Total organic carbon ( TOC ) is determined by a special instrument, the total organic carbon analyzer (hereinafter referred to as TOC analyzer). The TOC analyzer has simple process, good reproducibility, high sensitivity, stability and reliability, and the measurement process is generally not It consumes chemicals, basically does not produce secondary pollution, and has complete oxidation.

The measurement principle is based on the conversion of different forms of organic carbon into oxidatively determined carbon dioxide, using the corresponding relationship between the carbon content of carbon dioxide and total organic carbon, thereby quantitatively determining TOC in aqueous solution . According to different working principles, it can be divided into combustion oxidation - non-dispersive infrared absorption method, conductance method, gas chromatography, wet oxidation - non-dispersive infrared absorption method, etc., in which combustion oxidation - non-dispersive infrared absorption method only needs one-time conversion The process is simple, reproducible, high sensitivity, and the application is relatively common. China's national standard GB13193-91 is the combustion oxidation - non-dispersive infrared absorption method.

TOC assays are generally divided into direct assays and indirect assays. The direct measurement method is generally a method of measuring total carbon ( TC ) by removing inorganic carbon ( IC ) , and is suitable for measuring a water sample having a high IC content, but is liable to lose volatile organic carbon ( POC ) in the water sample .

The IC treatment method uses acidification aeration treatment to acidify the water sample to pH<3 , convert CO 3 2- and HCO 3 - to carbonic acid, generate carbon dioxide, and then remove CO 2 by aeration . Since the loss of volatile organic compounds ( VOC ) in the water sample during aeration , the direct method can only represent the non-blowing organic carbon ( NPOC ) content. When the inorganic carbon content in the sample is high, the interference of the organic carbon is determined by the interference, so the interference of the inorganic carbon in the wastewater must be eliminated, and the sample is pretreated before the measurement.

In the indirect method, TOC is obtained by subtracting IC from TC , and all carbon is oxidized to obtain TC . IC is obtained by measuring the amount of CO 2 decomposed by acid in the sample , and is suitable for measuring a water sample having a lower IC than TOC .

The subtraction method has the following shortcomings: ( 1 ) The subtraction method is used to measure the water sample twice, and the IC and TC are respectively obtained . Therefore, an external sampler is required to ensure the water quality of the IC and TC are the same. case where temporal and spatial distribution of pollutants is very difficult to achieve; (2) Since the water samples to be measured IC and TC, respectively, so that when the instrument is calibrated and TC IC also requires two kinds of standards, thus increasing the The complexity of the instrument; ( 3 ) the measurement period is long; ( 4 ) two integral measurements of the amount of CO 2 produced increase the error of the instrument.

1 , oxidation technology

The methods for determining the oxidation of organic pollutants used in TOC are classified into dry oxidation and wet oxidation . More specifically, there are mainly the following: high temperature catalytic combustion oxidation, persulfate oxidation, ultraviolet light ( UV ) / Persulfate oxidation, ultraviolet ( UV ) oxidation, and the like. Dry oxidation (high temperature catalytic combustion oxidation) is characterized by high detection rate, strong oxidizing ability, and simple and rapid operation. Wet oxidation is characterized by high accuracy, large injection volume, high sensitivity, and good safety performance, but it takes time.

( 1 ) High temperature catalytic combustion oxidation

It is the organic matter in the water sample that is burned at high temperature to convert it into CO 2 . If the temperature is controlled properly and the catalyst works well, this method is the most efficient method of oxidation and therefore is considered to be the most accurate. The method can be used as a standard for calibrating other methods.

The liquid sample can be injected directly into the combustion tube or it can be evaporated prior to combustion. If injected directly, volatile organic compounds ( VOC ) are swept through the combustion tube at high speed with the generated gas, resulting in a decrease in the measured organic carbon value; if evaporated prior to combustion, the organic compound may also volatilize.

When the temperature is higher than 1000 °C ~ 1100 °C , O 2 can oxidize the organic carbon in the sample; when the temperature is low, the catalytic furnace is required to completely carry out the oxidation reaction. At 950 °C , Cr 2 O 3 , CoO and CuO can be selected ; at 680 ° C , transition metal oxides such as platinum ( Pt ), copper ( Cu ), iridium ( Ir ) and nickel ( Ni ) can be selected.

