COD norms. Wastewater treatment of enterprises with high COD and BOD. Safety requirements, environmental protection
Introduction
Several thousand were found in the water of water supply sources. organic matter different chemical classes and groups. Organic compounds of natural origin (humic substances, various amines and others) are capable of changing the organoleptic properties of water, and for this reason they must be removed during water treatment.
There is no doubt that organic substances of technogenic origin, when they enter from drinking water may adversely affect the body. Analytical control of their content in drinking water is difficult not only because of their huge number, but also because many of them are very unstable and their continuous transformation takes place in water. Therefore, analytical control cannot identify all organic compounds present in drinking water.
However, many organic substances have pronounced organoleptic properties (smell, taste, color, foaming ability), which makes it possible to identify them and limit their content in drinking water. Examples of such substances are: synthetic surfactants (surfactants) that form foam in low (non-toxic) concentrations; phenols, which give water a specific odor; many organophosphorus compounds.
Organic substances are always present in the natural water of reservoirs. Their concentrations can sometimes be very low (for example, in springs and melt waters). Natural sources of organic substances are the decaying remains of organisms of plant and animal origin, both living in the water and falling into the reservoir from the foliage, through the air, from the shores. In addition to natural sources, there are also technogenic sources of organic substances: transport enterprises (petroleum products), pulp and paper and timber processing plants (lignins), meat processing plants (protein compounds), agricultural and fecal effluents, etc. Organic pollution enters the reservoir in different ways, mainly with sewage and rain surface washouts from the soil.
BOD and COD
The integral content of organic substances is estimated according to the indicators of BOD and COD.
Biochemical and Chemical Oxygen Demand - BOD and COD accepted in hygiene, hydrochemistry and ecology, integral indicators characterizing the content of unstable (non-conservative) organic substances in water that are transformed in water by hydrolysis, oxidation and other processes. The content of such substances is expressed in terms of the amount of oxygen required for their oxidation in a strongly acidic medium with permanganate (BOD) or bichromate (COD). These substances include aliphatic acids, some esters, amines, alcohols.
Under natural conditions, organic substances in water are destroyed by bacteria, undergoing aerobic biochemical oxidation with the formation of CO 2 . At the same time, oxygen dissolved in water (DO) is consumed for oxidation. In water bodies with a high content of organic matter, most of the oxygen is consumed for biochemical oxidation, thus depriving other organisms of oxygen. Therefore, the number of organisms more resistant to low oxygen content increases, and oxygen-loving species disappear. Thus, in the process of biochemical oxidation of organic substances in water, the oxygen concentration decreases, and this decrease is indirectly a measure of the content in water organic substances. The corresponding indicator of water quality, which characterizes the total content of organic substances in water, is called biochemical oxygen demand (BOD).
BOD is the amount of oxygen in (mg) required for the oxidation of organic matter in 1 liter of water under aerobic conditions, without access to light, at 20 ° C, for a certain period as a result of biochemical processes occurring in water.
The determination of BOD is based on measuring the concentration of RA in a water sample immediately after sampling, as well as after sample incubation. The sample is incubated without access to air in an oxygen flask (that is, in the same container where the DO value is determined) for the time necessary for the biochemical oxidation reaction to proceed. Since the rate of the biochemical reaction depends on temperature, the incubation is carried out in a constant temperature mode (20 ± 1) °C, and the accuracy of the BOD analysis depends on the accuracy of maintaining the temperature value. Usually determine the BOD for 5 days of incubation (BOD 5). Can also be determined BOD 10 for 10 days and BOD full. for 20 days (in this case, about 90% and 99% of organic substances are oxidized, respectively). It is tentatively accepted that BOD 5 is about 70% of the total BOD. , but can range from 10% to 90% depending on the oxidizing substance. error in determination of BOD Illumination of the sample can also be introduced, which affects the vital activity of microorganisms and can in some cases cause photochemical oxidation. Therefore, the incubation of the sample is carried out without access to light.
In surface waters, the value of BOD 5 ranges from 0.5 to 5.0 mg/l; it is subject to seasonal and daily changes, which mainly depend on temperature changes and on the physiological and biochemical activity of microorganisms. Very significant changes in BOD 5 natural waters oem when contaminated with sewage.
Table 1. Values of BOD 5 in water bodies with various degrees of pollution
The standard for BOD is full. should not exceed: for reservoirs of household and drinking water use - 3 mg/l; for reservoirs of cultural and household water use - 6 mg/l. Accordingly, it is possible to estimate the maximum permissible values of BOD 5 for the same water bodies, equal to 2 mg/l and 4 mg/l.
The value characterizing the content of organic and minerals oxidized by one of the strong chemical oxidizing agents under certain conditions is called oxidizability or COD. There are several types of water oxidizability: permanganate, bichromate, iodate, cerium.
Being an integral (total) indicator, COD is currently considered one of the most informative indicators of anthropogenic water pollution. This indicator, in one form or another, is used everywhere in monitoring the quality of natural waters, studying wastewater, etc. The results of determining oxidizability are expressed in milligrams of oxygen consumed per 1 liter of water (mgO / l).
In water bodies and streams subjected to strong impact economic activity a person, the change in oxidizability acts as a characteristic reflecting the mode of inflow of wastewater. For natural low-polluted waters, it is recommended to determine permanganate oxidizability; in more polluted waters, as a rule, dichromate oxidizability(COD).
In accordance with the requirements for the composition and properties of water in reservoirs at points of drinking water use, the value of COD should not exceed 15 mg O/dm 3 ; in recreation areas in water bodies, the COD value is allowed up to 30 mg O/dm 3 .
In monitoring programs, COD is used as a measure of the amount of organic matter in a sample that is susceptible to oxidation by a strong chemical oxidizer. COD is used to characterize the state of watercourses and reservoirs, household and (including the degree of their purification), as well as surface runoff.
Table 2. COD values in water bodies with various degrees of pollution
However, not all organic substances equally participate in the chemical oxidation reaction. Just as in biochemical oxidation, in chemical oxidation it is possible to distinguish groups of easily, normally, and heavily oxidized organic substances. Therefore, there is always a difference between theoretically possible and practically achievable COD values. interfere accurate determination of COD in the first place, chloride anions, as a rule, contained in natural and, especially, in sewage. The determination is also hindered by nitrites, which are often present in waters that have undergone biochemical treatment.
Standards for COD in the water of reservoirs: for drinking water - 5.0 mgO / l (for permanganate oxidizability), COD - 15 mgO / l.
