Environmental analysis method

The main methods are chemical analysis, instrumental analysis, biological analysis and molecular biology experiment. Among them, chemical analysis is divided into quality analysis and titration analysis. Instrumental analysis is divided into optical analysis, electrochemical analysis, chromatographic analysis, mass spectrometry analysis and so on. Gravimetric analysis, a classical method in quantitative analysis. /kloc-In the middle of the 8th century, lomonosov first used the balance weighing method to measure the quantity change of substances in chemical changes, which proved the law of conservation of mass and actually laid the foundation for the gravimetric analysis in quantitative analysis. Gravimetric analysis requires a precise analytical balance, and the weighing accuracy of analytical balance in19th century reaches 0.1mg; Micro-analytical balance and ultra-micro analytical balance appeared in the 20th century, and the weighing accuracy reached 0.00 1 and 0.000 1 mg respectively, which expanded the application scope of gravimetric analysis.

Gravimetric analysis is to accurately weigh a certain amount of samples, and then use appropriate chemical reaction to turn the components to be tested into pure compounds or monomers for precipitation, and separate them from other components by filtration and other methods, and then weigh them after drying or burning until they reach a constant weight, so as to find out the proportion of the components to be tested in the samples. In addition to this direct determination method, indirect determination method can also be used, that is, the components to be tested in the sample are volatilized to obtain the weight difference of the sample before and after volatilization, so as to obtain the content of the components to be tested. Gravimetric analysis is divided into precipitation, homogeneous precipitation, electrolysis, gas generation (absorption) and extraction. In the analysis of environmental pollutants, sulfate, silica, residue, suspended solids, grease, floating dust and dustfall are often determined by gravimetric method. Gravimetric analysis is widely used in chemical analysis. With the improvement of weighing tools, gravimetric analysis has been continuously developed, such as the determination of atmospheric dust and mercury vapor in the air by micro-gravimetric method with piezoelectric crystals in recent years. Volumetric analysis, also known as titration, is a classic method. /kloc-at the beginning of the 9th century, L. Guy Lussac put forward the gas law, which laid a theoretical foundation for gas volumetric analysis. Later, he applied the analytical method of measuring the volume of gas and liquid to practice. Volumetric analysis is the reaction of reagent solution with known concentration (called standard solution) with the test solution of the component to be tested. After the rapid quantification of the reaction is completed (that is, the reaction end point is reached), the content of the components to be detected in the test solution is calculated according to the concentration and volume of the standard solution (read from the burette) and their equivalent relationship. In addition to visual identification of indicator discoloration, various instrumental methods can also be used to identify the end point, such as potentiometric titration, photometric titration, high-frequency titration, current titration, conductometric titration and temperature titration. In recent years, various types of automatic titrators have been used for volumetric analysis.

Volumetric analysis has the advantages of simple operation, rapidity, accuracy and low cost, and is suitable for daily analysis. According to the type of reaction used, volumetric analysis can be divided into neutralization titration, redox titration, precipitation titration, complexometric titration and so on. In the analysis of environmental pollution, volumetric analysis is applied to the analysis of conventional indicators of water pollution such as biochemical oxygen demand, dissolved oxygen and chemical oxygen demand, as well as the analysis of pollutants such as volatile phenol, formaldehyde, cyanide, fluoride, sulfide, hexavalent chromium, copper ion and zinc ion. According to the color depth of the test solution, the test solution is compared with the known standard solutions with different color depths to determine the substance content.

1729 P. Baugel proposed Baugel's law, that is, when the liquid layer thickness is the same, the color intensity is the same. 1760, J.H. Lambert put forward Lambert's law, which is similar to Bogel's law, that is, the color intensity of colored solution with the same concentration is proportional to the thickness of liquid layer. 1852 A. Bill put forward Bill's law, that is, when the thickness of liquid layer is equal, the color intensity is directly proportional to the concentration of chromogenic solution. These laws laid a theoretical foundation for colorimetric analysis. In 1854, J. Di Bosek and J. Naisler applied these theories to the field of quantitative analytical chemistry. Vero first used spectrophotometry for photometric analysis. Photometric method does not compare the color intensity of chromogenic solution like colorimetric method, but determines the transmittance or absorbance of chromogenic solution. 1874 нг. yegorov first applied photoelectric effect to colorimetric analysis, and the photoelectric photometer he designed was the embryonic form of modern photoelectric colorimeter. 1894, the Puffrich photometer appeared; 19 1 1 year, Berger photoelectric colorimeter appeared. 194 1, Beckmann's spectrophotometer appears. Later, automatic recording spectrophotometer, oscilloscope spectrophotometer, dual-wavelength spectrophotometer and digital display spectrophotometer appeared. The sensitivity and accuracy of spectrophotometry are constantly improving, and the application scope is also expanding.

