Chemical oxygen requirements (cods) are the core indicators for measuring the level of organic contamination in aquatic bodies, and their detection techniques are based on precision quantification of the oxidizing reaction. The current mainstream cod analyser is based on the high-temperature decomposition of potassium chromic acid assay, and its technical principles can be broken down into three core links: oxidation, signal conversion and quantification。
The oxidation response phase is the technical basis. In a strong acid medium (heavy sulphuric acid), potassium chromiumate, as a strong oxide agent, reacts at a high temperature of 165°c with reductive substances (including organics, nitrites, sulphides, etc.) in water samples. In response, hexavalent chromium (cr6+) was reduced to trivalent chromium (cr3+) and the solution colour gradually changed from yellow to blue green. The key to this phase is to accelerate response rates through high-temperature, high-pressure environments, while silver sulfate catalysts can be effective in increasing the oxidation efficiency of straight-chain fatty human compounds and ensuring adequate response。
Quantitative analysis is achieved during signal conversion. The modern cod analyser https://harkerhk. Com/ uses spectrophotometry to measure the sorbence of cr3+ in the digested solution by a specific wavelength (usually 600-620 nm). According to the lambert-bill law, sorbence is linear with cr3+ concentration, while the generation of cr3+ directly corresponds to the consumption of potassium chromium acid, thus reflecting the oxygen consumption of reductive substances in the water sample. To eliminate diaphragm, some of the instruments use a dual radiance design to simultaneously measure 254 nm (absorbing peaks of organic matter) and 546 nm (coloured compensation wavelength) in order to correct the absorption values of pure organic matter through algorithms。
Quantification phase completes data conversion. The instrument contains a standard curve or converts the suction value to cod concentration through the faraday law (culent). For example, in the potassium chromium acid method, 1mol cr6+ has been restored to cr3+ correspondingly consume 1. 5mol oxygen, which can be calculated by measuring cr3+ increments for organic oxide in water samples. In order to increase accuracy, the instrument is usually equipped with an automatic calibration module, which can be regularly calibrated with a standard solution (theoretically cod value 500 mg/l) to ensure that the measurement error is within > 5%。