At present, most dry oxidation uses 950 °C high temperature plus catalyst, a few use 680 °C plus catalyst, 680 °C can extend the life of quartz tube and improve repeatability.

The minimum detection limit for combustion oxidation is 1.0 mg/L . If the injection volume is too small, it will affect the reproducibility and reduce the sensitivity of the method, but the injection volume should not be too much, otherwise it will affect the gasification efficiency. Generally, when testing several mg/L , the injection volume is preferably 30 to 50 microliters; when the test is above tens of mg/L , the injection volume can be selected within the range of 10 to 30 microliters. Due to the high TOC content in the wastewater , the different sewage samples should be appropriately diluted during the measurement process to make the measured value within the linear range of the standard curve, thus ensuring the accuracy of the measured values.

( 2 ) Persulfate oxidation

Persulfate is usually used for the determination of soluble organic carbon ( DOC ) in water samples. Persulfate is an oxidant. During the reaction, persulfate oxidizes organic substances to form CO 2 under high temperature and high pressure . The inorganic carbon must be removed prior to the addition of the persulfate because the amount of CO 2 produced by the reaction needs to be determined .

In natural water, when the working temperature of persulfuric acid is close to 100 °C, the recovery of organic molecules and biopolymers is >95% ; in seawater, the measured DOC is reduced by 50%~75% , and the apparent incomplete oxidation of seawater is Due to the particle size distribution of the soluble organic molecules and the weakening of the activity of the oxidized compound in the free radical reaction.

High concentrations of Cl - will interfere with the reaction, resulting in a low amount of DOC measured, which can be solved by using a higher concentration of persulfate or prolonging the reaction time, or by adding Hg 2+ to complex Cl- . However, as the temperature increases, the persulfate decomposes like dichromate and the rate of decomposition is faster than the rate of oxidation. Therefore, in order to shorten the reaction time of the organic substance with the persulfate, the concentration of the oxidant should be increased without increasing the temperature.

( 3 ) ultraviolet ( UV ) oxidation

Under the irradiation of ultraviolet light ( 185 nm ), the liquid sample can continuously generate an oxidant. It has been reported that all of the DOC can be oxidized using only ultraviolet light , but the particles and colloids cannot be completely oxidized . Nowadays, the application of flow systems in TOC measurement is very common, and many commercial analyzers include UV digestion units. UV oxidation of TOC in water and wastewater has been listed as an ISO standard and standard methods in Germany, the United States, Japan and other countries.

In order to obtain more accurate results, the inorganic carbon in the sample is removed before being injected into the UV reactor. In the ultraviolet oxidation process, in order to accelerate the decomposition of organic matter, titanium dioxide ( TiO 2 ) is commonly used as a catalyst for photooxidation. TiO 2 is currently recognized as the best performing photocatalyst. The TiO 2 photocatalytic oxidation method has been used practically, but it has not been certified by the American Society for Testing Materials.

( 4 ) Ultraviolet ( UV ) / persulfate oxidation

In the UV/ persulfate oxidation method, after adding persulfate to a water sample and mixing it uniformly, it is irradiated with ultraviolet light ( UV ). In this method, large particles in the water sample cannot be completely oxidized, and the oxidation efficiency thereof is affected by Morphological effects of organic pollutants in water samples. It is a synergistic effect of ultraviolet oxidation and wet oxidation, which complement each other and promote each other. The oxidative degradation effect is superior to any of the methods, but there are characteristics that the device is relatively complicated and the running cost is high. The bottleneck of persulfate supplemented by ultraviolet oxidation is the wavelength and intensity of ultraviolet light emitted by the UV lamp.

Oxidation methods include ozone oxidation, ultrasonic cavitation sonoluminescence, and the like. Among them, the oxidation capacity of ozone is only owed to fluorine in natural elements, and the ozone oxidation method is used as the TOC detection technology. The advantage is that the reaction speed is fast, there is no secondary pollution, and it has high application value. However, the utilization rate of ozone is low, and the ozone oxidation ability is often insufficient. In order to improve the ozone oxidation efficiency, hydrogen peroxide can be added to assist oxidation.

