To Measure COD of WasteWater using Open Reflux Method

History of COD :

KMnO₄ was used as oxidizing agent for many time pb with KMnO₄ was that different value of COD obtained due to strength change of KMnO₄. BOD value obtained greater than COD with KMnO₄ means KMnO₄ was not oxidizing all the substances. Tthen ceric sulphate potassium loadate and potassium dichromate all tested separately and at the end potassium sichromate was found practical.

Pottassium dichromate is used in excess a mount to oxidize all the organic matter, this excess aomunt can be found at the end by using ferrousiion.

Method for cod test :

1. open reflux (drawback: end product is dangerous and cannot be discharged in open draws)
2. close reflux (same chemicals as for open reflux but sample and chemicals used in less quantity) spectro photometric (septrophotometer) titremetric ( titration)

Chemicals/ regents in open reflux method:

1. Potassium di-chromate (oxidation agents)
2. Sulphuric acid.
3. Mercuri sulphate (Hgs04)
4. Ferrous ammonium sulphate (Fe NH4)2 (So4)2 0.25 N used as tritrante,
5. Fezroin indicator.

Limitations of COD:

• cannot differentiate between biodegradable and non-biodegradable material
• N-value cannot be accurately found.

Advantages of COD:

1. can be performed in short time i.e 30 min can be correlated with BOD with a factor.
2. More biological resistant matter, more will be the difference in Bod and Cod results,

Apparatus

1. Digestion vessels (vial)

2. COD Reactor

3. Spectro-photometer

4. Premixed Reagents in Digestion Vessel (vials)

5. K₂G₂O₇

6. Concentrated H₂SO4

7. HgSO4

8. Ag2SO4

Procedure:

1. Place Approximately 500ml Of Sample In a clean blender bowl and homogenize at high speed for two minutes. blending the sample ensures a uniform distribution of suspended solids and thus improves the accuracy of test results.

2. Pre heat the COD reaction to Iso c

3. Carefully remove the cap of COD digestion Reagent vial.

4. While holding The vial at a 45 degree angle carefully pipette 2 ml sample into the vial.

5. Replace and tighten the cap.

6. Holding the vial by the cap in an empty sink, gently invert several times to mix the contents they will become very hot during mixing.

7. Place the vial in preheated COD reaction.

8. Prepare a reagent blank by repeating step 3 through 6, substituting2 ml of distilled water in place of sample.

9. Incubate the vial for two hours at size.

10. Turn off the reaction off and allow the vials to cool to 120 degree and less. invert each vial several times while still warm place vial in a cooling reach and allow them to room temp.

11. Measure the COD using spetrcophotometer method.