Rigidity
Water hardness is a property of natural water, which depends on the presence in it of mainly dissolved salts of calcium and magnesium. The total content of these salts is called total hardness. The total hardness is subdivided into carbonate, due to the concentration of bicarbonates (and carbonates at pH> 8.3) of calcium and magnesium cations, and non-carbonate - the concentration of calcium and magnesium salts of strong acids in water. Since, when boiling water, bicarbonates turn into carbonates, which precipitate, carbonate hardness is called temporary or removable. The hardness remaining after boiling is called constant. The results of the determination of hardness are usually expressed in meq/dm 3 . Under natural conditions, ions of calcium, magnesium and other alkaline earth metals that cause hardness enter the water as a result of the interaction of dissolved carbon dioxide with carbonate minerals and during other processes of dissolution and chemical weathering of rocks. The source of these ions is also microbiological processes occurring in soils in the catchment area, in bottom sediments, as well as wastewater from various enterprises. Water hardness varies widely. Water with a hardness of less than 4 meq / dm 3 is considered soft, from 4 to 8 meq / dm 3 - medium hardness, from 8 to 12 meq / dm 3 - hard and above 12 meq / dm 3 - very tough. The total hardness ranges from units to tens, sometimes hundreds of mg-eq / dm 3, and the carbonate hardness is up to 70-80% of the total hardness. Usually predominates (up to 70%) hardness due to calcium ions; however, in some cases magnesium hardness can reach 50-60%. The hardness of sea water and oceans is much higher (tens and hundreds of meq/dm3). The hardness of surface waters is subject to noticeable seasonal fluctuations, reaching usually the greatest value at the end of winter and the smallest value during the flood period.
Oxidability: permanganate and dichromate (COD)
A value that characterizes the content in water of organic and mineral substances oxidized by one of the strong chemical oxidizing agents under certain conditions. There are several types of water oxidizability: permanganate, bichromate, iodate, cerium. The highest degree of oxidation is achieved by the methods of dichromate and iodate oxidation of water. It is expressed in milligrams of oxygen used for the oxidation of organic substances contained in 1 dm 3 of water,. The composition of organic matter in natural waters is formed under the influence of many factors. Among the most important are intra-aquatic biochemical processes of production and transformation, receipts from other water bodies, with surface and underground runoff, with atmospheric precipitation, with industrial and domestic wastewater. The organic substances formed in the reservoir and entering it from the outside are very diverse in nature and chemical properties, including resistance to the action of various oxidizing agents. The ratio of easily and hardly oxidizable substances contained in water significantly affects the oxidizability of water under the conditions of one or another method of its determination. In surface waters, organic substances are in dissolved, suspended and colloidal states. The latter are not taken into account separately in routine analysis, therefore, the oxidizability of filtered (dissolved organic matter) and unfiltered (total organic matter) samples is distinguished. The values of the oxidizability of natural waters vary from fractions of milligrams to tens of milligrams per liter, depending on the overall biological productivity of water bodies, the degree of contamination with organic substances and compounds of biogenic elements, as well as the influence of organic substances. natural origin coming from swamps, peat bogs, etc. Surface waters have a higher oxidizability compared to underground waters (tenths and hundredths of a milligram per 1 dm 3), with the exception of the waters of oil fields and groundwater, fed by swamps. Mountain rivers and lakes are characterized by an oxidizability of 2-3 mg O 2 / dm 3, flat rivers - 5-12 mg O 2 / dm 3, swamp-fed rivers - tens of milligrams per 1 dm 3. The oxidizability of unpolluted surface waters shows a fairly distinct physico- geographic zoning:
Oxidability is subject to regular seasonal fluctuations. Their nature is determined, on the one hand, by the hydrological regime and the influx of organic matter from the watershed, which depends on it, and, on the other hand, by the hydrobiological regime. In reservoirs and watercourses subjected to a strong impact of human activities, the change in oxidizability acts as a characteristic that reflects the regime of sewage inflow. For natural slightly polluted waters, it is recommended to determine the permanganate oxidizability; in more polluted waters, as a rule, bichromate oxidizability (COD) is determined. In accordance with the requirements for the composition and properties of water in reservoirs at points of drinking water use, the value of COD should not exceed 15 mgO 2 /dm 3; in recreation areas in water bodies, COD values up to 30 mgO 2 /dm 3 are allowed. In monitoring programs, COD is used as a measure of the amount of organic matter in a sample that is susceptible to oxidation by a strong chemical oxidizer. COD is used to characterize the state of watercourses and reservoirs, the inflow of domestic and industrial wastewater (including the degree of their purification), as well as surface runoff. To calculate the concentration of carbon contained in organic substances, the COD value (mg / dm 3) is multiplied by 0.375 (coefficient equal to the ratio of the amount of carbon equivalent substance to the amount of oxygen equivalent substance).
Biochemical oxygen demand (BOD)
The degree of water pollution by organic compounds is defined as the amount of oxygen required for their oxidation by microorganisms under aerobic conditions. Biochemical oxidation of various substances occurs at different rates. Easily oxidizing ("biologically soft") substances include formaldehyde, lower aliphatic alcohols, phenol, furfural, etc. The middle position is occupied by cresols, naphthols, xylenols, resorcinol, pyrocatechol, anionic surfactants, etc. "Biologically hard" substances hydroquinone, sulfonic acid are slowly destroyed , nonionic surfactants, etc. In laboratory conditions, along with BOD full. BOD 5 is determined - biochemical oxygen demand for 5 days. In surface waters, BOD 5 values usually vary within 0.5-4 mgO 2 /dm 3 and are subject to seasonal and daily fluctuations. The determination of BOD 5 in surface waters is used to assess the content of biochemically oxidizable organic substances, habitat conditions for aquatic organisms, and as an integral indicator of water pollution. It is necessary to use the values of BOD 5 when monitoring the efficiency of the treatment facilities. Seasonal changes depend mainly on changes in temperature and on the initial concentration of dissolved oxygen. The influence of temperature affects through its effect on the rate of the consumption process, which increases by 2-3 times with an increase in temperature by 10 o C. The influence of the initial oxygen concentration on the process of biochemical oxygen consumption is due to the fact that a significant part of microorganisms have their own oxygen optimum for development in in general and for physiological and biochemical activity. Daily fluctuations in BOD 5 values also depend on the initial concentration of dissolved oxygen, which can change by 2.5 mg/dm 3 during the day, depending on the ratio of the intensity of its production and consumption processes. Changes in BOD 5 values are quite significant depending on the degree of pollution of water bodies.
Values of BOD 5 in water bodies with varying degrees of pollution.
For reservoirs polluted mainly by domestic wastewater, BOD 5 is usually about 70% of BOD total. . Depending on the category of the reservoir, the value of BOD 5 is regulated as follows: not more than 3 mgO 2 /dm 3 for reservoirs of domestic and drinking water use and not more than 6 mgO 2 /dm 3 for reservoirs of domestic and cultural water use. For the seas (categories I and II of fishery water use), the five-day oxygen demand (BOD 5) at 20 ° C should not exceed 2 mgO 2 /dm 3.
BOD full
The total biochemical oxygen demand (BOD total) is the amount of oxygen required for the oxidation of organic impurities before the start of nitrification processes. The amount of oxygen consumed for the oxidation of ammonium nitrogen to nitrites and nitrates is not taken into account when determining BOD. For domestic wastewater (without a significant admixture of industrial) BOD 20 is determined, considering that this value is close to BOD full. Full biological oxygen demand BOD full. for inland waters for fishery purposes (categories I and II) at 20 ° C should not exceed 3 mgO 2 /dm 3.
Oxygen
Dissolved oxygen is found in natural water in the form of O 2 molecules. Its content in water is affected by two groups of oppositely directed processes: some increase the concentration of oxygen, others decrease it. The first group of processes enriching water with oxygen should include:
- the process of absorbing oxygen from the atmosphere;
- release of oxygen by aquatic vegetation during photosynthesis;
- entry into water bodies with rain and snow waters, which are usually supersaturated with oxygen.
