Definition
The Chemical Oxygen Demand (COD) and Ammonia Nitrogen Online Analyzer is an automated analytical instrument for water monitoring. It can continuously or intermittently measure and output real-time chemical oxygen demand and ammonia nitrogen concentration in water samples, and is usually used in industrial processes and environmental protection scenarios where continuous monitoring of water quality is required.
Measurement principle
The instrument is measured based on proven chemical analysis and optical detection technology. For the determination of chemical oxygen demand, the high-temperature digestion method of potassium dichromate or potassium permanganate method is generally used, the core of which is to oxidize the reducing substances in the water sample under specific conditions with a strong oxidant, and then measure the oxidation dose consumed by titration or colorimetric method, so as to calculate the corresponding oxygen demand. The relationship can be expressed as follows: the COD value is directly proportional to the consumption of oxidant. For the determination of ammonia nitrogen, Knott reagent spectrophotometry or salicylic acid spectrophotometry is often used. Taking the Knotts reagent method as an example, ammonia ions react with Knott's reagent under alkaline conditions to form a yellow-brown complex, and its color depth is directly proportional to the ammonia nitrogen concentration A = εbc Conduct a quantitative analysis, where:Ais absorbance,εabsorbance coefficient for molarity,bfor the optical path,cis the concentration of ammonia nitrogen.
Measurement method
The workflow of an online analyzer typically consists of automated sampling, pre-treatment, reagent addition, reaction, measurement, and cleaning. The instrument collects the water sample to be tested according to a preset cycle, and after pretreatment such as filtration or homogenization, the specific reagent is injected by the precision pump tube system. Digestion or chromogenic reactions are completed in a temperature-controlled reaction unit. An optical detection system (e.g., a photometer) then measures the change in absorbance of the reaction solution at a specific wavelength, and the built-in processor converts the signal into a chemical oxygen demand or ammonia nitrogen concentration value based on a calibration curve and automatically cleans the measurement cell in preparation for the next measurement cycle.
Influencing factors
The accuracy of the measurement results is influenced by several factors. Chloride ions, suspended solids, greases, or other reducing substances in water samples can interfere with the determination of chemical oxygen demand, resulting in high or low results. For ammonia nitrogen determination, turbidity, chromaticity, and calcium and magnesium ions may interfere with water samples. Ambient temperature and reagent stability affect the reaction rate and color rendering effect. The sampling representativeness, pretreatment effect, light source stability, detector sensitivity, and rationality of the calibration cycle are also key to maintaining long-term measurement reliability. Regular maintenance, calibration, and targeted pre-treatment of interfering substances are necessary.
Applications
This type of instrument is widely used in the monitoring of inlet and outlet of municipal sewage treatment plants, the compliance monitoring of industrial wastewater discharge outlets (such as chemical, papermaking, food processing and other industries), the automatic monitoring station of surface water such as rivers and lakes, and the water quality management of various industrial production processes (such as circulating cooling water and boiler make-up water). It provides continuous dynamic data support for environmental regulation, process control and scientific research.
Selection considerations
When choosing the right online analyzer, a comprehensive evaluation is required. First, it is necessary to clarify the monitoring objectives, and select the measurement principle of matching the anti-interference ability according to the chemical oxygen demand of the water to be measured, the expected concentration range of ammonia nitrogen, and the possible interference substances. Secondly, consider the needs of the site for measurement frequency, response time, and data output method. Instrument reliability, ease of maintenance, reagent consumption costs, and adaptability to the operating environment (e.g., protection level, power supply conditions) also need to be taken into account. It is recommended to refer to relevant national or international technical standards and evaluate the technical support and service capabilities of suppliers.