On line chloride and residual chlorine analyzer

Chlorides and residual chlorine: chlorides are found in almost all water and waste water. The content of chloride in fresh water is low, which is about mg / L, and the source is mainly brought in when the water flows through the stratum containing chloride; The content of chloride in seawater, salt lake and some groundwater can reach to ten grams / liter; There are also a lot of chloride ions in domestic and industrial wastewater. Chlorination is a reliable method to disinfect drinking water, kill pathogenic bacteria in water and prevent the spread of water medium diseases. Because of its strong ability of sterilization and algae killing, convenient operation and low price, it is still the main disinfection method of drinking water in the world. In order to ensure the sterilization effect of the water supply network terminal, a certain amount of residual chlorine must be used in the pipe network, but the high concentration of chloride will also damage the health of human body. The drinking water sanitation standard gb5749-85 stipulates that: after 30 minutes of contact with water, it shall not be less than 0.3mg/l, and the end water of the pipe network shall not be less than 0.05mg/l. Meanwhile, chlorination of drinking water will produce trihalomethane, haloacetic acid and other disinfection by-products, which leads to the risk of three causes (carcinogenesis, teratogenesis and mutation) in drinking water. The determination of disinfection by-products such as haloalkanes will be introduced in the section of the determination of organic compounds, and the determination of residual chlorine in water will be emphasized. Residual chlorine refers to the amount of chlorine left in water after being sterilized with chlorine for a certain time. The chlorine added less the residual chlorine is the chlorine demand of the water. The role of residual chlorine in drinking water is to characterize the disinfection effect and prevent the drinking water from being polluted again. There are three forms of residual chlorine: total residual chlorine, chemical residual chlorine and free residual chlorine. The determination methods of residual chlorine are as follows: the iodine method is used to determine the total residual chlorine and free residual chlorine (gb11898-89) by the total residual chlorine (gbj48-83), N, n-diethyl-p-phenylenediamine (DPD) - ammonium ferrous sulfate titration (gb11897-89). (1)    Iodimetry Under the acidic condition, the residual chlorine and potassium iodide release the iodine from the single substance, making the water brown yellow, and using standard sodium thiosulfate Titrate the solution to light yellow, add starch indicator and continue titration until blue disappears. The reaction formula is as follows The content of residual chlorine is calculated according to the amount of sodium thiosulfate consumed, and the calculation method is as follows: Total residual chlorine (Cl2, mg/l) = Where: C -- standard solution concentration, mol/l; ——Volume of consumption