Definition
A chemical oxygen demand analyzer is a laboratory or online testing instrument used to determine chemical oxygen demand in water bodies. Chemical oxygen demand refers to the amount of oxidant consumed when treating water samples with strong oxidants under strong acid and heating conditions, expressed in milligrams per liter of oxygen. This index is one of the key parameters for evaluating the degree of organic matter pollution in water bodies and measuring the effectiveness of wastewater treatment, and is widely used in environmental monitoring, municipal water affairs and industrial process control.
Principle
The core principle of the chemical oxygen demand analyzer is based on redox reactions. The organic matter in the water sample is oxidized by a strong oxidant (usually potassium dichromate or potassium permanganate) under specific conditions, and the amount of oxygen consumed in the water sample is indirectly calculated by measuring the consumption of the oxidant or the change in the reaction products. According to different measurement methods, it is mainly divided into titration method, colorimetric method and electrochemical method. Among them, the potassium dichromate method has been adopted by many standard methods at home and abroad due to its high oxidation rate and good reproducibility.
Measurement method
Common measurement methods mainly include standard reflow method, rapid digestion spectrophotometry and online monitoring method. The standard reflux method is often regarded as a classic method in which a water sample is heated and refluxed in a sulfuric acid medium with a known amount of potassium dichromate solution and silver sulfate catalyst, and the remaining potassium dichromate is titrated with a standard solution of ferrous sulfate after cooling. The results of this method are accurate but time-consuming.
The rapid digestion spectrophotometry method is based on measuring the absorbance of trivalent chromium ions at specific wavelengths, the concentration of which is proportional to the amount of potassium dichromate consumed by the water sample, resulting in the calculation of chemical oxygen demand values. This method significantly reduces analysis time and is suitable for rapid determination of bulk samples. The online monitoring method realizes continuous or intermittent real-time monitoring through automatic sampling, digestion and measurement units.
Influencing factors
The accuracy of chemical oxygen demand measurement results is influenced by a variety of factors. Chloride ions in water samples are common interfering substances that react with oxidants at high concentrations, resulting in high results, usually requiring the addition of mercury sulfate for masking. The control of digestion temperature and time is crucial, insufficient digestion will make oxidation incomplete, and excessive digestion may cause the loss of volatile substances. The concentration and amount of oxidant added, the effectiveness of the catalyst, and the uniformity and representativeness of the water sample body all have a direct impact on the measurement. In addition, for water samples with complex compositions, appropriate pretreatment methods should be selected according to their characteristics.
Application:
Chemical oxygen demand analyzers have a wide range of applications. In the field of environmental monitoring, it is used for routine monitoring of surface water, groundwater and various types of discharged wastewater to assess the status of organic pollution in water bodies. Municipal wastewater treatment plants use it to monitor chemical oxygen demand in incoming and outgoing water to optimize process operations and ensure compliant discharges. In industries such as food processing, papermaking, and chemicals, this instrument is used to monitor wastewater loads during production and the operational efficiency of internal treatment facilities. scientific research institutions use it to develop water treatment technology and related environmental science research.
Selection
When choosing a chemical oxygen demand analyzer, it is necessary to comprehensively consider the measurement needs and application scenarios. For intermittent sample analysis in the lab, a multi-channel digester with spectrophotometer capabilities with batch processing capabilities can be selected. If continuous monitoring of wastewater treatment processes or outlets is required, consider an online analyzer with automatic cleaning, calibration, and fault diagnosis. The measurement range of the instrument should cover the expected concentration of the water sample to be measured. Method compliance is a concern to ensure that the instrument uses a method that meets or is equivalent to the national or industry standards being implemented. At the same time, ease of operation, reagent consumption and waste disposal costs, and ease of instrument maintenance are also considerations in the selection process.