Basic meaning
TOC indicates the total organic carbon content in sewage, and it is also an indicator to characterize the degree of contamination of water by organic matter. The theoretical value determined by TOC and TOD method has high accuracy, which is an indispensable method for measuring various water quality indicators.
Folding principle
Folded ultraviolet oxidation
When the water sample is irradiated with a UV lamp, the water will be decomposed into hydroxyl and hydrogen groups, and the combination of hydroxyl and oxide will generate CO2 and water, and then the newly formed CO2 will be detected to calculate the total organic carbon content. When the ultraviolet oxidation method is used, the oxidizing ability can be improved by adding titanium oxide, persulfate or the like. The advantages of the ultraviolet oxidation method are high oxidation efficiency and simple maintenance. The disadvantage is that the UV lamp needs to be replaced periodically.
Folding combustion oxidation
Among them, the combustion oxidation-non-dispersive infrared absorption method has the advantages of only one-time conversion, simple process, good reproducibility and high sensitivity. The disadvantage is that the detector needs to be frequently calibrated, the volume is large and the preheating time is long, and acid, catalyst and Carrier gas.
The TOC analyzer is mainly composed of the following parts: inlet, inorganic carbon reactor, organic carbon oxidation reaction (or total carbon oxidation reactor), gas-liquid separator, non-dispersive infrared CO2 analyzer, and data processing part.
Combustion oxidation-non-dispersive infrared absorption method can be divided into two methods: differential subtraction method and direct method according to the different principles of determining TOC value.
1. Method of measuring the TOC value by subtraction method
The water samples were injected into a high temperature combustion tube (900 ° C) and a low temperature reaction tube (150 ° C), respectively. The water sample passing through the high temperature combustion tube is subjected to high temperature catalytic oxidation to convert both the organic compound and the inorganic carbonate into carbon dioxide. The water sample passing through the reaction tube is acidified to decompose the inorganic carbonate into carbon dioxide, and the carbon dioxide generated is sequentially introduced into a non-dispersive infrared detector to measure total carbon (TC) and inorganic carbon (IC) in the water, respectively. The difference between total carbon and inorganic carbon is total organic carbon (TOC).
2. Method principle of direct method for determining TOC value
After the acid sample is acidified and aerated, the various carbonates are decomposed to form carbon dioxide and then driven out, and then injected into a high-temperature combustion tube to directly measure the total organic carbon. However, since the measurement error occurs due to the loss of volatile organic compounds in the water sample during the aeration process, the measurement result is only the organic carbon value that cannot be blown out.
Folded supercritical water oxidation
Supercritical Water Oxidation (SCWO) technology was originally used to treat large volumes of wastewater, sludge, and contaminated supercritical oxidation. GE is the first company to apply this technology to commercial laboratory TOC analyzers. When the temperature and pressure are above the critical point of water (375 ° C and 3,200 psi), organic waste is rapidly oxidized by the oxidant in the water. The characteristics of supercritical water can make organic carbon extremely efficiently and rapidly oxidize to carbon dioxide, even if there are chlorides and other inorganic substances that cause negative interference when using non-supercritical oxidation. Moreover, the TOC analyzer using SCWO technology does not require high maintenance and calibration. The advantage of supercritical water oxidation is that the oxidation is completely rapid and can tolerate high-salt compounds; the disadvantage is that water samples with low TOC concentrations cannot be detected.
Folding conductivity test
TOC conductivity detection technology is capable of measuring liquid CO2. There are two main conductivity detection technologies used in the industry: one is direct conductivity method, and the other is thin film conductivity detection method (also known as selective conductivity method). The TOC analyzer with two conductivity methods has stable calibration results and high detection accuracy. The main difference between the two technologies is that the direct conductivity method is more susceptible to interference by hetero-acids, halogenated organics, etc., while the thin film conductivity detection technology is more resistant to interference.
The film conductivity test method is a test method used by the GE TOC analyzer. The membrane used in the TOC analyzer can prevent the passage of impurities, ensuring that only the CO2 content is detected, so that the TOC reading is more accurate.
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