Chemical Oxygen Demand (Chemical Oxygen Demand) test determines the oxygen requirement equivalent of organic matter that is susceptible to oxidation with the help of a strong chemical oxidant. It is an important, rapidly measured parameters as a means of measuring organic strength for streams and polluted water bodies.
The test can be related empirically to BOD, organic carbon or organic matter in samples from a specific source taking into account its limitations. The test is useful in studying performance evaluation of wastewater treatment plants and monitoring relatively polluted water bodies.
Chemical Oxygen Demand determination has the test is useful in studying performance evaluation of wastewater treatment plants and monitoring relatively polluted water bodies. Chemical Oxygen Demand determination has an advantage over BOD determination. Chemical Oxygen Demand results can be obtained in 3-4 hrs as compared to 3-5days required for BOD test.
Open Reflux Method
The open reflux method is suitable for a wide range of wastes with a large sample size. The dichromate reflux method is preferred over procedures using other oxidants (e.g. potassium permanganate) because of its superior oxidizing ability, applicability to a wide variety of samples and ease of manipulation. Oxidation of most organic compounds is up to 95-100% of the theoretical value.
The organic matter gets oxidized completely by potassium dichromate (K2Cr2O7) with silver sulphate as a catalyst in the presence of concentrated H2SO4 to produce CO2 and H2O. The excess K2Cr2O7 remaining after the reaction is titrated with ferrous ammonium sulphate [Fe (NH4)2(SO4)2]. The dichromate consumed gives the oxygen (O2) required for oxidation of the organic matter. The chemical reactions involved in the method are as under:
- 2K2Cr2O7 + 8 H2SO4 ® 2 K2 SO4 + 2Cr2(SO4)3 + 8 H2O + 3O
- C6H12O6 + 6O2 ® 6CO2 + 6H2O
- Cr2O7— + 6Fe++ + 14H+ ® 6Fe+++ + 2Cr3+ + 7H2O
Apparatus & Equipment
- 250 or 500mL Erlenmeyer flask with standard (24/40) tapered glass joints.
- Friedrich’s reflux condenser (12 inches) with standard (24/40) tapered glass joints.
- Electric hot plate or six-unit heating shelf.
- Volumetric pipettes (10, 25, and 50mL capacity).
- Burette, 50mL with 0.1mL accuracy.
- Burette stand and clamp.
- Analytical balance, accuracy 0.001g.
- Volumetric flasks (1000mL capacity).
- Boiling beads, glass.
- Magnetic stirrer and stirring bars.
Reagents & Standards
- Standard potassium dichromate solution, 0.25N (0.04167 M): Dissolve 12.259g K2Cr2O7 dried at 103°C for 24h in distilled water and dilute to 1000mL. Add about 120mg sulphamic acid to take care of 6 mg/L NO2 – N.
- Sulphuric acid reagent: Add 10g of Ag2SO4 to 1000mL concentrated H2SO4 and let stand for one to two days for complete dissolution.
- Standard ferrous ammonium sulphate approx. 0.25N (0.25M): Dissolve 98g Fe(NH4)2(SO4)2.6H2O in about 400mL distilled water. Add 20mL concentrated H2SO4 and dilute to 1000mL.
- Ferroin indicator: Dissolve 1.485g 1, 10-phenanthroline monohydrate and 695mg FeSO4.7H2O in distilled water and dilute to 100mL.
- Mercuric Sulphates: HgSO4, crystals, analytical grade.
- Potassium hydrogen phthalate (KHP) Standard: Dissolve 425mg lightly crushed dried potassium hydrogen phthalate (HOOC.C6H4.COOK) in distilled water and dilute to 1000mL. This solution has a theoretical Chemical Oxygen Demand of 500μg O2/mL. This solution is stable when refrigerated, up to 3 months in the absence of visible biological growth.
Sample Collection, Preservation & Storage
Preferably collect samples in glass bottles. Homogenise samples containing settleable solids. If there is a delay in collection and analysis, preserve sample by acidification to pH≤2 using concentrated H2SO4. Samples can be preserved for maximum 7 days.
All samples high in solids should be blended for 2 minutes at high speed and stirred when an aliquot is taken for analysis. Select the appropriate volume of the sample based on expected Chemical Oxygen Demand range, e.g. for Chemical Oxygen Demand range of 50-500 mg/L take 25-50mL of the sample.
Sample volumeless than 25mL should not be pipetted directly, but serially diluted and then a portion of the diluted sample taken. Dilution factor should be incorporated in calculations.
- 500mL of sample diluted to 1000mL = 0.5mL sample/mL of diluent, 50mL = 25mL of sample.
100mL of sample diluted to 1,000mL = 0.1mL sample/mL diluent, 50mL of diluent = 5mL of sample.
Reflux Of Samples
- Place 0.4g HgSO4 in a 250mL reflux sample.
- Add 20mL sample or an aliquot of sample diluted to 20mL with distilled water. Mix well.
- Add clean pumic stones or glass beads.
- Add 10mL 0.25N (0.04167M) K2Cr2O7 solution and mix.
- Add slowly 30mL concentrated H2SO4 containing Ag2SO4 mixing thoroughly. This slow addition along with swirling prevents fatty acids to escape due to the generation of high temperature. Alternatively, attach flask to the condenser with water flowing and then add H2SO4 slowly through the condenser to avoid escape of volatile organic substance due to the generation of heat.
- Mix well. If the color turns green, either take a fresh sample with lesser aliquot or add more potassium dichromate and acid.
- Connect the flask to the condenser. Mix the contents before heating. Improper mixing will result in bumping and blow out of flask content.
- Reflux for a minimum of 2 hours. Cool and then wash down condenser with distilled water.
- Disconnect reflux condenser and dilute the mixture to about twice its volume of distilled water. Cool to room temperature and titrate excess K2Cr2O7 with0.1M FAS using 2-3 drops of ferroin indicator. The sharp color change from blue green to reddish brown indicates end-point or completion of the titration. After a small time gap, the blue-green color may reappear. Use the same quantity of ferroin indicator for all titrations.
- Reflux blank, in the same manner, using distilled water instead of a sample. An alternate procedure for low Chemical Oxygen Demand samples less than 50mg/L: Follow similar procedure with two exceptions (i) use standard 0.025N (0.004167M) K2Cr2O7 and (ii) Titrate with standardizing 0.025M FAS. The sample volume should be 5.mL. Exercise extreme care with this procedure because even a trace of organic matter on the glassware or from the atmosphere may cause gross errors. Compute amount of HgSO4 to be added based on chloride concentrations. Carry blank reagent through the same procedure.
Chemical Oxygen Demand as mg/L = (a –b) x N x 8000 / mL sample
a = mL FAS used for blank
b = mL FAS used for sample
N = normality of FAS
8000 = Milieq. wt. of O2 x 1000