Colorimetric analysis, such as comparing the color of solution with the naked eye to determine the substance content, is called visual colorimetry. The method of measuring the intensity of transmitted light through colored solution with photocell and galvanometer to obtain the content of measured substance is called photoelectric colorimetry. The instrument used is called photoelectric colorimeter. Colorimetry and spectrophotometry are based on Lambert-Beer law (also known as light absorption law), that is, the absorbance of a solution is proportional to the product of the concentration of colored substances in the solution and the thickness of the liquid layer. The numerical relationship is LG (IO/I) = K C L, where IO is the incident light intensity; I is the intensity of transmitted light; L is the thickness through which light passes through the liquid layer of the colored solution; C is the concentration of colored substances in the solution; K is a constant (for the incident light of colored substances with a certain wavelength, K is a certain value), which is called extinction coefficient (also called absorption coefficient). The value of k varies with the units of L and C. If L is expressed in centimeters and C is expressed in moles/liter, this constant is called molar absorption coefficient (or molar extinction coefficient) and is often expressed as ε.

The main advantages of colorimetric analysis are accuracy, sensitivity, rapidity, simplicity and low cost. Generally, the lowest concentration of the tested substance can reach per liter 10- 10g, and if it is enriched by chemical methods, the sensitivity can be improved by 2-3 orders of magnitude. The relative error of determination is usually 65438 0 ~ 5%.

Colorimetry and spectrophotometry have been widely used in the analysis of environmental pollution, but pollutants must react with chromogenic agents and be converted into colored compounds before they can be determined. At present, various effective and sensitive organic color developers have been developed. Metal ions, nonmetallic ions and organic pollutants can all be determined by this method. An analytical method based on the functional relationship between the absorption of chemicals in ultraviolet region and the wavelength of ultraviolet light. The wavelength range of ultraviolet spectrum can be divided into near ultraviolet region (200 ~ 400 nm) and far ultraviolet region (10 ~ 200 nm). The former is often used for chemical analysis, while the latter is rarely used for analysis because the air absorbs ultraviolet rays with wavelength below 200 nm and the measurement must be carried out in vacuum.

The absorption of ultraviolet radiation by molecules is usually the result of the excitation of their outer electrons or valence electrons. The easier the electrons are excited, the longer the wavelength of the absorption peak.

Ultraviolet spectrophotometer generally uses hydrogen lamp as radiation source, timely prism or grating as monochromator and photomultiplier tube as detector. The material of the absorption pool is usually timely or silica, and the length is 1 ~ 10 cm. If deuterium is used instead of hydrogen, its emission intensity in the short wavelength of ultraviolet region can be increased by three times.

Simple inorganic ions, their complexes and organic molecules can be detected and measured in the ultraviolet spectrum. Effective solvents are water, saturated hydrocarbons, fatty alcohols and ethers. Organic compounds that can absorb ultraviolet rays must contain at least one unsaturated bond, such as C=C, C=O, N=N, S=O, in order to play the role of chromophore. The wavelength of absorption peak increases with the increase of chromophore unsaturation. Some compounds and their maximum absorption wavelengths are shown on the right.

