I am using closed reflux method for internal monitoring of water analysis. At the same time, we do send samples to accredited lab for analysis. The results from lab (titration method) and the digestion method (commercial digestion vials) differ greatly (~1000mg/L COD). What can be the reason for the vast differences? Samples are taken from same point, same time, same preservation method.Thank you.
I think its the same procedure but you can use one of them according to the range of pollutant you have and open reflux method have wide raneg of pollutant.
The open reflux method (5220B) is suitable for a wide range of wastes where a large sample size is preferred.The closed reflux method (5220C and D) are more economical in the use of metallic salt reagents and generate smaller quantities of hazardous waste, but require homogenization of samples containing suspended solids to obtain reproducible results.
I think your sample need more homogenization in case for The closed reflux method,
please read the two method carefully
please see these sites:
https://www.standardmethods.org/store/ProductView.cfm?ProductID=37
http://www.envexp.com/technical/method-downloads/cod-method-410
regards.
hello there I agree with Dr.Mohamed Hassaan ,But it depends on your samples nature
The closed reflux method we use magnetic stirrer to stir the samples for 10 minutes, then pipiette 2 ml of the sample and add into the reagent vials. Heated for 2 hours at 150 degree celcius, and read the COD reading using the colorimeter. Still got difference with open reflux method.
Chemical Oxygen Demand (COD) is a critical test in environmental and wastewater treatment analysis. It measures the amount of oxygen required to oxidize organic and inorganic compounds in water. There are two common methods for determining COD: the open reflux method and the closed reflux method. While both aim to measure the same parameter, there are some differences in their procedures and the results they produce.
Open Reflux Method
- Procedure: Involves refluxing the sample with a strong oxidizing agent, usually dichromate in an acidic medium, in an open system. The refluxing is done for a few hours.
- Accuracy: Can be very accurate but is more susceptible to interference from external factors like air.
- Application: Suitable for a wide range of concentrations but more commonly used for high concentration samples.
- Handling and Safety: Requires careful handling due to the use of open vessels with hot, corrosive chemicals. More labor-intensive and time-consuming.
Closed Reflux Method
- Procedure: Similar chemical reaction as the open method, but it occurs in a sealed container, typically in small, digestible vials heated in a block digester.
- Efficiency: Generally quicker and requires less reagent than the open method.
- Safety: Safer and more convenient due to the closed system, which minimizes exposure to toxic reagents and vapors.
- Accuracy: Provides consistent results and is less prone to external contamination. However, it might be less accurate for very high concentration samples.
Comparison in Results
- Consistency: The closed method tends to be more consistent due to reduced chances of external contamination and evaporation losses.
- Sensitivity to Sample Type: The open method might be better for certain types of samples, especially those with high COD concentrations.
- Standardization: Both methods are standardized (e.g., by the EPA or ISO), but the exact protocol must be carefully followed for accurate results.
- Interferences: Each method has its own set of potential interferences and requires proper sample preparation to minimize them.
Conclusion
- While both methods aim to measure the same chemical oxygen demand in a water sample, their results might not always be directly comparable due to differences in procedure, sensitivity, and potential for interference. The choice between open and closed methods depends on the specific requirements of the analysis, including sample concentration, available equipment, and safety considerations. For precise comparison or regulatory compliance, it's essential to use the same method consistently.
l Check out this protocol list; it might provide additional insights for resolving the issue.
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