Absorption of oxygen from the atmosphere occurs on the surface of a water body. The rate of this process increases with a decrease in temperature, with an increase in pressure and a decrease in salinity. Aeration - enrichment of deep layers of water with oxygen - occurs as a result of mixing of water masses, including wind, vertical temperature circulation, etc. Photosynthetic release of oxygen occurs during the assimilation of carbon dioxide by aquatic vegetation (attached, floating plants and phytoplankton). The process of photosynthesis proceeds the stronger, the higher the water temperature, the intensity of sunlight and the more biogenic (nutrient) substances (P, N, etc.) in the water. Oxygen production occurs in the surface layer of the reservoir, the depth of which depends on the transparency of the water (for each reservoir and season it can be different - from a few centimeters to several tens of meters). The group of processes that reduce the oxygen content in water includes reactions of its consumption to the oxidation of organic substances: biological (respiration of organisms), biochemical (respiration of bacteria, oxygen consumption during the decomposition of organic substances) and chemical (oxidation of Fe 2+ , Mn 2+ , NO 2 -, NH 4 +, CH 4, H 2 S). The rate of oxygen consumption increases with increasing temperature, the number of bacteria and other aquatic organisms and substances undergoing chemical and biochemical oxidation. In addition, a decrease in the oxygen content in water can occur due to its release into the atmosphere from the surface layers and only if the water at a given temperature and pressure turns out to be supersaturated with oxygen. In surface waters, the content of dissolved oxygen varies widely - from 0 to 14 mg/dm 3 - and is subject to seasonal and daily fluctuations. Daily fluctuations depend on the intensity of the processes of its production and consumption and can reach 2.5 mg / dm 3 of dissolved oxygen. In winter and summer periods, the distribution of oxygen has the character of stratification. Oxygen deficiency is more often observed in water bodies with high concentrations of polluting organic substances and in eutrophicated water bodies containing a large number of biogenic and humic substances. The oxygen concentration determines the magnitude of the redox potential and, to a large extent, the direction and speed of the processes of chemical and biochemical oxidation of organic and inorganic compounds. The oxygen regime has a profound effect on the life of the reservoir. The minimum content of dissolved oxygen that provides normal development fish, is about 5 mgO 2 /dm 3. Lowering it to 2 mg / dm 3 causes mass death (freeze) of fish. The supersaturation of water with oxygen as a result of photosynthesis processes with insufficiently intensive mixing of water layers also adversely affects the state of the aquatic population. In accordance with the requirements for the composition and properties of water in reservoirs at points of drinking and sanitary water use, the content of dissolved oxygen in a sample taken before 12 noon should not be lower than 4 mg / dm 3 in any period of the year; for reservoirs for fishery purposes, the concentration of oxygen dissolved in water should not be lower than 4 mg / dm 3 in winter (during freezing) and 6 mg / dm 3 - in summer. Determination of oxygen in surface waters is included in the observation programs in order to assess the living conditions of hydrobionts, including fish, and also as an indirect characteristic of assessing the quality of surface waters and regulating the process of wastewater treatment. It is essential for aerobic respiration and is an indicator of biological activity (i.e. photosynthesis) in a water body.
Water pollution level and quality class. | dissolved oxygen | ||
summer, mg/dm 3 | winter, mg / dm 3 | % saturation | |
very clean, I | 9 | 14-13 | 95 |
clean, II | 8 | 12-11 | 80 |
moderately polluted, III | 7-6 | 10-9 | 70 |
contaminated, IV | 5-4 | 5-4 | 60 |
dirty, V | 3-2 | 5-1 | 30 |
very dirty, VI | 0 | 0 | 0 |
GOST 31859-2012
INTERSTATE STANDARD
Method for determining chemical oxygen demand
water. Method for determination of chemical oxygen demand
ISS 13.060.50
TN VED 220100000
220110000
Introduction date 2014-01-01
Foreword
The goals, basic principles and basic procedure for carrying out work on interstate standardization are established by GOST 1.0-92 "Interstate standardization system. Basic provisions" and GOST 1.2-2009 "Interstate standardization system. Interstate standards, rules and recommendations for interstate standardization. Rules for the development, adoption, application, renewal and cancellation.
About the standard
1 PREPARED BY "Protektor" Limited Liability Company together with "Lumex" group of companies
2 INTRODUCED by the Federal Agency for Technical Regulation and Metrology (Technical Committee for Standardization TK 343 "Water Quality")
3 ADOPTED by the Interstate Council for Standardization, Metrology and Certification (Minutes of November 15, 2012 N 42)
Voted to accept:
Short name of the country according to MK (ISO 3166) 004-97 | short name national authority for standardization |
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Armenia | Agency "Armstandard" |
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Kazakhstan | State Standard of the Republic of Kazakhstan |
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Belarus | State Standard of the Republic of Belarus |
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Kyrgyzstan | Kyrgyzstandart |
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Moldova | Moldova-standard |
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Russia | Rosstandart |
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Uzbekistan | Uzstandard |
4 This standard complies with the international standard ISO 15705:2002* Water quality - Determination of the chemical oxygen demand index (ST-COD) - Small-scale sealed-tube method
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* Access to international and foreign documents mentioned hereinafter can be obtained by clicking on the link to the site http://shop.cntd.ru
The degree of conformity is non-equivalent (NEQ).
This standard has been prepared on the basis of the application of GOST R 52708-2007 "Water. Method for determining the chemical oxygen demand"
5 By order of the Federal Agency for Technical Regulation and Metrology of November 29, 2012 N 1618-st, the interstate standard GOST 31859-2012 was put into effect as a national standard Russian Federation from January 1, 2014.
6 INTRODUCED FOR THE FIRST TIME
Information about changes to this standard is published in the annual information index "National Standards", and the text of changes and amendments - in the monthly information index "National Standards". In case of revision (replacement) or cancellation of this standard, a corresponding notice will be published in the monthly information index "National Standards". Relevant information, notification and texts are also placed in information system general use - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet
1 area of use
1 area of use
This International Standard specifies a method for the determination of chemical oxygen demand (COD) in water using photometry. The method applies to all types of water (drinking, natural, waste) in the range of COD values from 10 to 800 mgO/dm. The method can be used to analyze water samples with higher COD values provided they are diluted, but not more than 100 times.
Interfering factors during the determination include the presence of chlorides in the water sample at their content of more than 1000 mg/dm3 and manganese (II) at its content of more than 50 mg/dm3. Interfering factors are eliminated by diluting the water sample.
2 Normative references
This standard uses normative references to the following interstate standards:
GOST 17.1.5.05-85 Nature protection. Hydrosphere. General requirements for sampling surface and sea waters, ice and precipitation
GOST 1770-74 (ISO 1042-83, ISO 4788-80) Measuring laboratory glassware. Cylinders, beakers, flasks, test tubes. General specifications
GOST 4204-77 Reagents. Sulfuric acid. Specifications
GOST 4220-75 Reagents. Potassium dichromate. Specifications
GOST ISO 5725-6-2003 Accuracy (correctness and precision) of measurement methods and results. Part 6: Using Accuracy Values in Practice*
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GOST R ISO 5725-6-2002 "Accuracy (correctness and precision) of measurement methods and results. Part 6. Use of accuracy values in practice".
