Definition
The Chemical Oxygen Demand (COD) and Ammonia Nitrogen Analyzer is a multi-parameter detection equipment for water quality analysis, which can quantitatively determine the chemical oxygen demand and ammonia nitrogen content in water samples. Chemical oxygen demand reflects the total amount of reducing substances in water samples that can be oxidized by chemical oxidants, and is usually used to assess the degree of water body being affected by organic pollutants. Ammonia nitrogen refers to nitrogen in the form of free ammonia and ammonium ions in water, which is one of the key indicators for evaluating eutrophication and pollution in water bodies. The instrument is widely used in environmental monitoring, industrial production process control, and scientific research experiments, providing data support for water quality management and pollution control.
Principle
The determination of chemical oxygen demand is usually based on the redox reaction principle. Under strong acidic conditions, oxidants such as potassium dichromate or potassium permanganate are used to oxidize the reducing substances in the water sample during the heating digestion process, and the corresponding chemical oxygen demand value is calculated by measuring the oxidation dose consumed or the absorbance of trivalent chromium ions produced. Its reaction can be expressed as:
Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O
The determination of ammonia nitrogen is mostly by Knott reagent spectrophotometry or salicylic acid spectrophotometry. Taking the Knotts reagent method as an example, ammonia nitrogen reacts with Knott's reagent under alkaline conditions to form a yellowish-brown complex, the color shade of which is directly proportional to the ammonia nitrogen concentration, and can be measured at specific wavelengths (such as 420 nm) to quantitatively analyze ammonia nitrogen content.
Measurement method
The measurement of chemical oxygen demand mainly includes potassium dichromate digestion method and rapid digestion spectrophotometry. The operation steps of potassium dichromate method are more complex, involving the digestion, cooling and titration of water samples, but the results have high accuracy and applicability. The Fast Digestion Spectrophotometry method combines prefabricated reagents with micro-digestion devices to reduce analysis time and is suitable for rapid screening of bulk samples. In addition to the Knott reagent spectrophotometry, the measurement of ammonia nitrogen can also be done by distillation-titration or electrode method, which is widely used due to its ease of operation and high sensitivity. Modern analyzers often integrate digestion, reaction, and detection modules for automated or semi-automated operation.
Influencing factors
The results of chemical oxygen demand measurement were affected by factors such as oxidant type, digestion temperature and time, chloride ion interference and water sample uniformity. For example, high chloride concentrations can lead to high measured values, often masked by the addition of mercury sulfate. Ammonia nitrogen determination is interfered with by aqueous pH, turbidity, chromaticity, and coexisting ions (e.g., calcium and magnesium ions), and pretreatment steps such as flocculation precipitation or distillation can reduce interference. The calibration status of the instrument itself, the stability of the optical path and the quality of the reagent will also play a role in the measurement accuracy. Operators are required to follow standard procedures, regularly maintain instruments, and verify reagent effectiveness.
Application:
The instrument is suitable for water quality monitoring in a variety of scenarios. In the field of environmental monitoring, it is used for routine testing of surface water, groundwater and domestic sewage to assess the pollution load and self-purification capacity of water bodies. In industrial production, assist in monitoring the operation of wastewater treatment facilities, such as emission control in chemical, paper, food processing and other industries; In agriculture and aquaculture, it can be used to monitor ammonia nitrogen changes in aquaculture water and prevent eutrophication risks. In addition, in scientific research and educational institutions, it also provides analytical means for related experimental research. Through continuous or intermittent monitoring, it helps to realize the dynamic management of water quality and the traceability of pollution.
Selection
When choosing a chemical oxygen demand ammonia nitrogen analyzer, it is necessary to comprehensively consider the measurement range, accuracy, analysis speed and ease of operation. For routine water quality monitoring, the instrument should cover the expected concentration range of the target water sample, such as the chemical oxygen demand range can be set from low to high concentrations. In terms of accuracy, you can refer to national or international standards (such as ISO, EPA, or GB methods) that the instrument complies with, and pay attention to its repeatability and detection limit indicators. The speed of analysis involves the digestion time and detection cycle, and the rapid digestion instrument is suitable for large sample volumes. In addition, the degree of automation, reagent consumption, data storage function, and maintenance requirements of the instrument are also reference factors when selecting the instrument. Users should conduct a comprehensive evaluation based on actual application scenarios, budgets, and personnel operation levels.