The application of ultraviolet spectrophotometry in environmental pollution analysis mainly includes the following aspects: ① Vacuum ultraviolet gas analyzer has been used to analyze automobile exhaust in air pollution analysis; Ultraviolet gas analyzer can be used to analyze ozone, nitrogen dioxide and chlorine. Gaseous ammonia has several strong absorption bands at the wavelength of 190 ~ 230 nm, which can be used to directly determine the concentration of ammonia. ② Some polycyclic aromatic hydrocarbons and benzo (a) pyrene have strong absorption peaks in the ultraviolet region, so this method is often used for determination. ③ Some oils containing * * * yoke system have characteristic absorption peaks in the ultraviolet region, so this method can be used to determine the oil contamination. ④ This method can also be used to determine carcinogens, pesticide residues, nitrates and phenols in food, beverages, cigarettes, water quality, organisms, soil and other samples. ⑤ This method can also be combined with chromatographic analysis. The sample to be tested first passes through the chromatographic column, and then the eluent of the chromatographic column flows through the absorption tank of the ultraviolet spectrophotometer to detect the trace pollutants contained in the sample. In recent years, the rapid development of high-speed liquid chromatograph is equipped with ultraviolet detector. Also called infrared spectrum analysis, it is an instrumental analysis method. Under the irradiation of infrared light, a substance can only absorb infrared light with the same vibration and rotation frequency as its molecules, so different substances can only absorb incident light of a certain wavelength to form their own characteristic infrared spectra, and the infrared absorption intensity of a certain wavelength is related to the concentration of the substance. According to this principle, we can make qualitative and quantitative analysis of substances and study the structure of complex molecules.

In environmental analytical chemistry, infrared spectrophotometry is mainly used to absorb gas, liquid and solid pollutants in the infrared region of 450 ~ 1000 cm ~ 1. When measuring air pollution, volatile gases (acetylene, amine, ethylene, formaldehyde, hydrogen chloride, hydrogen sulfide, methane, propylene, benzene, phosgene, etc. ) range from ppm to ppb. A new compound, peroxyacetyl nitrate, found in the atmosphere was identified by infrared spectroscopy and mass spectrometry. Infrared spectrum also found the existence of ozone in the air of Los Angeles. The concentrations of organic pollutants and pesticides in water below 1ppb can be determined by Fourier transform infrared spectroscopy. Compared with mass spectrometry, infrared spectroscopy can easily distinguish various isomers of pollutants. Infrared spectroscopy is one of the main methods to identify oil pollution in water. Infrared spectrum can be used to determine the chemical reaction of air pollution. Gas chromatography-infrared spectroscopy can be used to determine low boiling point and volatile organic pollutants. Because of the resolution of gas chromatography, the limitation that infrared spectrum is only applicable to pure compounds is broken through, so the combination of gas chromatography and infrared spectrum can also be applied to the determination of mixtures. Quantitative analysis method of absorbing light from sharp light source (hollow cathode lamp or electrodeless discharge lamp) by atomic vapor of elements (produced by flame or graphite furnace). Main advantages: ① When analyzing complex environmental samples, it has good selectivity, less interference and easy to obtain reliable analysis data. ② The operation of the instrument is simple and the cost is low. ③ High sensitivity, which can be used for micro-sample analysis. The sample content can be determined by flame atomic absorption spectrometry and microgram by graphite furnace method, which has higher sensitivity than high frequency coupled plasma method. ④ The measuring range is wide, which can not only analyze trace elements, but also determine the content of matrix elements. The accuracy of a stable atomic absorption spectrophotometer can reach 0. 1 ~ 0.3%, which is comparable to the classical volumetric method.

After adding accessories such as mercury and hydride generator, the sensitivity of atomic absorption spectrum is higher than that of graphite furnace, and the determination range of mercury, arsenic, selenium, tellurium, bismuth, antimony, germanium, tin and lead can be increased by 1 ~ 2 orders of magnitude. Atomic absorption spectrometry has been widely used to determine heavy metal elements in water, floating dust, soil, grain and various biological samples. More than 70 elements were determined by atomic absorption spectrometry. Flame method is mature and widely used in atomic absorption spectrometry, but its sensitivity to environmental samples is not high enough. Although the graphite furnace method is not mature enough, it is a highly sensitive analysis method.