GOST 6709-72 Distilled water. Specifications
GOST 12026-76 Laboratory filter paper. Specifications
GOST ISO/IEC 17025-2009 General requirements for the competence of testing and calibration laboratories
GOST 24104-2001 Laboratory balance. General technical requirements*
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* In the Russian Federation, GOST R 53228-2008 "Non-automatic scales. Part 1. Metrological and technical requirements. Tests" is in force.
GOST 25336-82 Laboratory glassware and equipment. Types, basic parameters and dimensions
GOST 29169-91 (ISO 648-77) Laboratory glassware. Pipettes with one mark
GOST 29227-91 (ISO 835-1-81) Laboratory glassware. Pipettes graduated. Part 1. General requirements
GOST 30813-2002 Water and water treatment. Terms and Definitions
GOST 31861-2012 Water. General Sampling Requirements
GOST 31862-2012 Drinking water. Sample selection
Note - When using this standard, it is advisable to check the validity of reference standards in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet or according to the annual information index "National Standards", which was published as of January 1 of the current year, and on issues of the monthly information index "National Standards" for the current year. If the reference standard is replaced (modified), then when using this standard, you should be guided by the replacing (modified) standard. If the referenced standard is canceled without replacement, the provision in which the reference to it is given applies to the extent that this reference is not affected.
3 Terms and definitions
This standard uses the terms according to GOST 30813 and the following term with the corresponding definition:
4 Essence of the method
The essence of the method is to treat a water sample with sulfuric acid and potassium dichromate at a given temperature in the presence of silver sulfate, an oxidation catalyst, and mercury (II) sulfate, used to reduce the effect of chlorides, and to determine COD values in a given concentration range by measuring the optical density of the test solution at set value wavelength using the calibration dependence of the optical density of the solution on the COD value.
COD values in the range from 10 to 160 mgO/dm inclusive are determined by measuring the optical density of the solution at a wavelength of (440±20) nm.
COD values in the range from 80 to 800 mgO/dm inclusive are determined by measuring the optical density of the solution at a wavelength of (600±20) nm.
COD values in the range from 80 to 160 mgO/dm inclusive can be determined both at a wavelength of (440±20) nm and at a wavelength of (600±20) nm.
Safety requirements for measurements are given in Appendix A.
5 Measuring instruments, auxiliary equipment, reagents, materials
Photometer, spectrophotometer or photometric analyzer (hereinafter - analyzer) equipped with an adapter for measuring the optical density of water and aqueous solutions directly in the reaction vessels in the wavelength range from 400 to 700 nm.
Reaction vessels made of heat-resistant glass (tubes with screw caps with a capacity of 10 to 15 cm) designed for processing water samples and measuring the optical density of water and aqueous solutions.
Heating unit (thermoreactor) designed to heat reaction vessels, maintaining the temperature of the contents of the reaction vessels (150 ± 5) °C.
Stirring device such as magnetic stirrer, desiccator or ultrasonic bath.
Laboratory scales according to GOST 24104 of a high or special accuracy class with a division value (reading resolution) of 0.1 mg and a maximum weighing limit of 220 g.
Volumetric flasks according to GOST 1770 of the 2nd accuracy class with a capacity of 25, 50, 1000 cm3.
Dimensional cylinders according to GOST 1770, 2nd accuracy class.
Chemical heat-resistant glasses according to GOST 25336 with a capacity of 1000 cm.
Graduated pipettes of the 2nd class of accuracy according to GOST 29227 or pipettes with one mark of the 2nd class of accuracy according to GOST 29169, or pipette dispensers with a permissible maximum dosing error of ± 5%.
State (interstate) standard sample (GSO) of bichromate oxidizability with an error of the certified value of not more than ±2%.
Distilled water according to GOST 6709.
Sulfuric acid according to GOST 4204, chemically pure
Mercury sulfate (II), chemically pure or h.d.a.
Silver sulfate, chemically pure or h.d.a.
Potassium dichromate (potassium dichromate) according to GOST 4220, chemically pure. or standard-titer (fixanal).
Laboratory filter paper according to GOST 12026.
6 Sampling
Water samples are taken according to GOST 31861, GOST 31862, GOST 17.1.5.05.
For the collection, transportation and storage of water samples, containers made of glass or polymeric materials with a screw-on or ground stopper are used. Containers made of polymeric materials are used only for storing frozen water samples at a temperature of minus 20 °C. The volume of the water sample taken is at least 100 cm3.
Sampling is carried out on the day of the analysis. If water samples are stored until analysis, they are acidified to a pH less than 2 with dilute sulfuric acid (see 7.3.3), adding 10 ml of acid per 1000 ml of sample. At the same time, water samples are stored at a temperature of 2 °C to 8 °C for no more than 5 days in a place protected from light.
The shelf life of water samples frozen to minus 20 °C is no more than 1 month.
If the sample contains sediment visible to the naked eye, suspended matter or undissolved organic matter such as fats, the sample must be vigorously agitated using any agitator (such as a magnetic stirrer, extractor, or ultrasonic bath) to ensure homogeneity before taking an aliquot of the water sample.
7 How to prepare for measurements
7.1 Preparation of the analyzer for operation is carried out in accordance with the manual (instruction) for operation.
7.2 Preparation of reaction vessels
From a new batch of reaction vessels, from 5% to 10% of the total number of reaction vessels is selected by random sampling, but not less than three pieces. Place 5 cm of distilled water into each vessel. The reaction vessel is closed with a lid and checked for the absence of air bubbles visible to the naked eye in distilled water. If bubbles are present, they are removed by light tapping on the wall of the reaction vessel. Measure the absorbance of distilled water in the reaction vessel at 440 nm or 600 nm, depending on the intended COD measurement range (see Section 4).
If the measured values of the optical density of distilled water in each reaction vessel differ by no more than 0.01 absorbance units, then the entire batch of reaction vessels is used for COD measurements.
If the measured values of the optical density of distilled water in the reaction vessels differ by more than 0.01 units of optical density, then a complete control of the entire batch of reaction vessels is carried out, selecting for COD measurements those that differ from each other in terms of optical density by no more than than 0.01 optical density units.
Subsequent checks of the suitability of reaction vessels for measurements are carried out at intervals of at least once a month, similarly to checking a new batch of reaction vessels.
7.3 Preparation of auxiliary solutions
7.3.1 Potassium dichromate solution for measuring COD values in the range 10 to 160 mgO/dm
Potassium dichromate is dried at (105 ± 5) ° C for 2 hours. A weighed portion of 4.90 g of dried potassium dichromate is dissolved in distilled water in a volumetric flask with a capacity of 1000 cm 3 and the volume of the solution in the flask is adjusted to the mark with distilled water. The molar concentration of the equivalent of potassium bichromate is 0.1 mol/dm.
7.3.2 Potassium bichromate solution for measuring COD values in the range 80 to 800 mgO/dm
Potassium dichromate is dried at (105 ± 5) ° C for 2 hours. A weighed portion of 24.52 g of dried potassium dichromate is dissolved in distilled water in a volumetric flask with a capacity of 1000 cm 3 and the volume of the solution in the flask is adjusted to the mark with distilled water. The molar concentration of the equivalent of potassium bichromate is 0.5 mol/dm.