standard solution, ml; V - water sample volume, ml; 35.45 - molar mass of chlorine, g/mol This method is suitable for the determination of water samples with total residual chlorine (calculated by Cl2, mg/l) content greater than 1mg/l. （2） N, n-diethyl-p-phenylenediamine (DPD) - ammonium ferrous sulfate titration When iodine free ions exist in water, the free residual chlorine reacts with DPD to form red compounds, Titrate with ammonium ferrous sulfate solution until red disappears, and record the titration volume as V1 (ML). The content of free chlorine is: Where: C -- concentration of free chlorine, mg / L; C1 - concentration of standard solution of ammonium ferrous sulfate, mg/l; V0 - volume of water sample, ml. If 1g of potassium iodide is added after DPD is added, at this time, both NH2Cl, nhcl2 and NCl3 react with DPD to form red compounds. The results are total chlorine content at the same time as the determination of free chlorine. The results can be calculated by the following formula: Where: C2 - volume of standard solution of ammonium ferrous sulfate consumed by titration after iodization, ml. If the conical bottle containing buffer solution and DPD test solution is added to the sample, a small amount of potassium iodide is added, the reaction will be limited to the free chlorine and the chloroamine in the chlorine; Before the sample is added with the buffer value and DPD, a small amount of potassium iodide is added. At this time, the reaction occurs between the free chlorine, the one chloroamine and the nitrogen trichloride in the chlorine; Dichloroamine does not react in both cases. Therefore, the concentrations of monochloramine, dichloroamine and nitrogen trichloride in the chemical chlorine can be calculated according to different reaction conditions. This method is suitable for the determination of free chlorine or total chlorine (calculated by Cl2) from 0.0004-0.07mmol / L (0.03-5mg / L). This method is not available for determination when the following oxides are present in water samples: bromine, iodine, bromine, iodamine, ozone, hydrogen peroxide, chromate, manganese oxide, nitrite, copper ion and iron ion. The interference of copper ion < 8mg / L and Fe < 20mg / l can be masked by EDTA disodium. （3） N, n-ethyl-p-phenylenediamine (DPD) colorimetry The principle of this method is basically the same as method 2, but it is not quantitative by titration with ammonium ferrous sulfate solution, but the red compound formed by DPD and chlorine is compared in spectrophotometer. The determination wavelength is 515nm, and quantitative analysis is carried out according to the standard curve. This method is suitable for the determination of free chlorine or total chlorine (calculated by Cl2) from 0.0004-0.07mmol / L (0.03-5mg / L). This method is applicable in the absence of the following oxidants: bromine, iodine, bromine, iodamine, ozone, hydrogen peroxide, chromate, manganese oxide, nitrite, copper ion and iron ion. The interference of copper ion < 8mg / L and Fe < 20mg / l can be masked by EDTA disodium.

