Chemical Oxygen Demand (COD) is a critical analytical parameter used to measure the total quantity of oxygen required to chemically oxidize all organic and oxidizable inorganic matter present in a water sample.
It serves as a rapid and robust indicator of the overall pollution strength of wastewater, effluents, and surface water, reflecting the total load of oxidizable pollutants.
What Does the COD Test Measure
Unlike other tests that target specific fractions, the COD test provides a comprehensive oxidation capacity by quantifying the oxygen equivalent needed to decompose.
Biodegradable Organic Matter: Substances that can be broken down by microorganisms.
Non Biodegradable Organic Matter: Refractory organics that resist biological degradation.
Oxidizable Inorganic Substances: Reduced chemical species in states such as chloride (Cl⁻), cyanide (CN⁻), and nitrite (NO₂⁻) are also oxidized during the strong chemical digestion.
Key Insight: COD delivers a complete picture of chemically oxidizable material, making it invaluable for assessing industrial wastewater where non-biodegradable organics are prevalent.
COD vs BOD Understanding the Critical Difference
A common point of comparison is the Biochemical Oxygen Demand (BOD) test. While both gauge organic pollution, their mechanisms and applications differ significantly.
Feature Chemical Oxygen Demand (COD) Biochemical Oxygen Demand (BOD) Oxidation Method Strong chemical oxidants (Potassium Dichromate) Microbial respiration (aerobic bacteria) Time Required 3 to 4 hours for results 5 days (BOD₅) standard incubation Scope Oxidizes nearly all organic & some inorganic matter Measures only the biodegradable fraction of organics Ammonia (NH₃) Oxidation Not oxidized under standard conditions (similar to BOD) Not oxidized in the standard 5-day test.
The Major Advantage of COD: The most prominent benefit of COD analysis is speed. Obtaining results within 4-5 hours, compared to the 5-day wait for BOD, enables real-time process control, rapid troubleshooting in treatment plants, and timely regulatory compliance checks.
Key Applications and Importance of COD Testing
1. Wastewater Treatment Plant: Monitoring Used to assess treatment efficiency, optimize chemical dosing, and control sludge activity.
2. Industrial Effluent Control: Essential for monitoring discharges from industries like pharmaceuticals, textiles, and chemicals that contain complex, non-biodegradable waste.
3. Pollution Load Assessment: Determines the total oxidizable pollutant load entering a water body, crucial for environmental impact studies.
4. Correlation with BOD: Established COD:BOD ratios can help quickly estimate the biodegradable fraction and BOD for routine monitoring.
5. Regulatory Compliance: A standard parameter in discharge permits worldwide to enforce water quality standards.
Principle
The organic matter gets oxidized completely by K2Cr2 O7 in the presence of H2SO4 (50% of the total volume) to produce CO2 + H2O at an elevated temperature. The excess K2Cr2O7 remaining after the oxidation of organic matter is titrated with standard Fe(NH4)2(SO4)2.
The dichromate consumed gives the O2 required for oxidation of organic matter.
InterferencesFatty acids, straight chain aliphatic compound, chlorides, nitrites and iron are the main interfering radicals.
1mg/lit Cl- exert 0.23 mg/L COD while 1mg/L NO2 exerts 1.1 mg/L COD.
These interferences can be eliminated as described below
Addition of HgSO4 eliminates Cl interference by precipitating Cl as HgCl2.
Addition of silver sulfate as a catalyst to concentrated H2SO4 stimulates oxidation of straight chain aliphatic and aromatic compound. Sulphamic acid in the amount of 10mg/mg NO2 may be added to K2Cr2O7 solution to avoid interference caused by NO2.
Regents
1. Standard potassium dichromate (0.250 N): Dissolve 12.259g K2Cr2O7 dried at 103oC for 24 hrs in distilled water and dilute to 1000ml. Add about 120mg sulphamic acid to take care of 6mg/L NO2-N
2. Sulphuric acid reagent: 10g of Ag2SO4 (AR grade) to 1000ml concentrated H2SO4 and keep overnight for dissolution.
3. Standard ferrous ammonium sulfate (0.25 M): Dissolve 98g Fe(NH4)2(SO4)2 6H2O in distilled water. Add 20 ml conc. H2SO4, cool and dilute to 1000ml and standardize.
Note: For standardization of ferrous ammonium sulfate use 10ml std.
K2Cr2O7 diluted to 100ml and acidify by adding 30ml H2SO4.
Titrate with ferrous ammonium sulfate to be standardized using ferroin indicator. Calculate normality.
4. Ferroin indicator: Dissolve 1.485g 1,10 phenanthroline monohydrate and 695 mg FeSO4. 7H2O and dilute to 100ml with distilled water.
5. Mercuric sulfate: HgSO4 crystals, analytical grade.
6. Standard solution for COD: 425 mg potassium hydrogen phthalate in 1L of distilled water exert 500 mg/l COD or 469 mg glucose in 1L distilled water exert 500mg/L COD.
Procedure
1) Place 0.4g HgSO4 in a reflux flask.
2) Add 20 ml sample or an aliquot of sample diluted to 20ml with distilled water mix well.
3) Add pumic stone or glass beads followed by 10ml std. K2Cr2O7.
4) Add slowly 30 ml H2SO4 contained Ag2SO4 mixing thoroughly. This slow addition along with swirling prevents fatty acids to escape out due to high temperature.
5) Mix well; if the colour turns green, either take fresh sample with lesser aliquot or diluted sample.
6) Connect the flask to condenser. Mix the contents. Improper mixing result in bumping and sample may be blown out.
7) Reflux for a minimum of 2 hrs, then cool and wash down the condenser with distilled water.
8) Dilute for a minimum 150 ml cool and titrate excess K2Cr2O7 with 0.25 N ferrous ammonium sulfate using ferroin indicator. Sharp color change from blue green to red indicates end point or completion of the titration.
9) Reflux blank in the same manner using distilled water instead of sample.
Note: For small volumes i.e. 10,20,30,40 ml of sample, proportionate reduction of potassium dichromate and sulphuric acid may be done.
Calculation
Calculate COD from formula
COD mg/l = (a-b) x Mx 8000/mlsample
a = ml Ferrous Ammonium Sulfate for blank
b = ml Ferrous Ammonium Sulfate for sample
M = Molarity of Ferrous Ammonium Sulfate
Reactions
1) CnHaOb + c Cr2 O 7 2- +8cH + a + 8c H2O + 2c Cr3+
where
c = 2/3n + a/6 = b/3
2) 6 Fe2+ + Cr2 O7 2- + 14H+ 6 Fe3+ + 2Cr3 + 7H2O
3) 6 Cl- + Cr2 O7 2- 14H+ 3Cl2 + 2 Cl2 + 2 Cr3+ + 7H2O
4) Hg2+ + 2Cl- HgCl2
Conclusion
The COD test is an indispensable, rapid tool for quantifying the total polluting potential of water. By providing a faster and more comprehensive measure of oxidizable matter than the BOD test, it empowers engineers, scientists, and environmental professionals to make informed decisions for effective water quality management and pollution control. Its ability to detect both biodegradable and non biodegradable organics makes it particularly vital for managing complex industrial wastewater streams.