Chemical oxygen demand

Generally, sulfuric acid and silver sulfate are used as catalysts; the addition of mercuric sulfate is primarily intended to eliminate the interference of chloride ions. However, when the chloride ion concentration is too high, dilution is necessary

Chloride ions can generally be handled without dilution as well :)

Excessively high chloride ions can have a masking effect on COD, leading to inaccurate measurement values; therefore, it is necessary to remove the chloride ions.

Sharing some brief information: Analysis of methods for determining chemical oxygen demand. Chemical oxygen demand refers to the amount of oxidizing agent required to oxidize the reducing substances in one liter of water sample under certain conditions, expressed in mg/L of oxygen. Reducing substances in water include organic compounds as well as inorganic substances such as nitrites, sulfides, and ferrous salts. Chemical oxygen demand reflects the degree of contamination in water by reducing substances. Given that water bodies are frequently polluted by organic substances, this indicator is also used as one of the comprehensive measures for the relative content of organic matter; however, it can only reflect those organic substances that can be oxidized by oxidizing agents. To determine the chemical oxygen demand in wastewater, China’s standards specify the potassium dichromate method (GB 11914—89). Other methods include the industry standard chlorine correction method (HJ/T 70—2001), the potassium iodide-potassium permanganate method (HJ/T 132—2003), and the rapid digestion spectrophotometry method (HJ/T 399—2007). 1. Potassium dichromate method (GB 11914—89): Under certain conditions, it refers to the mass concentration of oxygen corresponding to the amount of dichromate consumed by the soluble substances and suspended solids in the water sample during oxidation with potassium dichromate. 1.1 Principle: A known amount of potassium dichromate solution is added to the water sample. In a strong acid medium, with silver salts as catalysts, after boiling and refluxing, ferroin is used as an indicator to titrate the unreduced potassium dichromate in the sample using ammonium ferrous sulfate. The mass concentration of oxygen consumed is determined from the amount of ammonium ferrous sulfate used. Under acidic potassium dichromate conditions, aromatics and pyridine are difficult to oxidize, resulting in a low oxidation rate. Under the catalysis of silver sulfate, straight-chain aliphatic compounds can be effectively oxidized. 1.2 Scope of application: Applicable to various types of water samples with a COD value greater than 30 mg/L; the upper limit for measuring undiluted water samples is 700 mg/L. This standard is not applicable to saline waters with a chloride concentration greater than 1000 mg/L (after dilution). 1.3 Instruments: Common laboratory instruments, a reflux apparatus, a heating device, and 25ml or 50ml acid burettes. 1.4 Disadvantages: When using the potassium dichromate method to analyze high-concentration saline wastewater, it is necessary to dilute the water sample. For samples with high levels of Cl- and relatively low COD values, the measurement errors are significant. This standard does not apply to brine with a chloride ion concentration greater than 1000 mg/L (after dilution). When the chloride ion concentration exceeds 1000 mg/L, the minimum allowable value for COD is 250 mg/L; accuracy becomes unreliable below this value. Therefore, GB11914-89 is not applicable to high-chloride wastewater, such as the determination of COD in oilfield exploration, development, and production wastewater. Furthermore, the potassium dichromate method for determining chemical oxygen demand also leads to an increase in mercury- and silver-contaminated wastewater, causing secondary environmental pollution. 2. Chlorine correction method (HJ/T 70—2001) 2.1 Principle: A known amount of potassium dichromate solution and mercury sulfate solution is added to the water sample. In a strong acidic medium, silver sulfate is used as a catalyst; after 2 hours of boiling under reflux, 1,10-phenanthroline is used as an indicator to titrate the unreduced potassium dichromate in the sample using ammonium ferrous sulfate. The mass concentration of oxygen consumed is calculated from the amount of ammonium ferrous sulfate used, which represents the apparent COD value. The chlorine gas formed from the chloride ions in the water sample that were not complexed and thus oxidized was extracted; this gas was then absorbed using a sodium hydroxide solution. Potassium iodide was added, and the pH was adjusted to around 3–2 using sulfuric acid. With starch as an indicator, it was titrated using a standard sodium thiosulfate solution. The amount of sodium thiosulfate consumed was converted into the mass concentration of oxygen consumed, which represents the correction value for chloride ions. The difference between the apparent COD and the chloride correction value represents the true COD of the tested water sample. 2.2 Scope of application This method is applicable to the determination of chemical oxygen demand (COD) in high-chloride wastewater with a chloride ion content of less than 20,000 mg/L. The method’s detection limit is 30 mg/L. Suitable for the determination of COD in wastewater from oil fields, coastal refineries, oil depots, chlor-alkali plants, and deep-sea discharge of wastewater. 2.3 Instruments: Common laboratory instruments, a reflux absorption apparatus, a heating device, a nitrogen flow meter (a float-type flow meter with a flow range of 5–40 ml/min), and 25 ml or 50 ml acid burettes. 2.4 Disadvantages: Long detection time and high costs. 