Do online dissolved oxygen instrument

Dissolved oxygen (do): the molecular oxygen dissolved in water is called dissolved oxygen. Dissolved oxygen in the atmosphere and Photosynthesis of aquatic organisms such as aquatic algae are the sources of dissolved oxygen in water. The content of dissolved oxygen in water is related to atmospheric pressure, water temperature and salt content. The decrease of atmospheric pressure, the increase of water temperature and the increase of salt content will lead to the decrease of dissolved oxygen content. Dissolved oxygen mainly has the following changes: (1) diurnal variation. The oxygen content is high in the daytime, and the dissolved oxygen in the water is often oversaturated at 2-4 PM. The dissolved oxygen in the water is low at night and reaches the lowest value before dawn（ 2) Vertical variation. In general, the dissolved oxygen in the upper water is much higher than that in the lower water in the daytime. At night, due to the convection of pool water, the dissolved oxygen difference between the upper and lower water gradually decreases, and the oxygen difference is the largest in the afternoon of the whole day（ 3) Level change. Generally, due to the effect of wind, the dissolved oxygen in the downwind is higher than that in the upwind during the day, but the change of dissolved oxygen in the morning is opposite, that is, the dissolved oxygen in the upwind is higher than that in the downwind（ 4) Seasonal change. Generally, low dissolved oxygen occurs in summer and autumn, especially in rainy weather. The dissolved oxygen of clean surface water is nearly saturated. When a large number of algae propagate, do may be oversaturated; When the water body is polluted by organic and inorganic reducing substances, the dissolved oxygen content will decrease or even tend to zero. At this time, the anaerobic bacteria are active and the water quality will deteriorate. When dissolved oxygen in water is lower than 3-4mg / L, many fish have difficulty breathing; If it continues to decrease, it will suffocate and die. It is generally stipulated that the dissolved oxygen in water should be at least 4 mg / L. The content of dissolved oxygen in water can be used as an indirect indicator of organic pollution and self purification. The dissolved oxygen content of rivers, lakes and reservoirs in China is more than 4 mg / L. the dissolved oxygen content of some rivers south of the Yangtze River is generally higher, up to 6-8 mg / L.   Dissolved oxygen is also an important control index in the process of wastewater biochemical treatment. Because the content of dissolved oxygen has a great relationship with the atmosphere, temperature and other factors, the sample collection of dissolved oxygen should use special sampling bottles, such as double oxygen bottle and dissolving bottle. When sampling, pay attention not to make the water sample contact with the air, and the sampling action should be gentle to minimize the disturbance. When sampling, the sampling bottle must be filled, and then the bottle stopper must be tightly closed. At the same time, pay attention not to leave bubbles. When collecting water samples from pipes and water taps, a rubber tube or other hose can be used to guide the water to flow along the bottle wall to overflow, and the water can be collected continuously for a few minutes, and then the water can be plugged tightly without bubbles. In order to prevent the change of dissolved oxygen in water samples, the collected water samples must be fixed on site (adding manganese sulfate and basic potassium iodide) or determined directly on site with oxygen electrode. The methods to determine dissolved oxygen in water include iodometry and its correction method (gb7489-87) and oxygen electrode method (gb11913-89). Clean water can be measured by iodometry; The modified iodometric method or oxygen electrode method must be used for polluted surface water and industrial wastewater. In addition, in order to realize the automatic monitoring of dissolved oxygen, the National Environmental Protection Bureau formulated the technical requirements for dissolved oxygen (do) water quality automatic analyzer (HJ / t99-2003). (1)    Iodimetry Iodometry is the standard method for the determination of dissolved oxygen in water. In the absence of interference, this method is suitable for all kinds of water samples with dissolved oxygen concentration greater than 0.2mg/l and less than twice of oxygen saturation concentration (about 20mg / L). Easily oxidized organic compounds, such as tannic acid, humic acid and lignin, will interfere with the determination; Oxidable sulfides, such as thiourea, can also cause interference. When the water sample contains the above substances, the oxygen electrode method should be used. The principle of iodimetry is: manganese sulfate and basic potassium iodide are added to the water sample, and the dissolved oxygen in the water will oxidize the divalent manganese to the tetravalent manganese, and generate hydroxide precipitation. After adding acid, the precipitate is dissolved, and tetravalent manganese can oxidize iodide ion and release free iodine equivalent to dissolved oxygen. With starch