It is allowed to prepare a solution of potassium dichromate from the standard titer according to the instructions attached to it.
The shelf life of the solution is no more than 6 months.
7.3.3 4 mol/l sulfuric acid solution
About 700 ml of distilled water are placed in a glass beaker with a capacity of 1000 ml, 220 ml of concentrated sulfuric acid are carefully added with stirring, cooled and the volume of the solution in the beaker is brought to the mark with distilled water.
7.3.4 Sulfuric acid solution, molar concentration 1.8 mol/l
180 ml of distilled water are placed in a glass beaker with a capacity of 1000 ml, 20 ml of concentrated sulfuric acid are carefully added with stirring.
Shelf life of the solution - no more than 12 months.
7.3.5 Solution of mercury(II) sulfate in sulfuric acid
Dissolve in a glass container 50 g of mercury(II) sulfate in 200 ml of sulfuric acid solution (see 7.3.4). The shelf life of the solution in a glass container is no more than 12 months.
7.3.6 Solution of silver sulfate in sulfuric acid
Dissolve in a glass container 3.25 g of silver sulfate in 250 ml of concentrated sulfuric acid. The solution is stirred and left in a place protected from light for 12 hours at room temperature. Then the solution is again intensively stirred until complete dissolution of silver sulfate.
The solution is stored in a dark glass container in conditions that exclude exposure to direct sunlight, no more than 12 months.
7.3.7 Reagent for filling reaction vessels when measuring COD values in the range from 10 to 160 mgO/dm
Before starting work, add 0.5 ml of potassium dichromate solution (see 7.3.1) to the reaction vessel with a pipette or dosing device, carefully add 2.5 ml of silver sulfate solution (see 7.3.6), then 0.2 ml of mercury sulfate solution (II) (see 7.3.5). It is allowed to add 0.05 g of dry salt of mercury (II) sulfate instead of a solution of mercury (II) sulfate. The mixture is gently stirred rotational movements or using any stirring device, then close the vessel with a lid. Reaction vessels filled with the reagent are stored in a light-tight container in a place protected from light at a temperature of 2 °C to 8 °C.
The shelf life of the reaction vessel filled with the reagent is no more than 12 months. The contents of the reaction vessel are stirred before use.
7.3.8 Reagent for filling reaction vessels when measuring COD values in the range of 80 to 800 mgO/dm
Prepare the reagent according to 7.3.7 using potassium dichromate solution (7.3.2).
Conditions and shelf life of the reaction vessel filled with the reagent according to 7.3.7. The contents of the reaction vessel are stirred before use.
7.3.9 When using reagents (see 7.3.7 and 7.3.8), it is allowed to increase the volumes of solutions of potassium dichromate and silver sulfate by 2 times while increasing the volume of an aliquot portion of the water sample to 4 cm3 (see 8.1), provided that after introduction of a water sample, the free space in the reaction vessel above the liquid is at least 10%-15% of the height of the vessel.
7.4 Preparation of calibration solutions
7.4.1 Preparation of a stock solution with a COD value of 1000 mgO/l
The main solution for measuring COD is prepared from GSO of bichromate oxidizability in accordance with the instructions for use. For example, when using GSO of bichromate oxidizability with a certified COD value of 10,000 mgO / dm, 5 cm3 of GSO of bichromate oxidizability are added to a volumetric flask with a capacity of 50 cm 3 and the volume in the flask is brought to the mark with distilled water. The solution is stable for 1 month when stored in a stoppered flask at 2°C to 8°C.
7.4.2 Preparation of calibration solutions for the range of COD values from 10 to 160 mgO/dm
In volumetric flasks with a capacity of 50 cm3, 0.5 is added with volumetric pipettes; 1.0; 2.0; 3.5; 5.0; 8.0 ml stock solution (see 7.4.1) and dilute the volumes in the flasks to the mark with distilled water. The COD values of the prepared solutions are respectively 10; 20; 40; 70; 100; 160 mgO/dm. Solutions are used on the day of preparation.
7.4.3 Preparation of calibration solutions for the range of COD values from 80 to 800 mgO/dm
In volumetric flasks with a capacity of 25 ml, volumetric pipettes make 2; 5; ten; 20 ml stock solution (see 7.4.1) and dilute the volumes in the flasks to the mark with distilled water. The COD values of the prepared solutions are respectively 80; 200; 400; 800 mgO/dm.
Solutions are used on the day of preparation.
7.5 Analyzer calibration
The analyzer is calibrated in accordance with the operation manual (instruction) using calibration solutions (see 7.4.2 and 7.4.3) depending on the range of measured COD values. Distilled water is used as a zero sample. Calibration solutions and a zero water sample are prepared for measurements similarly to the analyzed samples (see 8.5-8.7), the values of the optical density of the solutions in the reaction vessels are measured at wavelengths (see Section 4) and the calibration dependence of the optical density of the solutions on the COD value is established (calibration characteristic ) using the analyzer software and/or software designed for processing calibration dependencies. The calibration characteristic is recognized as stable if the absolute value of the correlation coefficient set by the software is not less than 0.98. If the correlation coefficient is less than 0.98, the analyzer calibration is repeated.
The stability control of the calibration characteristic is carried out at least once every three months in accordance with the frequency established in the Quality Manual of the laboratory*, using at least two newly prepared calibration solutions with different meanings COD (see 7.4.2 and 7.4.3). The control of the stability of the calibration characteristic is also carried out when changing the batch of the reagent.
________________
*Document not cited. Behind additional information refer to the link. - Database manufacturer's note.
8 How to take measurements
8.1 Analyze at least two aliquots of the water sample at the same time (parallel samples). The volume of the selected aliquot portion of the water sample is 2 cm3. It is allowed to increase the volume of the water sample up to 4 cm3 under the conditions specified in 7.3.9.
8.2 Fill reaction vessels with reagent (see 7.3.7 or 7.3.8).
If the expected value of COD is in the range from 80 to 160 mgO/l, then it is allowed to use the reagent according to both 7.3.7 and 7.3.8.
8.3 Conduct a visual inspection of the reaction vessels and their contents. If cracks, damage of any type, or signs of a green color of the solution are found in the vessel, the reaction vessel is not used.
8.4 Turn on the heating block, heat it up to 150 °C and keep it at this temperature for at least 10 minutes.
8.5 Remove the lid from the reaction vessel and immediately introduce a sample of water into it with a dispenser or volumetric pipette, if necessary, thoroughly mixed beforehand (see Section 6).
NOTE It is recommended to take an aliquot of the sample of water containing suspended solids, after mixing, with a 5 ml graduated pipette with an extended spout or dispenser.
8.6 Screw the lid tightly onto the reaction vessel and mix the contents by gently inverting several times. Wipe the outside of the reaction vessel with filter paper. Place the reaction vessel in the heating block and hold for (120 ± 10) min.
8.7 Carefully, for example, with special grips, remove the reaction vessels from the heating block and cool at room temperature to a temperature not exceeding 60 °C. Stir the contents by inverting the reaction vessels. The reaction vessels are then cooled to room temperature. Reaction vessels in which a visually noticeable decrease in the volume of contents has occurred are not used for measurements. The analysis of the water sample in this case is repeated (see 8.1-8.6).
