Five Detection Methods and Principles of COD

Five Detection Methods and Principles of COD

What is COD?

Chemical Oxygen Demand (COD), also known as chemical oxygen consumption, is a measure of the amount of oxygen required to oxidize organic and inorganic matter in water using a chemical oxidant such as potassium permanganate. COD is a critical parameter indicating the level of water pollution. The higher the COD value, the greater the pollution by organic matter. COD is expressed in milligrams per liter (mg/L). According to COD values obtained from water quality monitoring instruments, water quality is classified into five categories: Class I and Class II (COD ≤ 15 mg/L) are suitable for drinking water standards; water exceeding Class II is unsuitable for drinking. Class III (COD ≤ 20 mg/L), Class IV (COD ≤ 30 mg/L), and Class V (COD ≤ 40 mg/L) represent increasing levels of pollution.

Five Detection Methods for COD

  1. Dichromate Reflux Method:

    • Principle: In a sulfuric acid medium, potassium dichromate is used as the oxidant, with silver sulfate as the catalyst and mercury sulfate as the chloride ion masking agent. The digestion reaction solution has a sulfuric acid acidity of 9 mol/L and is heated to a boiling point of 148℃±2℃ for 2 hours. After cooling, the remaining potassium dichromate is titrated with ferrous ammonium sulfate to calculate the COD value.
    • Advantages and Disadvantages: This method requires a large experimental setup, high water and electricity consumption, significant reagent use, and is inconvenient for rapid and large-scale determinations.

  2. Potassium Permanganate Method:

    • Principle: Potassium permanganate is used as the oxidant, and the COD measured is referred to as the permanganate index (CODMn). After acidifying the water sample with sulfuric acid, potassium permanganate is added and heated in a boiling water bath for 30 minutes. Excess sodium oxalate is then added to reduce the remaining permanganate, followed by titration with potassium permanganate to calculate the permanganate index.
    • Advantages and Disadvantages: The method produces less pollution than the national standard method but requires significant time for titration. Additionally, it has lower oxidation efficiency, resulting in a CODMn value lower than the CODCr value by a factor of 3-8. CODCr is preferred for wastewater with high reducing pollutants, while CODMn is suited for river and surface water with lower pollutants.

  3. Spectrophotometry:

    • Principle: Similar to the national standard method, this involves reducing substances in the sample reacting with potassium dichromate to produce trivalent chromium ions. These ions have strong absorption at 600 nm, and their absorbance follows Lambert-Beer’s law. The COD value is indirectly measured by the absorbance of trivalent chromium.
    • Advantages and Disadvantages: This method saves time on reagent preparation and avoids titration, making it easy to operate. However, the digestion process still requires 2 hours.

  4. Rapid Digestion Method:

    • Principle: To expedite analysis, this method increases oxidant concentration, sulfuric acid acidity, reaction temperature, and co-catalysts. This reduces digestion time from 2 hours to 10-15 minutes.
    • Advantages and Disadvantages: Increased sulfuric acid acidity (from 9.0 mg/L to 10.2 mg/L) and reaction temperature (from 150℃ to 165℃) speeds up the process. However, microwave ovens used vary in power and time settings.

  5. Rapid Digestion Spectrophotometry:

    • Principle: This method uses a sealed tube for digestion, where a small water sample and reagents are heated in a constant temperature dish. COD is measured by spectrophotometry.
    • Advantages and Disadvantages: It requires less space, lower energy consumption, minimal reagent use, and produces minimal waste. It is simple, safe, stable, accurate, and suitable for mass determinations.

Common Issues and Solutions in COD Testing

  1. How to Remove Chlorine Interference:

    • Prepare a blank sample with deionized water containing the same chlorine concentration as the sample. Subtract the chlorine influence from the test result.

  2. Sources of Error in COD Testing:

    • Chlorine is the main source of error. Mercury sulfate in the dichromate method and vacuum pretreatment in the MnIII method can remove chlorine interference.

  3. Reusing COD Blanks:

    • The same blank sample can be reused for multiple tests if stored in the dark. Monitor absorbance over time to ensure stability.

  4. Verifying COD Test Accuracy:

    • Use a COD standard solution within the concentration range of your samples. If test results match the standard concentration, the instrument and reagents are working correctly.

  5. Diluting Samples for COD Determination:

    • Samples can be diluted, and the diluted sample is added to the test bottle. Multiply the final result by the dilution factor.