Definition
The Chemical Oxygen Demand Water Quality Analyzer is an analytical instrument used to determine the chemical oxygen demand in water bodies. Chemical oxygen demand refers to the amount of oxidant consumed by reducing substances in a water sample that can be oxidized by strong oxidants under certain conditions, expressed in milligrams per liter of oxygen. This index is an important parameter for evaluating the degree of water pollution by reducing substances and measuring the relative content of organic matter in water bodies, and is widely used in environmental monitoring, municipal drainage, industrial process control and scientific research.
Principle
The core principle of the Chemical Oxygen Demand Water Quality Meter is based on redox reactions. In the presence of strong acidic media and catalysts, strong oxidants (usually potassium dichromate or potassium permanganate) are used to digest and oxidize the reducing substances in the water sample at high temperature. After the reaction, the oxidation dose consumed by the water sample is indirectly calculated by measuring the remaining amount of the oxidant that has not been reduced, or by measuring the concentration of products such as trivalent chromium ions produced by the reduced oxidant, so as to obtain the chemical oxygen demand value. The basic relationship can be expressed as follows: the oxidation dose consumed is directly proportional to the content of reducing substances (mainly organic matter) in the water sample.
Measurement method
According to the oxidants and detection methods used, common instrumental measurement methods are mainly divided into potassium dichromate method and alternative methods. The potassium dichromate method is currently recognized as the standard method, in which the instrument digests the water sample at high temperature and pressure, and then uses titration, spectrophotometry or electrochemical methods to determine the concentration of the remaining potassium dichromate or the resulting trivalent chromium ions. Spectrophotometry, based on Lamber-Beale's law, determines the concentration by detecting the absorbance of a specific wavelength (e.g., around 610 nm), and the relationship can be expressed as:A = εbc, where A is the absorbance, ε is the molar absorbance coefficient, b is the optical path, and c is the concentration. In addition, some instruments also use rapid assay techniques based on the principles of UV absorption or ozone oxidation to meet the needs of online or on-site rapid screening.
Influencing factors
The results of chemical oxygen demand measurement are affected by a variety of factors. Chloride ions in water samples are the main interfering substances, which will be oxidized at high concentrations and cause high results, usually with mercury sulfate for masking. The temperature, time and oxidant concentration of the digestion process need to be strictly controlled to ensure that the oxidation reaction is complete. The uniformity and representativeness of the water sample, as well as the presence of suspended solids, can also affect measurement accuracy. In addition, the calibration status, reagent purity, and operational standardization of the instrument are all important links to ensure data reliability.
Application:
Chemical oxygen demand water quality meters have a wide range of uses in water quality management and pollution control. In the field of environmental monitoring, it is used for water quality evaluation and trend analysis of surface water, groundwater and seawater. In municipal wastewater treatment plants, the instrument is used to monitor influent loads, control treatment process efficiency, and assess effluent quality. In industrial fields, such as papermaking, printing and dyeing, food processing, etc., it is used to monitor whether production wastewater discharge meets regulatory requirements. In agriculture and scientific research, the instrument can be used to assess the load of organic matter in soil leachate or experimental samples.
Selection considerations
Choosing a suitable chemical oxygen demand water quality meter requires comprehensive consideration of measurement needs and application scenarios. For routine laboratory analysis, benchtop instruments with standard method compliance, good reproducibility, and the right amount of sample handling are common choices. Online monitoring scenarios need to consider the long-term stability of the instrument, automatic cleaning and calibration functions, and anti-interference capabilities. The measurement range needs to cover the chemical oxygen demand concentration of the intended water sample, from clean water to highly concentrated wastewater. Operational complexity, maintenance requirements, reagent consumption costs, and data output and management capabilities are also important evaluation factors. Users should refer to the applicable standards in their country or region to ensure that the principles and methods of the selected instruments meet the specifications.