as indicator, the content of dissolved oxygen can be calculated by titrating the released iodine with sodium thiosulfate standard solution. The reaction equation is as follows MnSO4＋2NaOH＝Na2SO4＋Mn(OH)2↓ (white precipitate) 2Mn(OH)2+O2＝2MnO(OH)2↓ (brown precipitate) MnO(OH)2＋2H2SO4＝Mn(SO4)2＋3H2O Mn(SO4)2＋2KI＝MnSO4＋K2SO4＋I2 2Na2S2O3＋I2＝Na2S4O6＋2Nal （2） Modified iodometry In the determination of dissolved oxygen in water samples by iodometry, if there are some reducing substances in water samples, it will be interfered. At this time, some reagents can be added for correction. The common correction methods are sodium azide correction method and potassium permanganate correction method. one   Sodium azide correction method Nitrite in water sample will interfere with the determination of dissolved oxygen by iodometry. Sodium azide can be used to decompose nitrite and then determine it by iodometry. The decomposition reaction of nitrite is as follows 2NaN3＋H2SO4＝2HN3+Na2SO4 HNO2＋NH3＝N2O＋N2＋H2O Nitrite mainly exists in wastewater and river water after biochemical treatment. It can react with potassium iodide to release free iodine and produce positive interference 2HNO2＋2KI＋H2SO4＝K2SO4＋2H2O＋N2O2＋I2 If the reaction is up to this point, the introduction error is not big; However, when the water sample comes into contact with air, the newly dissolved oxygen reacts with N2O2 to form nitrite 2N2O2＋2H2O＋O2＝4HNO2 Such a cycle, continuous release of iodine, will introduce considerable error. When the content of ferric ion in water sample is high, the interference can be eliminated by adding potassium fluoride or acidifying with phosphoric acid instead of sulfuric acid. The measurement results are calculated as follows: Where: m -- concentration of sodium thiosulfate standard solution, mol / L; V -- volume of sodium thiosulfate standard solution consumed in titration, ml; V water -- volume of water sample, ml; 8 -- oxygen conversion value, G. It should be noted that sodium azide is a highly toxic and explosive reagent. It is not allowed to acidify the alkaline potassium iodide sodium azide solution directly to avoid toxic azide mist. two   Potassium permanganate correction method The method is suitable for water samples containing a large amount of ferrous ions and without other reducing agents and organic compounds. Potassium permanganate is used to oxidize ferrous ion to eliminate interference. Excess potassium permanganate is removed by sodium oxalate solution, and the generated high valent iron ion is masked by potassium fluoride. Others are the same as iodometry. （3） Oxygen electrode method The widely used dissolved oxygen electrode is polytetrafluoroethylene film electrode, which is a typical oxygen electrode. According to its working principle, it can be divided into polarographic type and galvanic cell type. The structure of polarographic oxygen electrode is shown in Fig. 3-27. It is composed of gold cathode, silver silver chloride anode, polytetrafluoroethylene film and shell. The electrode cavity is filled with potassium chloride solution, the PTFE film separates the electrolyte from the water sample, and the dissolved oxygen diffuses through the film. When a fixed polarization voltage of 0.5-0.8V is applied between the two electrodes, the dissolved oxygen in the water sample diffuses through the film and is reduced on the cathode, resulting in a diffusion current proportional to the oxygen concentration. The electrode reaction is as follows Figure 3-27 dissolved oxygen electrode structure 1. Gold cathode; 2. Silver wire anode; 3. Thin film; 4. KCl solution; 5. Shell Cathode: O2 + 2H2O + 4E = 4OH- Anode: 4ag + 4CL - = 4agcl + 4e The reduction current I can also be expressed as: Where: K - proportional constant; N -- number of electrons gained and lost in electrode reaction; F -- Faraday constant; A -- cathode area; PM -- permeability coefficient of membrane; L -- thickness of film; C0 -- partial pressure or concentration of dissolved oxygen. Therefore, as long as the reduction current is measured, the concentration of dissolved oxygen in water sample can be calculated. All kinds of dissolved oxygen meters work according to this principle (see Figure 3-28). In the process of determination, the zero point is first corrected with the anaerobic water sample, and then the calibration value of the instrument is calibrated with the chemical method. Finally, the dissolved oxygen concentration of the water sample can be directly displayed. The instrument is equipped with automatic or manual temperature compensation device to compensate the measurement error caused by temperature change. 1. Polarized voltage source; 2. Dissolved oxygen electrode and measuring cell; 3. Operational amplifier; 4. Indicator The determination of do by do electrode method is not affected by the color and turbidity of water sample and the interfering substances in chemical titration; The method is rapid, simple and suitable for on-site determination; It is easy to realize automatic continuous measurement. However, if the water sample contains algae, sulfide, carbonate, oil and other substances, the film will be blocked or damaged, so the film should be replaced in time.