Regarding oxidation technology, it is generally believed that the development trend of high-temperature combustion method is getting better and better. It has the following two advantages: first, high oxidation efficiency and strong sample adaptability; second, the function of the instrument is expandable, such as combustion method can do For solid samples, the total nitrogen index can be expanded. However, there are some problems in the high-temperature combustion method. First, the injection volume is very small, only a few tens of microliters, it is difficult to obtain representative samples. Second, China's pollution sources require monitoring of water samples containing suspended solids ( SS ), using micro-injection. It is difficult to inject. The wet oxidation does not have the above problems.

2 , detection technology

The detection methods of CO 2 produced by organic carbon combustion include non-dispersive infrared absorption method ( NDIR ), conductance method, coulometric method, and gas chromatography.

( 1 ) Non-dispersive infrared absorption method ( NDIR )

Except for diatomic gases and monoatomic gases with symmetric structures and no polarities, most gases , including water vapor , have the ability to absorb infrared light. Each gas has a strong absorption capacity only for the radiant energy of its respective characteristic wavelength, while radiant energy outside this wavelength is rarely absorbed. Such as carbon dioxide at 4.26 μ m, the carbon monoxide in the 4.65 μ m, it has a strong absorptive capacity methane at 3.35 μ m wavelength. Since the infrared ray of a certain wavelength is selectively absorbed by carbon dioxide, the intensity of the absorption of infrared ray by carbon dioxide is proportional to the concentration of carbon dioxide in a certain concentration range, so that the water sample TC and IC can be quantitatively determined.

The NDIR consists of a light source, a gas chamber and a detector. NDIR has excellent stability, wide measuring range, high precision, high sensitivity and low detection limit. It is an ideal carbon dioxide detector. Water, high content of SO 2 and SO 3 , halides and other ionic solutions will interfere with the detection.

Non-dispersive infrared absorption method is also the standard method specified in GB13193-91 in China . The TOC analyzer localization of technical difficulties mainly NDIR.

( 2 ) Conductance method

Conductivity cell consists of reference electrode, measuring electrode, gas

- a liquid separator, an ion exchange resin, a reaction coil, a NaOH conductivity liquid, and the like. Its measuring principle is: when the reaction gas flows through the conductivity cell coil, the CO2 and the NaOH in the reaction coil are fully contacted in the reaction coil to form Na2CO3 . Because the conductivity of NaOH is different from the conductivity of Na2CO3 , the signals detected by the measuring electrodes change, and their difference is proportional to CO2 , thus completing the measurement of CO2 . When Na2CO3 passes through the ion exchange resin, OH- is reduced and the OH- concentration is kept constant. The advantage of the conductivity cell is that it is low in price and easy to popularize, but its stability is poor and susceptible to environmental factors.

( 3 ) Coulomb counter

The coulomb counter consists of a light source, a photoelectric sensor, a platinum electrode, a silver electrode, ethanol ammonia, an indicator, and the like. Its measurement principle is: when the reaction gas flow passes through Coulomb timing, the CO 2 reacts with ethanol ammonia to form titratable ethanol carbamic acid, and the indicator is discolored. Photoelectric sensors monitor color changes and send measurement data. The platinum electrode and the silver electrode are automatically titrated and the titration current data is transmitted to complete the measurement of CO 2 . The advantage of the coulomb counter is that it does not require a standard curve, and the accuracy, speed and precision are also improved.

While gas chromatography detects CO 2 with high sensitivity and precision, the equipment is complicated and expensive.

At present, there are only two types of CO 2 detection methods certified by the American Society for Testing Materials Science (ASTM) . One is non-dispersive infrared detection ( NDIR ), and the other is thin film conductivity detection. Among them, NDIR is the most mature and convenient application. The mainstream of technology, China's current national standard recommended is the non-dispersive infrared absorption method.

Yingnuo Instrument (Shanghai) Co., Ltd.

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