Chemical Oxygen Demand (cod-mn)

Environmental Engineering Laboratory By: 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: (b − a ) × f ×1000 × 0.2 COD ( mgO / L) =

V

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 10 mixture with KMnO4 0.025N, record b ml. f = ( a − b) 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)