3. Potassium iodide-alkaline potassium permanganate method (HJ/T 132—2003): Under alkaline conditions, potassium permanganate is used to oxidize reducing substances in wastewater (with the exception of nitrites); the remaining potassium permanganate is then reduced using potassium iodide. The mass concentration of oxygen is determined based on the amount of potassium permanganate consumed by the water sample. 3.1 Principle Under alkaline conditions, a certain amount of potassium permanganate solution is added to the water sample, and the mixture is heated in a boiling water bath for a specified period of time to oxidize the reducing substances present in the water. An excess of potassium iodide is added to reduce the remaining potassium permanganate; starch is used as an indicator, and sodium thiosulfate is used to titrate the iodine released. The concentration of oxygen is then calculated and expressed as CODOH,KI. 3.2 Scope of application This method is applicable to the determination of chemical oxygen demand in high-chloride wastewater from oil and gas fields and refining enterprises, where the chloride ion concentration can range from tens of thousands to over a hundred thousand mg/L. The method has a minimum detection limit of 0.20 mg/L and a maximum measurement limit of 62.5 mg/L. 3.3 Instruments: Common laboratory apparatus, boiling water bath, brown acid burette, G-3 glass frit funnel. 3.4 Disadvantages: Due to the different oxidation conditions used in the potassium iodide-permanganate method and the potassium dichromate method, the measurement values for the same sample vary. Moreover, the COD parameter specified in China’s comprehensive wastewater discharge standards refers to the results obtained using the potassium dichromate method. Therefore, it is necessary to convert CODOH and KI into CODcr, and this conversion can be accomplished by determining the ratio K between the potassium iodide alkaline permanganate method and the potassium dichromate method. Since it is high-chloride wastewater, errors are inevitable in the calculation of CODcr during the determination of the K value, which leads to inaccuracies in the K value. Errors also occur when converting CODOH and KI into CODcr. 4. Rapid digestion spectrophotometry: Environmental water samples contain complex components, and the concentrations of most pollutants are low; moreover, these pollutants exist in various forms. Therefore, treatment is often required prior to analysis. The purpose of the digestion process is to destroy organic matter, dissolve suspended solids, and oxidize the elements of interest in various oxidation states to a single higher oxidation state or convert them into inorganic compounds that are easy to separate. 4.1 Principle: A potassium dichromate solution is added to the sample; in a strong sulfuric acid medium, with silver sulfate as a catalyst, high-temperature digestion is carried out, followed by the determination of the COD value using spectrophotometry. When the COD value of the sample is between 100 mg/L and 1000 mg/L, the absorbance of trivalent chromium resulting from the reduction of potassium dichromate is measured at a wavelength of 600 nm ± 20 nm. There is a direct proportional relationship between the COD value of the sample and the increase in absorbance of trivalent chromium; the absorbance of trivalent chromium is then converted into the COD value of the sample. When the COD value of the sample is between 15 mg/L and 250 mg/L, the total absorbance of both hexavalent chromium, which remains unreduced by potassium dichromate, and trivalent chromium, which is formed as a result of reduction, is measured at a wavelength of 440 nm ± 20 nm. The COD value in the sample is proportional to the decrease in absorbance of hexavalent chromium, proportional to the increase in absorbance of trivalent chromium, and proportional to the overall decrease in absorbance; the total absorbance value is then converted into the COD value of the sample. 4.2 Scope of application It is applicable to the determination of chemical oxygen demand (COD) in surface water, groundwater, domestic wastewater, and industrial wastewater. For undiluted water samples, the lower limit for COD determination is 15 mg/L, and the upper limit is 1000 mg/L; the chloride ion concentration should not exceed 1000 mg/L. For water samples with a chemical oxygen demand (COD) greater than 1000 mg/L or a chloride content greater than 1000 mg/L, measurements can be taken after appropriate dilution. 4.3 Instruments: digestion tubes, heaters, photometers (with a photometric measurement range of at least 0–2 absorbance units, and a digital display sensitivity of 0.001 absorbance units), digestion tube holders, centrifuges, manual pipettes, Class A volumetric pipettes, flasks and measuring cylinders, and stirrers. 4.4 Chemical Oxygen Demand Analyzer: This method utilizes the technical principle of a chemical oxygen demand analyzer, which is based on spectrophotometry. A specified amount of potassium dichromate is used to oxidize water under certain conditions, causing hexavalent chromium to be converted quantitatively into trivalent chromium. The COD level is then determined using the absorbance peak at 610 nm for trivalent chromium or the absorbance peak at 420 nm for hexavalent chromium. Such instruments consist of a digestion furnace section and a measurement section. As stated in 1.1/1.2 above, sulfuric acid-mercuric sulfate can be used as a catalyst when measuring chemical oxygen demand. However, the chloride ion content should be properly controlled. Not very familiar with it; just for reference. This post was last edited by fengxuemei on 2009-4-18 21:16.]