The disadvantages of atomic absorption spectrometry are as follows: ① The special lamp must be replaced to determine each element, and multi-element analysis cannot be done at the same time. (2) Various interference effects are greater than those of high frequency coupled plasma method. ③ It is difficult to determine the elements whose vibration lines are located in the vacuum ultraviolet region. ④ Solid samples are difficult to determine. ⑤ The sensitivity of some high-temperature elements such as uranium, thorium, zirconium, hafnium, niobium, tantalum, tungsten, beryllium and boron is too low. A quantitative analysis method using the spectrum generated by atomic vapor under electric or thermal excitation, recorded by spectrometer or directly read by photometer. Its main feature is that it can determine multiple metal elements at the same time, with good selectivity and less interference. It can directly analyze liquid and solid samples and is suitable for qualitative and quantitative analysis of multiple elements. The analytical range of liquid is from mg/L to μ g/L, and the analytical sensitivity of solid is from 1% to 0.00 1%. After chemical separation and enrichment, the sensitivity can be increased by 1 ~ 2 orders of magnitude. It can be used for the analysis of water, floating dust, soil, grain and various biological samples in environmental protection. Disadvantages are the need for photographic dry plate records and long analysis period; For the quantitative analysis of ultra-trace elements, the sensitivity is not enough; When analyzing solid samples directly, the error is large.

The traditional emission spectrum analysis is that the solution is analyzed by dry slag method and the solid is analyzed by carbon canister powder method. Use AC arc or direct arc as excitation light source; Use medium-sized time spectrometer or grating spectrometer to take photos and record on dry plate. The influence of matrix will increase the analysis error. Recently, lithium salt was introduced into the solution dry residue method as a buffer, which reduced the influence of matrix and greatly improved the analysis accuracy, so emission spectrometry became a general quantitative analysis method to some extent. Carbon cell powder method has not become a general quantitative analysis method because of its low slope and large error.

In recent years, DC and high-frequency coupled plasma light sources have been developed. Combined with photoelectric recording, the accuracy, sensitivity and speed of analysis have been improved, the matrix effect has been reduced, the reproducibility is good, the linear dynamic range is wide, and various elements can be determined simultaneously. This is a new analytical method. However, the instrument using high-frequency coupled plasma as light source is expensive, argon gas consumption is high, analysis cost is high, and the analysis sensitivity of environmental samples is not enough. Although the sensitivity of DC plasma light source is not as good as that of high-frequency coupled plasma light source, it has developed rapidly in recent years because of its low price, low argon consumption and little impact on human health. The basic principle of X-ray fluorescence analysis is to bombard the sample with high-energy X-rays (primary X-rays) to expel electrons from the inner shell of the atom of the element to be measured, so that the atom is in an excited state. 10-12-10-15 seconds later, the atoms in the atoms recombine, and the vacancies of the inner electrons are supplemented by the outer electrons. The characteristic X-ray wavelength λ has a certain relationship with the atomic number Z: λ ∝ 1/Z2. Determine the wavelength or energy of these characteristic spectral lines for qualitative analysis; The content of this element can be obtained by measuring the intensity of the spectral line.

The excitation sources used in X-ray fluorescence analysis are X-ray tubes, radioisotopes, electrons, protons or alpha particles. There are two measurement methods: wavelength dispersion method and energy dispersion method. The wavelength dispersion method is a classical method. Energy dispersion method adopts silicon (or lithium) semiconductor detector and multichannel analyzer, which can simultaneously determine all elements above sodium. Its resolution is lower than that of wavelength dispersion method, but it can be applied to multi-element analysis.

X-ray fluorescence analysis has the advantages of rapidity, accuracy, wide measurement range, simultaneous determination of multiple elements, high degree of automation and no damage to samples, so it has been widely used in environmental pollution monitoring. For example, determination of trace metal compounds in atmospheric dust; With the help of electronic computer, it can automatically monitor the floating dust, sulfur dioxide and sulfur adsorbed by aerosol in the atmosphere, and is also suitable for the determination of heavy metals in suspended particles in various water bodies and trace elements dissolved in water. After a substance absorbs light with a certain wavelength (excitation light), it causes energy level transition and emits light with a wavelength slightly longer than the excitation light, which is called fluorescence. The method of determining the substance and its content by measuring the characteristics and intensity of fluorescence spectrum is called fluorescence analysis. If the concentration of the test sample is very low, its fluorescence intensity is directly proportional to the concentration of the substance. According to this characteristic, the substance can be quantitatively analyzed. Different substances have different fluorescence excitation spectra and emission spectra, and substances can be qualitatively analyzed according to the characteristics of the spectra. In particular, fluorescence spectrophotometer can obtain two kinds of spectra (excitation spectrum and emission spectrum), which are more reliable than absorption spectrum to identify substances.