Determination of pH value by colorimetry

PH value The pH value is the negative logarithm of the hydrogen ion activity in the solution When the concentration of H + is small, the concentration of H + can be used to calculate pH instead of activity. PH value is one of the most commonly used water quality indicators. Generally, the pH value of drinking water should be between 6.5 and 8.5; The pH value of surface water is in the range of 6.5-8.5; In order to ensure the normal operation of pipelines and equipment, the pH value of some industrial water must be kept between 7.0 and 8.5; In the biochemical treatment of wastewater, pH value is also one of the important indexes to evaluate the toxicity of toxic substances. In addition, pH value has great influence on the migration and transformation of pollutants in water, so it should be controlled. PH value is not only related to but also different from acidity and alkalinity. The pH value indicates the strength of the acidity and alkalinity of water, and the acidity or alkalinity is the content of acid or alkali substances in water. For solutions with the same acidity, such as 0.1mol hydrochloric acid and 0.1mol acetic acid, the acidity of both solutions is 100mmol / L, but their pH values are quite different. Hydrochloric acid is a strong acid, which is almost 100% ionized in water with pH of 1; Acetic acid is a weak acid, its ionization degree in water is only 1.3%, and its pH is 2.9. The pH value of water is mainly determined by glass electrode method (gb6920-86) and colorimetry. 1. Colorimetry Different pH aqueous solutions show different colors in various acid-base indicators, and each indicator has a certain range of color change. Add appropriate indicator into the standard buffer solution with known pH value to prepare the standard color column, and compare with the solution to be tested to determine the pH value of water sample. This method is not suitable for colored, turbid or high free chlorine, oxidant and reductant water samples. If the pH value of water sample is roughly determined, pH test paper can be used. 2. Glass electrode method The pH glass electrode was used as indicator electrode and the saturated calomel electrode as reference electrode Where:    Calomel: the electrode potential of saturated calomel electrode does not change with the activity of hydrogen ion (ah +) in the measured solution, so it can be regarded as a constant value; The electrode potential of glass pH glass electrode changes with the activity of hydrogen ion in the solution.   Glass can be expressed by Nernst equation, so the above equation is (at 25 ℃): E battery =    Calomel-（   0+0.0591gaH+）＝K+0.059pH It can be seen that as long as the E-Cell is measured, the pH of the solution to be measured can be obtained. In the actual determination, it is difficult to obtain the K value accurately, so the calculation method is not used. Instead, the solution with known pH value is used as the standard for calibration, and the pH of the solution to be measured is directly measured with a pH meter. The internal resistance of pH glass electrode is usually as high as tens to hundreds of megohm, so the matching pH meters are transistor millivoltmeter or electronic potentiometer with high impedance input. In order to correct the influence of temperature on pH measurement, the pH meter is equipped with temperature compensation device. In order to simplify the operation and make it convenient to use and suitable for field use, a variety of pocket pH meters and pen pH meters have been made by widely using composite pH electrodes. At present, more advanced pH meters at home and abroad can measure - 2.00 ～ + 19.00, some can even measure the range of - 9.00 ～ + 23.00, and the measurement accuracy is 0.01 pH value. The glass electrode method is accurate, rapid, and less affected by the factors of water color, turbidity, colloidal substances, oxidant, reducing agent and salinity.

Redox potential

Redox potential: for a water body, there are often a variety of redox pairs, forming a complex redox system, and its redox potential is the comprehensive result of redox reaction between a variety of oxidizing substances and reducing substances. Although this index can not be used as an index of the concentration of certain oxidizing and reducing substances, it can help us understand the electrochemical characteristics of water and analyze the properties of water. It is a comprehensive index. The redox potential of water must be measured on site. The determination method is to use platinum electrode as indicator electrode, saturated calomel electrode as reference electrode, and water sample as primary battery. The redox potential of platinum electrode relative to calomel electrode is measured by transistor millivoltmeter or universal pH meter, and then converted into redox potential relative to standard hydrogen electrode as report result. The formula is as follows: En=Eind+Eref Where: en -- redox potential (MV) of water sample; Eind -- measured redox potential (MV); Eref -- electrode potential (MV) of saturated calomel electrode at measured temperature, which can be found in physical chemistry manual or relevant data.

