What is the difference between the theoretical value and the measured value of chemical oxygen demand?
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Ladies and gentlemen, third-party testing shows that in water containing 2 wt.% dichloromethane, the COD level is 1000 mg/L, which is equivalent to 500 mg/L of COD per 1 wt.% of dichloromethane； Theoretical calculations for the complete oxidation of dichloromethane yield a relationship between dichloromethane and COD: 5000 mg/L of COD per 1 wt.% of dichloromethane. Let’s discuss with everyone the reasons for the differences. 1. According to the Solvent Manual, the solubility of dichloromethane in water is 2 wt.%(at 20 degrees Celsius), which indicates that the gas chromatography method can be used to determine the dichloromethane content in water samples without any problems ； 2. The theoretical COD value was calculated with reference to the source document \"Differences between theoretical COD and experimentally measured COD using potassium dichromate\", and multiple calculations confirmed the result, ruling out any theoretical factors ； 3. Equipment issues with third-party testing agencies? (This company is a partner; tests have been conducted over the years, and in the presence of oil, detergents, and dichloromethane, the maximum level is 10,000 mg/L.) No other reasons have been identified at present; experienced colleagues, please share your insights. Thank you
This post was last edited by qugd on 2017-5-10 at 11:40. Could the original poster list the products into which dichloromethane is converted during its complete oxidation? This is key to calculating COD. Based on what you have listed, it seems that you are using the potassium dichromate method for sample digestion; in that case, you should consider the scope of application of this method as well as its limitations. COD determination is a conditional test; for a considerable number of organic compounds, especially those with halogen substitutions and aromatic structures, the oxidation or degradation process during measurement is incomplete. In other words, the theoretical oxidation value is not achieved, as the concentration of sulfuric acid and the concentration of the oxidizing agent, dichromate, decrease as the oxidation reaction proceeds during COD determination, preventing the reaction from reaching the extent expected by theoretical chemical thermodynamics. Furthermore, the volatility of dichloromethane also affects the accuracy and recovery rate of COD determination for this component. This limitation shows that conventional COD detection methods for environmental testing are not well suited for such samples. It is recommended that you use the TOC method for testing, as this may yield better representativeness of the results. Alternatively, a VOC analysis method can be used to directly detect the amount of dimethoxane or the amount of the target component.
Hello: 1. K2Cr2O7 + CH2Cl2 + hydrogen ions → H2O + CO2 + Cl2 Explanation: (1) Standard electrode potentials: E(Cl2/Cl-) = 1.36 eV; E(Cr2O72-/Cr3+) = 1.33 eV; under strongly acidic conditions, E(Cr2O72-/Cr3+) = 1.55 eV. It indicates that Cl2 can be generated. 2. COD is determined using the potassium dichromate titration method as specified by national standards, carried out by a third-party testing laboratory. The dichloromethane content is measured by gas chromatography during the test, and the sample is then sent to the third party for analysis; the water sample is sealed during shipping. This COD testing method is the standard for environmental impact assessment tests. 3. The volatilization issue you mentioned has been taken into account; the wastewater in question is a sample that has been left outdoors for about 1 day, meaning that the COD present in the sample is due to dichloromethane, which exists in a stable state. 4. Assuming it is due to incomplete reaction, then if only 1/10 of the dichloromethane reacts, the COD value of the resulting water sample will still be 1200 (while the COD of the water sample without dichloromethane remains stable at 200). ) The above is the actual situation; thank you
Hello, I reviewed the national standards for the potassium dichromate titration method yesterday, and found that the oxidation reaction using potassium dichromate takes place under full reflux for two hours. Due to the low concentration of dichloromethane, it is difficult to cool this solvent under full reflux, which results in an excessively low COD value in the tests. Thank you for the reply.
We use the potassium dichromate method to determine COD, and the digestion process takes 2 hours and 20 minutes. Various factors can affect the results of COD testing; in our experiments we found that if the chloride ion concentration is high, the amount of mercury sulfate added has a significant impact on the results. Generally, when the chloride ion level is high, the standard amount to add is 0.4 g; we usually add between 0.45 and 0.48 g. Moreover, the degree of boiling during digestion also affects the measurement results
Indeed, upon closer inspection, the national standard titration method for measuring COD fails to take many factors into account. I have a question: are there any requirements regarding the temperature and flow rate of the condensate water under full reflux conditions?
There doesn’t seem to be any standard requirements for that. We use a digestion apparatus, and it’s all automated. During my experiments, I conducted comparisons, and there were differences in the results obtained using an electric furnace compared to those obtained with the digestion apparatus. Also, the amount of sulfuric acid and silver sulfate used will affect the results. What do you use to dissolve it?
Thank you for sharing. I discovered during sewage testing that the COD values determined by third-party laboratories differed from those calculated theoretically; we don’t conduct COD tests on our own