The instruments used in fluorescence analysis include visual fluorometer, photoelectric fluorometer and fluorescence spectrophotometer. Each instrument consists of light source, filter or monochromator, liquid tank and detector.

The sensitivity of fluorescence analysis is very high, which is 2 ~ 3 orders of magnitude higher than that of ordinary spectrophotometry. It can detect10-11~12g of trace substances. Fluorescence analysis also has the advantages of simple experimental method, easy sampling and less sample consumption, and is an important analytical technology. At present, more than 60 elements and hundreds of compounds have been determined by fluorescence analysis. In environmental pollution analysis, fluorescence analysis has been widely used to determine carcinogens and other poisons, such as polycyclic aromatic hydrocarbons such as benzo (a) pyrene, β-naphthylamine, aflatoxin, pesticides, mineral oil, sulfide, selenium, boron, beryllium, uranium and thorium. Gas chromatography-mass spectrometry

A new and complete analytical technique combining gas chromatography and mass spectrometry can be used for qualitative and quantitative analysis of complex mixed compounds. An electronic computer is usually equipped to form a gas chromatography-mass spectrometry-computer system. The combination of gas chromatograph and mass spectrometer usually solves the problem of pressure drop transition between the outlet of chromatographic column (usually atmospheric pressure) and the ion source of mass spectrometer (vacuum degree is 10-4 ~ 10-7) through an interface device (molecular separator). Molecular separators can also concentrate chromatographic fractions entering the mass spectrometer. The combination of capillary column chromatography and mass spectrometer also adopts direct coupling mode without molecular separator. Commonly used molecular separators are nozzle, porous glass, porous silver, porous stainless steel, PTFE capillary, silicone rubber diaphragm, variable conductivity slit, silver-palladium alloy tube coated with silicone, diaphragm-porous silver and other types. When the sample fraction enters the molecular separator with the carrier gas, most of the carrier gas and the sample fraction are separated in the molecular separator due to the great difference in molecular weight between the fraction and the carrier gas and the different spatial diffusion ability. A typical molecular separator with two nozzles is shown in Figure 3, and the schematic diagram of gas chromatography-mass spectrometry device is shown in Figure 4.

Mass spectrometer is an instrument used to analyze isotopes of various elements and measure their mass and content percentage. It consists of ion source, analyzer and collector. There are two kinds of mass spectrometers used in gas chromatography-mass spectrometry: magnetic mass spectrometer and quadrupole mass spectrometer. The former has high resolution (r = 1000 ~ 150000), high sensitivity (10-9 ~10-/3g), wide mass range, and can add functions such as peak matching and metastable technology, but Quadrupole moment mass spectrometer is light and fast, especially suitable for narrow peaks in capillary column chromatography, but the resolution can only reach r = 1000 ~ 3000, and the mass range is narrow, which has the function of quality discrimination. The commonly used mass spectrometry techniques of gas chromatography-mass spectrometry are as follows: ① electron bombardment technique, which is used to understand the structural information and molecular composition of samples, is the most commonly used technique in mass spectrometry. ② Chemical ionization technology can obtain some molecular information of compounds that cannot be obtained by electron bombardment technology. (3) Single-ion detection technology, through single-ion detection of the characteristic ion mass of the tested compound, a quality chromatogram with high signal-to-noise ratio can be obtained, and the sensitivity is 2-3 orders of magnitude higher than that of scanning the whole spectrum, and at the same time, unresolved chromatographic peaks can be distinguished. This method is especially suitable for the identification of suspicious chromatographic peaks. Combined with the retention value of gas chromatography, reliable qualitative results can be given directly. (4) Mass fragmentation technology, in which a plurality of selected characteristic ions are simultaneously scanned by skip scanning technology. This technique has strong specificity, and its sensitivity is 2 ~ 3 orders of magnitude higher than the total ion current (generally up to10-12 g). Combined with computer, it can be developed into strength matching technology and computerized quality fragmentation technology.