8.8 If the solution is clear after cooling, measure the absorbance of the water sample at a working wavelength of 440 nm using the reagent (7.3.7) or at 600 nm using the reagent (7.3.8).
If the solution is cloudy, then it is allowed to settle, then its optical density is measured as described above. If, after settling, the solution remains cloudy, then the analysis of the water sample is repeated, after diluting it with distilled water.
9 Rules for processing measurement results
9.1 From the value of the optical density of the solution, measured according to 8.8, for each aliquot portion of the water sample (see 8.1), using the calibration curve (see 7.5), determine the COD value.
If the COD value is outside the calibration range, then the tests of Section 8 are repeated either by diluting the sample with distilled water or using a reagent to work with a different range of COD values.
If the water sample was subjected to dilution during the measurement process, then the obtained COD value is multiplied by the dilution factor of the water sample, which is calculated by the formula
where is the volume of the water sample after dilution, cm;
is the volume of an aliquot of the water sample before dilution (see 8.1), see
9.2 The measurement result is taken as the arithmetic mean of at least two parallel determinations of the COD of a water sample, mgO/dm (see 9.1), provided that the following condition is met:
where is the maximum COD value from two parallel determinations (see 9.1), mgO/dm;
- the minimum COD value from two parallel determinations (see 9.1), mgO/l;
- the relative value of the repeatability limit according to table 1,%.
Table 1
Range of measured COD values, mgO/dm | Repeatability limit (relative value of the allowable discrepancy between two results of parallel determinations at 0.95), % | Reproducibility limit (relative value of the allowable discrepancy between two determination results obtained under reproducibility conditions at 0.95), % | Accuracy indicator (limits* of permissible relative error with a probability of .95), % |
From 10 to 50 incl. | |||
St. 50 "200" | |||
* The established numerical values of the limits of the permissible relative error correspond to the numerical values of the expanded uncertainty (in relative units) with a coverage factor of 2. |
9.3 If condition (2) is not met, methods for checking the acceptability of the results of parallel determinations and establishing final result measurements are carried out in accordance with the requirements of GOST ISO 5725-6 (clause 5.2).
10 Metrological characteristics
The method provides measurement results with metrological characteristics not exceeding the values given in Table 1, with a confidence level of 0.95.
11 Rules for reporting measurement results
The measurement results are recorded in the test report in accordance with GOST ISO / IEC 17025. The measurement result is presented in the form
MgO/dm, (3)
where is the COD value determined according to 9.2 or 9.3, mgO/l;
- the limits of the absolute measurement error of the COD value, mgO/dm, at a confidence level of 0.95.
Values are calculated by the formula
where - the limits of the permissible relative error of the results of measuring the COD value at a confidence level of 0.95 according to table 1, %.
It is allowed to represent the measurement result in the form , mgO/dm, with a confidence level of 0.95, provided that measurement results.
12 Quality control of measurement results
12.1 Monitoring the stability of measurement results in the laboratory includes monitoring the stability of the standard deviation of repeatability, monitoring the stability of the standard deviation of intermediate precision and monitoring the stability of the accuracy of routine analysis according to GOST ISO 5725-6 (section 6) using GSO of bichromate oxidizability.
12.2 Checking the compatibility of the measurement results obtained in two laboratories is carried out in accordance with GOST ISO 5725-6 (clause 5.3). The results are considered compatible under the condition
where is the maximum value of two COD measurement results obtained in two laboratories according to 9.2 or 9.3, mgO/dm;
- the minimum value of two COD measurement results obtained in two laboratories according to 9.2 or 9.3, mgO/dm;
- arithmetic mean of the measurement results obtained in two laboratories, mgO/dm;
- relative value of the reproducibility limit according to table 1, %.
If condition (5) is not met, to check the precision under reproducibility conditions, each laboratory must perform the procedures in accordance with GOST ISO 5725-6 (clauses 5.2.2; 5.3.2.2).
APPENDIX A (mandatory). Safety requirements
APPENDIX A
(mandatory)
A.1 The method of this International Standard uses hot, concentrated solutions of sulfuric acid and potassium dichromate. Personnel must be trained in acid safety and wear protective clothing and insulated gloves. A protective screen is installed in front of the heating block.
A.2 Sample preparation may release toxic gases (hydrogen sulfide, hydrogen cyanide). All operations must be carried out in a fume hood.
A.3 Reaction vessel contents include toxic mercury(II) and silver sulfates and potassium dichromate. Disposal of the contents of the reaction vessels is carried out in compliance with the rules for handling toxic waste.
A.4 Reaction vessels that have completely consumed potassium dichromate may contain mercury vapor. Such vessels should only be opened in a fume hood.
A.5 Capped reaction vessels develop pressure during heating and should therefore be carefully inspected before use. In order to avoid explosions, vessels with cracks, chips and other defects are not used.
A.6 Until the contents of the reaction vessels have completely cooled to room temperature, it is forbidden to unscrew the lids of the vessels in order to avoid ejection of the contents.
Bibliography
UDC 663.6:006.354 MKS 13.060.50 TN VED 220100000 NEQ
Key words: water, water quality, chemical oxygen demand, bichromate oxidizability, photometric method
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M.: Standartinform, 2014
Theoretical value of chemical oxygen demand. Practiced methods for determining COD. Disadvantages of permanganate oxidation. Bichromatic arbitration method. Application of the photometric method at low concentrations of organic substances.
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Xchemical oxygen demand(COD)
Theoretical COD
The theoretical value of COD is the amount of oxygen (or oxidizing agent in terms of oxygen) in mg / l, necessary for the complete oxidation of organic substances contained in the sample, at which carbon, hydrogen, sulfur, phosphorus and other elements (except nitrogen), if they are present in organic matter, are oxidized to CO 2, H 2 O, P 2 0 5, S0 3, and nitrogen is converted into ammonium salt. At the same time, oxygen included in the composition of oxidizable organic substances, participates in the oxidation process; and the hydrogen of these compounds donates 3 atoms for each nitrogen atom in the formation of the ammonium salt.
Practically applied methods of determination COD give results very close to COD theor, but they may deviate somewhat from one side or the other. Thus, the method in which the loss of oxygen is determined during the combustion of a dried sample in a flow of oxygen leads to the formation of nitric oxide, and the resulting COD value is somewhat higher COD theor. In the dry combustion method, in which carbon is converted to CO and the latter is determined by IR spectrometry, nitrogen in the free state is released, and the resulting COD value will also be somewhat higher COD theor. If the oxidation of organic substances has not passed completely, then the result, of course, will be underestimated. In addition, for any method of determining COD along with organic substances, inorganic reducing agents are also oxidized, if they were in the sample. The content of inorganic reducing agents in the sample is then determined separately special methods and the results of these determinations are subtracted from the found COD value.
For the determination of COD, there are "dry" methods, in which the organic matter of the sample is burned in a stream of oxygen or CO 2 . These methods have already been mentioned, they lead to results close to theoretical, but necessary devices, not yet produced by our industry. Good results are also obtained by the method in which organic substances are oxidized with ammonium persulfate. This is the "wet" method. The results are slightly increased due to nitrogen oxidation for nitrate ions.