conductivity

Conductivity: conductivity is the ability of a substance to transmit current, which is relative to the resistance value. Conductivity is the ability of conducting current in water solution. The conductivity of water solution is directly proportional to the concentration of dissolved solid quantity. Moreover, the higher the solid concentration, the greater the conductivity. The relationship between conductivity and dissolved solid concentration is approximately expressed as follows: 1.4 μ S/cm=1ppm or 2 μ S/cm=1ppm (CaCO3 per million units) measured in siemens/cm (s/cm), 10-6 of this unit is μ S/cm, ms/cm at 10-3. Among them, 1ppm is equal to 1mg/l, which is the unit of measurement of total solid solution. The conductance (L) is the reciprocal of the resistance (R). Under certain conditions (temperature, pressure, etc.), the resistance of conductor is not only determined by the nature of matter, but also depends on its cross-sectional area and length. The conductivity (L) of the conductor can be expressed as follows: Where: K-1/ ρ， It is called conductivity or specific conductivity;        Q-1 / A, known as electrode constant or conductivity cell constant. For electrolyte solution, conductivity refers to the conductivity of 1cm3 solution filled between two parallel electrodes with a distance of 1cm. It can be seen from the above formula that when the electrode constant (q) is known and the solution resistance (R) is measured, the conductivity can be obtained. The electrode constant is usually determined by the standard potassium chloride solution with known conductivity. The conductivity of potassium chloride solution with different concentrations (25 ℃) is shown in table 3-13. The conductivity of the solution is related to the temperature, polarization phenomenon on the electrode, the capacitance of the electrode distribution, etc. the compensation or elimination measures are generally adopted in the instrument. Table 3-13 conductivity of potassium chloride solution with different concentrations The conductivity of water body is mainly measured by conductance meter (hj/t97-2003), which is composed of conductivity pool system and measuring instrument. The conductivity cell is a container for holding or sending the solution under test. In the conductivity cell, the conductivity electrode and temperature sensing element are installed. The common flat electrode in laboratory, such as type 260 conductivity electrode, is to make two plates with an area of 5 × The smooth platinum plate of 10mm2 or platinum sheet coated with platinum black is fused to the ring glass, and the distance between poles is 6mm. The smooth platinum electrode is used to measure the solution with low conductivity, and the platinum electrode with platinum black plating is used to measure the solution with high conductivity. The electrode of industrial conductance meter is made of stainless steel or graphite in cylinder or ring; For the determination of conductivity of strongly corrosive media, non-contact electrodes can be used. According to the principle of measuring conductance, conductance meter can be divided into balanced bridge conductometer, resistance voltage divider conductivity meter, current measuring conductivity meter, electromagnetic induction conductivity meter, etc.

Physical monitoring index chromaticity

Chroma: pure water is colorless and transparent. There are soil, organic matter, plankton and inorganic minerals in natural water, which make it present certain color. Industrial wastewater contains dyes, biological pigments, colored suspended solids and so on, which is the main source of environmental water coloring. Colored water can weaken the transparency of water, affect the growth of aquatic organisms and reduce the ornamental value of water. The color of water can be divided into true color and surface color. True color refers to the color of water after removing suspended solids, that is, chromaticity; The color of water without removing suspended solids is called surface color. For clean or low turbidity water, its true color and surface color are similar; For industrial wastewater with deep coloring, the difference between them is great. The color of water is usually determined by the following methods. （1） Platinum cobalt standard colorimetry In this method, potassium chloroplatinate and cobalt chloride are dissolved in water to form standard color column (gb11903-89), and then the color of water sample is determined by visual colorimetry. It is stipulated that the color of 1mg platinum and 0.5mg cobalt per liter of water is 1 degree. As a standard color unit, this standard solution is stable and can be stored for a long time. However, potassium chloroplatinate is more expensive. It can be made into standard series by dissolving potassium dichromate and cobalt sulfate in water, but it cannot be preserved for a long time. If the water sample is turbid, it should be placed for clarification. Centrifugation or pore size 0.45 can also be used μ M membrane filtration to remove suspended solids, but not filter paper filtration. The method is suitable for the determination of natural water and drinking water with yellow color. If there is soil or other fine dispersed suspended solids in the water sample, and it is still opaque after being treated by clarification, centrifugation and other methods, the "surface color" shall be determined. If the water sample is not yellow, it can not be compared with the platinum cobalt color column, only the appropriate words can be used to describe its color and chromaticity. （2） Dilution multiple method The method is suitable for the determination of the color of surface water and industrial wastewater polluted by industrial wastewater. During the determination, the type and depth of the color of the water sample should be described in words, such as dark blue, brown yellow, dark black, etc. Then take a certain amount of pretreated water sample and dilute it with distilled water until the color just can't be seen. The chromaticity of the water sample is expressed according to the dilution ratio (gb11903-89), and the unit is times. The water sample should be free of leaves, dead branches and other debris; After sampling, it should be determined as soon as possible, otherwise it should be refrigerated. In addition, CIE has also formulated the standard for determining the chromaticity of water samples by spectrophotometer.