The gas chromatography technology used for gas chromatography-mass spectrometry is different from ordinary gas chromatography technology, and it is more sensitive to the flow rate of carrier gas and the loss of stationary liquid. Due to the vacuum degree of mass spectrometer, it is difficult to optimize the flow rate of carrier gas. At the same time, due to the principle of molecular separator, only those light gases with diffusion coefficients far from the sample compounds can be selected. Helium or hydrogen is usually used. The stationary liquid for gas chromatography-mass spectrometry should have high separation efficiency and good thermal stability, and the content of stationary liquid in the column should be low to ensure high efficiency and low loss. Commonly used fixatives are: SE-30, SE-52, SE-54, OV- 1, F-60, QF- 1, Dexsil 300, Dexsil 400, PPE-20, SF-96, etc. In recent years, the timely capillary elastic column has been widely used in gas chromatography-mass spectrometry.

The computer system in gas chromatography-mass spectrometry technology can process the collected information, compare and retrieve the measured spectrum with the standard spectrum library stored in the computer, and automatically give the final determination result.

Gas chromatography-mass spectrometry (GC-MS) is used in environmental analysis to determine pesticide residues, polycyclic aromatic hydrocarbons, halogenated hydrocarbons, other organic pollutants and carcinogens in air, precipitation, soil, water and its sediments or sludge, industrial wastewater and waste gas. In addition, it is also used to study the migration and transformation of photochemical smog and organic pollutants.

Gas chromatography-mass spectrometry (GC-MS) plays an extremely important role in the analysis of environmental organic pollutants, because the samples of environmental pollutants have the following characteristics: ① The sample system is very complex, and the qualitative method of retaining data by ordinary chromatography is not reliable enough, so special qualitative tools are needed to provide reliable qualitative results. ② The content of environmental pollutants in the sample is extremely small, generally in the order of ppm to ppb, and the analytical tools must have extremely high sensitivity. ③ The composition of pollutants in environmental samples is unstable, which is often influenced by factors such as sample collection, storage, transfer, separation and analysis methods. In order to improve the reliability and reproducibility of analysis, it is required that the analysis steps should be as simple and fast as possible, and the pretreatment process should be as few as possible. Gas chromatography-mass spectrometry can meet the requirements of these environmental analysis. With the high separation ability of chromatograph and high sensitivity of mass spectrometer (10- 1 1g), it has become a powerful tool for trace organic matter analysis. Mass spectrometry in the United States found trace amounts of peroxyacetyl nitrate and dioxane in the atmosphere. Polarographic analysis is an analytical method based on the principle of polarography. In this analysis method, an active small mercury dropping electrode and a large depolarization electrode are immersed in the solution to be measured, and the applied voltage between the two electrodes is gradually changed, thus the corresponding current-voltage curve (polarogram) is obtained. By analyzing and measuring the current-voltage curve, the concentration of corresponding ions in the test solution can be obtained.

The sensitivity of traditional polarographic analysis is generally 10-4 ~ 10-5 mol. In recent years, many new polarographic analysis methods have been proposed. Among them, oscillopolarography, square wave polarography, pulse polarography, polarographic catalysis and reverse stripping voltammetry are widely used. Among them, reverse stripping voltammetry is widely used in environmental analysis.

Reverse stripping voltammetry is also called anodic stripping method. In this method, under appropriate conditions, the measured substance is electrolytically enriched on the microelectrode, and then the potential of the electrode is changed to re-dissolve the enriched substance. According to the polarization curve obtained in the process of electrolytic dissolution, the analysis was carried out. The sensitivity of this method is very high, which can generally reach10-7 ~10-10 mol. It can be used to determine copper, lead, cadmium, indium, thallium, bismuth, arsenic, selenium and tin in natural water, seawater and biological samples. Electrical analysis method based on the change of solution conductivity. In water quality monitoring, water conductivity is an important index to evaluate water quality. It can reflect the degree of electrolyte pollution in water and is a common detection item in water quality monitoring.