The previously used permanganate oxidation method is completely unsuitable for the analysis of wastewater (it is still used in the analysis of natural waters). Permanganate is not a strong enough oxidizing agent: the oxidation of organic substances is incomplete and many of them are not oxidized at all. In addition, when boiling solutions containing an excess of permanganate, the latter decomposes to a large extent with the formation of manganese dioxide and oxygen. This decomposition occurs in both acidic and alkaline environments. The precipitated manganese dioxide catalytically accelerates the process. The amount of precipitate formed varies depending on the conditions and composition of the sample. Correction and a blank experiment is not possible here, since during a blank determination, the manganese dioxide precipitate usually completely precipitates.
Bichromate arbitration method for determining COD
It is possible to analyze the pre-filtered sample and the whole sample together with the sediment present in it (depending on the goal). If the analysis of the sample should show the effectiveness of the applied method of cleaning wastewater from organic substances (the completeness of the subsequent clarification of water in the sump should not be taken into account), then the sample must be filtered by analysis. On the other hand, if the treated waste water that has passed through the sump is analyzed immediately before it is discharged into the reservoir, then it often becomes necessary to analyze the water along with the particles of sediment remaining in it. In the latter case, the wastewater sample must be homogenized. When filtering the sample through a paper filter, the possible influence of the filter paper must be avoided. The filter must be pre-washed hot water and, while filtering, discard the first portion (200-250 ml) of the filtrate. However, it is not possible to filter wastewater containing substances that can escape during filtration or be oxidized by atmospheric oxygen. In such cases, filtration is replaced by prolonged settling of waste water and the upper transparent layer is taken for analysis with a pipette.
The essence of the method. Organic substances are oxidized with potassium dichromate in 18 N. (dilution 1:1) sulfuric acid. The bichromate is then reduced according to the equation
The oxidation of organic substances under these conditions is accelerated and covers almost all organic substances if a catalyst, silver sulfate, is introduced into the reaction mixture. Oxidation reactions of individual elements of organic substances proceed in accordance with what we have indicated above when formulating the concept of CODtheor, but the result obtained is 95-- 98% of CODtheor, (with a few exceptions). The loss (2-5%) is mainly due to the formation of volatile, oxidation-resistant decomposition products (CO, CH 4). It is possible, on the other hand, that some nitrogen-containing organic substances form N 2 instead of NH 3 during oxidation, which leads to an error with the opposite sign.
Pyridine and its homologs, pyrrole, pyrrolidine, proline, nicotinic acid and some other nitrogen-containing heterocyclic compounds, benzene, toluene and other aromatic hydrocarbons, paraffin, naphthalene are not oxidized.
If the analyzed sample contains inorganic reducing agents, then their amount, determined separately by appropriate methods, must be subtracted (in terms of oxygen) from the result of the COD determination.
However, it should be taken into account that H 2 S from sulfides and S0 2 from sulfites, hydrosulfites, etc. volatilize when determining COD (you only need to pour sulfuric acid into the flask before bichromate) and, therefore, a correction for their presence should not be introduced.
interfering substances. The interfering effect of chlorides (which are oxidized to elemental chlorine during the determination) is eliminated by masking them with mercury(II) sulfate in an amount of 22.2 mg HgSO 4 per mg CI - . The resulting very little dissociated mercury (II) chloride is sufficiently stable even in the presence of a high concentration of sulfuric acid and dichromate.
If there is confidence in the absence of organic substances for the oxidation of which a silver sulfate catalyst is required, then this determination can be carried out without a catalyst and without mercury. Chloride ions are then quantitatively oxidized to free chlorine, and a correction will have to be subtracted from the obtained result of the determination: 0.23 mg of oxygen is consumed per 1 mg of chloride ions.
Nitrites interfere with the determination (often present in wastewater that has undergone biochemical treatment). To eliminate them, 10 mg of sulfamic acid per 3 mg of NC are introduced into the flask. When the solution is boiled, nitrite ions are removed in the form of nitrogen, and an excess of sulfamic acid passes into ammonium sulfate.
Reagents
Sulfuric acid sq. 1.84 g/cm 3 hours. silver sulfate hard chda.
N- Phenyl anthranilic acid, 0.25 g of acid is dissolved in 12 ml of 0.1 sodium hydroxide solution and diluted with water to 250 ml.
ferroin, Dissolve 1.485 g of 1,10-phenanthroline and 0.695 g of FeSO 4 -7H 2 0 in water and dilute the solution with water to 100 ml.
potassium bichromate, 0.25 n. standard solution. 12.258 g of potassium bichromate, previously dried for 2 hours at 105 ° C, are dissolved in distilled water and the solution is diluted with water to 1 liter.
Mora Salt, approximately 0.25 n. solution. Dissolve 98 g of salt M in distilled water, add 20 ml of concentrated sour sulfuric acid, and dilute the solution with distilled water to 1 liter.
The titer of this solution is set according to the standard solution of potassium bichromate. After taking 25 ml of a standard solution of potassium bichromate, dilute with distilled water to 250 ml, add 20 ml of concentrated sulfuric acid and allow to cool. Then add 3-4 drops of ferroic solution or 5-10 drops of N-phenylanthranilic acid solution and titrate Mohr's salt solution.
mercury sulfate(I) crystalline chda.
Definition progress. Such a volume of analyzed standing water is taken so that no more than 20 ml of a standard solution of potassium dichromate is consumed for oxidation and that it contains no more than 40 mg of chloride ions, diluted to 50 ml with distilled water and transferred to a 300 ml round-bottomed flask. 1 g of mercury(II) sulfate is added, 5 ml of sulfuric acid is stirred until mercury sulfate dissolves, then 25.0 ml of a standard solution of potassium dichromate is poured in, 70 ml of sulfuric acid are poured in very small portions, pouring 0.4--0.5 g silver sulfate, several glass beads or pieces of pumice are introduced into the flask, closed with a stopper connected to a reflux condenser, and heated to a slight boil, which is maintained for 2 hours. conical flask with a capacity of 500 ml, washing the walls of the first flask several times with distilled water. After adding distilled water to a volume of 350 ml, 3-4 drops of a solution of ferroin (10-15 drops of a solution of N-phenylatranilic acid) are introduced and the excess of dichromate is titrated with a titrated solution of Mohr's salt.
Carry out a blank experiment; To do this, take 50 ml of distilled water and carry it through all stages of analysis.
Calculation. Chemical oxygen uptake (COD), expressed as the number of milligrams of oxygen per 1 liter of wastewater, is calculated by the formula
where a- the volume of "Mohr's salt solution used for titration in a blank experiment: ml; b-- the volume of the same solution used for titration of the sample, ml; N -- normality of the titrated solution of Mohr's salt; V-- volume of analyzed waste water, m; 8 is the equivalent of oxygen.
In the presence of sulfides (as well as mercaptans, organic sulfides and disulfides), when mercury (II) sulfate is added, a black precipitate of mercury sulfide precipitates, which does not dissolve during further processing. In these cases, it is recommended to slightly change the order of addition of reagents compared to that described above.
