 primary standard grade, in

water and dilute to 1000ml, dilute 10 times before using)