Conductivity analysis can also be used to determine dissolved oxygen in water. Because some non-conductive elements or compounds can react with dissolved oxygen to produce ions and change the conductivity of the solution, the content of dissolved oxygen in water can be determined by measuring the conductivity change of water. For example, thallium reacts with dissolved oxygen in water to produce Tl+ ions and OH- ions. For every 0.035 microsievert/cm increase in conductivity (Siemens in the west, conductivity unit), the corresponding dissolved oxygen is 1ppb.

Sulfur dioxide in the atmosphere is also commonly determined by conductivity method. The principle is as follows: sulfur dioxide reacts with water to generate sulfurous acid, and partially dissociates to generate hydrogen ions and sulfite ions, which are conductive;

SO2+H2O─→H2SO3

Sulfuric acid 32h++sulfuric acid

Therefore, the content of sulfur dioxide in the gas sample can be continuously determined by contacting the gas sample with a certain proportion of solution with a certain conductivity and absorbing sulfur dioxide by increasing the conductivity of the solution. This method has a wide measurement range, but if the gas sample contains other gases that are easily soluble in water and will produce conductivity, it will affect the correctness of the measurement results. Include potentiometric titration and direct potentiometric method. Potentiometric titration is an instrumental analysis method and a capacitance analysis method. In this method, the standard solution which can react with the substance to be detected is dropped into the test solution, and the change of indicator electrode potential is observed during titration. According to the potential jump caused by the sudden change of the concentration of the substance to be measured when the reaction reaches the equivalence point, the titration end point is determined, so as to carry out quantitative analysis. This method can be used for acid-base titration, redox titration, precipitation titration and complexometric titration of industrial wastewater in environmental analysis. The direct potentiometric method is a quantitative analysis method by directly measuring the potential of indicator electrode, and the indicator electrode is responsive to the ion concentration in the solution to be measured. In water quality monitoring, both pH value and redox potential are measured by direct potential method.

In recent years, due to the appearance and development of ion-selective electrodes, direct potentiometry has been widely used in environmental monitoring. For example, the application of fluoride ion selective electrode to determine fluoride ion in air, natural water and industrial wastewater has the advantages of rapidity, accuracy, convenience and sensitivity. Cyanide ion selective electrode, nitrate electrode, halogen ion and sulfur ion electrode have also been applied to environmental monitoring.

The solid membrane lead ion and cadmium ion selective electrode can determine 10-7 moles of lead ion and cadmium ion. This method has been used to determine lead and cadmium in water, air, food and biological samples in the laboratory.

There are many kinds of ion-selective electrodes used in direct potentiometry. There are more than 20 kinds of electrodes developed and produced in China, some of which have been used for environmental monitoring and pollution control. An electrochemical analysis method developed on the basis of electrolytic analysis. It calculates the result by measuring the electric energy consumed by electrolytic reaction. Coulomb analysis is based on Faraday's law of electrolysis. The amount of substance that reacts with the electrode under the action of current is directly proportional to the amount of electricity passing through the electrolytic cell. Every time 1 Faraday passes, 1 gram equivalent substance is deposited or dissolved on the electrode surface. If the molecular weight or atomic weight of the reaction substance is m, the number of electron transfer when the electrode reacts is n, and the amount of electricity passing through the electrolytic cell is q, the weight w of the measured substance can be calculated by Faraday's law (figure 1).

In coulometric analysis, the measured substance can react directly on the electrode under the control potential, or an auxiliary substance can react on the electrode under the action of constant current to produce coulometric intermediate, and then interact with the measured substance. The former is called controlled potential coulometric analysis, and the latter is generally called constant current coulometric titration. Coulomb analysis is widely used in environmental monitoring. Sulfur dioxide, carbon monoxide, nitrogen oxides, ozone and total oxidants in the atmosphere, biochemical oxygen demand, chemical oxygen demand, halogen, phenol, cyanide, arsenic, manganese and chromium in water can be determined by this method.