Definition
The Chemical Oxygen Demand Rapid Tester is an instrument used to determine the chemical oxygen demand in water samples. Chemical oxygen demand refers to the oxidation dose consumed when treating water samples with a certain strong oxidant under strong acid and heating conditions, expressed in milligrams per liter of oxygen. This index is an important comprehensive parameter for evaluating the relative content of organic matter in water bodies. The Rapid Tester is designed to provide reliable test results in less time than traditional standard methods through optimized processes and designs.
Principle
The core principle of the Chemical Oxygen Demand Rapid Tester is based on redox reactions. It accelerates the oxidation of organic matter in water samples by strong oxidants (usually potassium dichromate) through heating and catalysis. In strong acid media, potassium dichromate oxidizes organic matter and some inorganic reducing substances in water samples at high temperature. After the reaction, the oxidation dose consumed is calculated by measuring the remaining oxidation dose that has not been reduced or by measuring the concentration of trivalent chromium ions produced by reduction, so as to convert the corresponding chemical oxygen demand value. Instruments typically employ techniques such as colorimetry, titration, or electrochemistry to determine the reaction endpoint.
Taking the colorimetric method as an example, its basic basis is the Lamber-Beale law, that is, the absorbance of a solution to a specific wavelength of light is directly proportional to its concentration. The concentration of trivalent chromium ions in the reaction solution is related to the oxidation dose consumed (i.e., chemical oxygen demand), and the chemical oxygen demand value of the water sample can be calculated by measuring the absorbance of trivalent chromium at a specific wavelength (e.g., 610 nm), using a calibration curve. The relationship can be expressed as: A = εbc, where A is the absorbance, ε is the molar absorbance coefficient, b is the path length, and c is the trivalent chromium ion concentration.
Measurement method
The typical operating process for a Chemical Oxygen Demand Rapid Tester consists of several key steps. First, the sample is pretreated, and an appropriate amount of representative water sample is added to a special reaction tube or cuvette. This is followed by the addition of matching prefabricated reagents, which usually contain digestive fluids (a mixture of sulfuric acid and potassium dichromate), catalysts (such as silver sulfate), and masking agents (such as mercury sulfate to eliminate chloride ion interference). Then, the mixed samples are placed in the built-in digestion device of the instrument for constant temperature heating and digestion, and the digestion temperature and time are optimized, which is generally lower than the standard reflow method of two hours. Once digestion is complete, the reaction tube is cooled to a set temperature. Finally, the reaction tube is placed into the measuring chamber of the instrument, and the instrument automatically performs colorimetric analysis or potentiometric titration, and directly displays or outputs the chemical oxygen demand concentration results according to the built-in calibration curve. The entire process significantly reduces analysis time.
Influencing factors
The measurement results of the Chemical Oxygen Demand Rapid Tester can be influenced by a variety of factors. Chloride ions in water samples are common interfering substances, and high concentrations of chloride ions will be oxidized by potassium dichromate, resulting in high results, which need to be inhibited by adding masking agents such as mercury sulfate. The uniformity and representativeness of water samples directly affect the accuracy of sampling, and samples with high suspended solids content need to be homogenized. Temperature uniformity and time control in the digestion process are crucial, and temperature fluctuations or insufficient time may lead to incomplete oxidation. The quality and ratio of reagents, especially the concentration and acidity of oxidants, have a direct impact on the reaction efficiency. The cleanliness of the instrument's optical system, the stability of the detector, and the accuracy of the calibration curve are also fundamental to ensuring data reliability. Operators need to follow standard operating procedures to control these variables.
Application:
The Chemical Oxygen Demand Rapid Tester is widely used in fields where a quick understanding of the organic pollution status of water bodies is required. In environmental monitoring, it is used for routine monitoring and emergency monitoring of surface water, groundwater, domestic sewage and industrial wastewater. In industrial production process control, such as paper, textile, chemical, food processing, and other industries, it is used to monitor the operational efficiency of process drainage and treatment facilities. In municipal wastewater treatment plants, it is used to quickly assess influent load and outlet water quality. It is also used in the field of agricultural environmental monitoring and scientific research. Its rapid nature makes it suitable for scenarios such as on-site screening, process adjustments, and initial compliance judgments.
Selection
When choosing a rapid chemical oxygen demand tester, multiple technical parameters and actual needs need to be comprehensively considered. The measurement range should cover the expected concentration range of the sample to be tested, and common instruments are available in different models from low to high ranges. Digestion and measurement time are related to the efficiency of analysis, and the time consumption varies between models. The detection limit and repeatability of an instrument are the basic parameters to measure its performance. The ability to resist chloride ion interference is of practical significance for the detection of chlorine-containing water samples. Ease of operation, cost of reagent consumption, and complexity of instrument maintenance are also factors to consider. In addition, whether the instrument complies with the relevant standard methods in the country or region (e.g., comparability with the standard reflow method) and whether it has the necessary data storage and output functions should be within the scope of selection evaluation. It is recommended to make a comprehensive trade-off based on the main application scenarios and laboratory conditions.