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Thank you all. We use the methanecromic acid method, with sulfuric acid and mercuric sulfate as catalysts; no other reagents are needed to remove chloride ions. Is this method good?

Did you read the previous replies? The potassium dichromate method (GB 11914—89) refers to the mass concentration of oxygen corresponding to the dichromate consumed by the soluble substances and suspended solids in the water sample during oxidation treatment with potassium dichromate under certain conditions. 1.1 Principle: A known amount of potassium dichromate solution is added to the water sample. In a strong acid medium, with silver salts as catalysts, after boiling and refluxing, ferroin is used as an indicator to titrate the unreduced potassium dichromate in the sample using ammonium ferrous sulfate. The mass concentration of oxygen consumed is determined from the amount of ammonium ferrous sulfate used. Under acidic potassium dichromate conditions, aromatics and pyridine are difficult to oxidize, resulting in a low oxidation rate. Under the catalysis of silver sulfate, straight-chain aliphatic compounds can be effectively oxidized. 1.2 Scope of application: Applicable to various types of water samples with a COD value greater than 30 mg/L; the upper limit for measuring undiluted water samples is 700 mg/L. This standard is not applicable to saline waters with a chloride concentration greater than 1000 mg/L (after dilution). 1.3 Instruments: Common laboratory instruments, a reflux apparatus, a heating device, and 25ml or 50ml acid burettes. 1.4 Disadvantages: When using the potassium dichromate method to analyze high-concentration saline wastewater, it is necessary to dilute the water sample. For samples with high levels of Cl- and relatively low COD values, the measurement errors are significant. This standard does not apply to brine with a chloride ion concentration greater than 1000 mg/L (after dilution). When the chloride ion concentration exceeds 1000 mg/L, the minimum allowable value for COD is 250 mg/L; accuracy becomes unreliable below this value. Therefore, GB11914-89 is not applicable to high-chloride wastewater, such as the determination of COD in oilfield exploration, development, and production wastewater. Furthermore, the potassium dichromate method for determining chemical oxygen demand also leads to an increase in mercury- and silver-contaminated wastewater, causing secondary environmental pollution. If the chloride ion content in your sample is no more than 1000 mg/L, it is acceptable ； If it is too high, the chloride content still needs to be removed or reduced.

The purpose of adding mercuric sulfate is mainly to eliminate the interference of chloride ions; however, when the chloride ion concentration is too high, analytical methods for high-chloride wastewater can be employed