Definition progress. To 50 ml of a sample (or a smaller volume diluted with distilled water to 50 ml), first add 25.0 ml of a titrated dichromate solution, then pour 5 ml of concentrated sulfuric acid and allow to stand for 10-20 minutes at room temperature to oxidize easily oxidizing substances, including sulfur compounds. Then 1 g of mercury(II) sulfate is added, 70 ml of concentrated sulfuric acid, 0.5 g of silver sulfate are added and continue as described above.
Accelerated method for determining COD
This method is intended for continuous daily analyzes carried out to monitor the operation of a treatment plant or the condition of water in a reservoir. The results of the determination, as a rule, are somewhat lower than in the analysis of the arbitration method, but they are usually quite reproducible. It is recommended to periodically carry out determinations by both methods, accelerated and arbitration, in order to find an approximate conversion factor. It should be taken into account that the discrepancies between the results of both methods are especially large when the sample contains substances that are difficult to oxidize, such as acetic acid, alanine, benzene, etc.
The main feature of the accelerated method is the increased concentration of H2SO4. Heating from the outside is not required, the temperature rises due to the heat released when water is mixed with concentrated sulfuric acid.
Definition progress. If the COD of the analyzed water is within 500--400 mg/l of oxygen, 1 ml of the sample is taken for analysis, if the COD is 50--500 mg/l, 5 ml of the sample is taken, if the COD is above 4000 mg/l, the sample is preliminarily diluted if COD is below 60 mg/l, this method cannot be used.
Introduce 2.& ml of 0.25 n. potassium dichromate solution, then 0.2 g of mercury (II) sulfate and, with stirring, concentrated sulfuric acid (7.5 ml per 1 ml of sample, 15 ml per 5 ml of sample). In this case, the temperature of the solution rises above 100 ° C. After 2 minutes, cool the solution to room temperature, add 100 ml of distilled water and titrate the excess of bichromate, as in the arbitration method.
Photometric method for determining COD at low concentrations of organic substances
The above arbitration method does not give reproducible results when analyzing waters containing small amounts of organic matter, such as wastewater that has passed through treatment facilities, and many natural waters. The reason is that when ordinary amounts of dichromate are added to the sample, almost all of the introduced dichromate has to be titrated back, and when small amounts of dichromate are added to the sample, the reactions of oxidation of organic substances by it are very slow and incomplete.
We encounter essentially the same difficulty when applying the photometric methods proposed by some authors. With an unavoidable large excess of dichromate ions in solution, with the accuracy not required, neither the weakening of the color of dichromate ions resulting from the reactions of these ions with organic substances of the sample, nor the concentration of green chromium (III) ions formed against the background of a high concentration of Cr 2 07 ions will be determined photometrically. ~.
In the proposed method, the amount of dichromate introduced is the same as in the arbitration method, but after the completion of the reaction, the formed chromium(III) ions are separated from the excess dichromate (see Sections 6.25) and then chromium(III) is determined photometrically by its reactions with EDTA.
The method allows to determine COD from 2 to 100 mg/l.
Determination when the content of chloride ions is below 25 mg/l
Reagents
Potassium dichromate -- see Sec. 5.6.2. Silver sulfate -- see sec. 5.6.2.
Caustic soda, 45% solution. Dissolve chemically pure reagent in double-distilled and boiled water.
Sulfuric acid, 2 N solution.
Magnesium oxide, chemically pure. The sales powdered reagent is calcined for about 1 hour in a muffle furnace, stored in a jar equipped with a ground stopper. Universal indicator paper or Congo paper. Ammonium chloride EDTA, 10% water solution. Rochelle salt, 20% solution.
Ammonia, commercial 25% solution, diluted (1: 1). Acetic acid, 2 N solution. Phenolphthalein, 1% alcohol solution.
Distilled water, double distilled (added with H2SO4 - H KMpO) in a glass device on thin sections.
Chrome alum. Basic standard solution. Weigh 4.8024 g of unweathered crystals of chromic alum KCr(SO 4 )2-12H 2 0 analytically, dissolve in distilled water and dilute to 500 ml; 1 ml of the resulting solution contains 1 mg of chromium.
Working standard solution. In a volumetric flask with a capacity of 1 l, using a burette, pour 216.7 ml of the basic standard solution of chromium alum and dilute with distilled water to the mark; 1 ml of the resulting solution contains 0.2167 mg of chromium, which corresponds to 0.1 mg of oxygen in the process of determining COD.
Calibration chart. In volumetric flasks with a capacity of 100 ml pour 0.5; 1, 2, ... 20 ml of a working standard solution of chromic alum, which corresponds to an oxygen concentration of 2; 4; 8, ... 80 mg/l, dilute each solution to 25 ml with distilled water, add ammonium chloride, Rochelle's salt, EDTA solution, etc. and continue as in the analysis of the sample. After staining and diluting each solution to the mark, the optical density is measured at - 536 nm and a layer thickness of 5 cm and a calibration graph is built.
Definition progress. Place 25 ml of analyzed water into the flask of the device for determining COD, add 10 ml of potassium dichromate solution, 35 ml of concentrated sulfuric acid and pour 0.1 g of silver sulfate. Then several glass beads are lowered into the flask and, having connected it to a reflux condenser, boil for 2 hours. At the same time, a blank experiment is carried out, taking 25 ml of doubly distilled water for it.
After cooling the solution, it is transferred to a volumetric flask with a capacity of 200 ml, washing the walls of the original flask with double-distilled water, diluted with the same water to the mark and mixed.
Having taken 100 ml of the resulting solution, transfer it to a beaker with a capacity of 400-450 ml, dilute with distilled water to 300 ml and carefully neutralize. First, about 30 ml of sodium hydroxide solution is poured, mixed, then NaOH solution is added dropwise to pH = 5--7. The pH value is determined by indicator paper, touching it with a glass rod moistened with the analyzed solution.
The neutralized solution is heated to boiling; 0.1 g of magnesium oxide is added to it and boiled for 20 minutes at a low boil. The precipitate is allowed to collect at the bottom of the beaker, then the solution is filtered through a dense filter, previously washed with hot water, trying not to stir up the precipitate. The precipitate is transferred to a filter and washed with hot water until a colorless filtrate is obtained. A funnel with a precipitate is placed on a small conical flask in the filter, a hole is made and the precipitate is washed through it with hot water into the flask. The filter is then treated with 3 ml of 2N. sulfuric kitty? lots, after washing the walls of the glass with it to dissolve the adhering traces of sediment. The filter and beaker are washed with hot water, collecting the washings in the same flask, and the contents of the flask are boiled until the precipitate dissolves.
The resulting solution is transferred into a volumetric flask with a capacity of 100 ml, filtering it, if necessary, through a dense filter. Add 3 g of ammonium chloride, 2 ml of Rochelle's salt solution (to bind iron, if it is present in the complex), 2 ml of EDTA solution, 2-3 drops of phenolphthalein; ammonia solution to a slightly pink color and 5 ml of acetic acid (pH of the resulting solution should be close to 4). The contents of the volumetric flask are heated and boiled for 5 minutes, cooled and diluted with distilled water to the mark.
The optical density of the obtained color solution is determined at l = 536 nm and the thickness of the liquid layer in the cuvette 5 relative to the blank solution.
Calculation. The CP value in mg/l found from the calibration curve is multiplied by 2, since during the analysis half of the volume of the solution obtained after oxidation with dichromate was taken.
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