1、BRITISH STANDARD BS 6068-2.34: 1988 Water quality Part 2: Physical, chemical and biochemical methods Section 2.34 Method for the determination of the chemical oxygen demand UDC 556.11 + 614.777 + 628.1/.3 + 663.63 53/54BS6068-2.34:1988 This British Standard, having been prepared under the directiono
2、f the Environment andPollutionStandards Committee, was published underthe authorityof the BoardofBSI andcomes into effect on 31October1988 BSI 02-1999 The following BSI references relate to the work on this standard: Committee reference EPC/44 Draft for comment 84/50067 DC ISBN 0 580 16696 1 Forewor
3、d This Section of BS 6068, which has been prepared under the direction of the Environment and Pollution Standards Committee, is related to ISO 6060-1986 “Water quality Determination of the chemical oxygen demand”. The international standard was prepared by sub-committee 2, Physical, chemical and bio
4、chemical methods, of Technical Committee 147, Water quality, of the International Organization for Standardization (ISO) with the active participation of the UK. This Section of BS 6068 differs from ISO 6060 principally in the use of single reagents in place of mixed reagents, which are unacceptable
5、 to the UK on technical and safety grounds. This has necessitated changes to clause 4 and7.4. In addition the maximum recommended concentration of chloride in the sample or diluted sample is 1000 mg/L, not 2000 mg/L as given in the international standard (see clause 1), and relevant reproducibility
6、data are given in 8.2. This British Standard is being published in a series of Parts subdivided into Sections that will generally correspond to particular international standards. Sections are being, or will be, published in Parts 1 to 6, which, together with Part 0, are as follows. Part 0: Introduc
7、tion; Part 1: Glossary; Part 2: Physical, chemical and biochemical methods; Part 3: Radiological methods; Part 4: Microbiological methods; Part 5: Biological methods; Part 6: Sampling. The tests described in this British Standard should only be carried out in laboratories with suitable facilities an
8、d by suitably qualified persons with an appropriate level of chemical expertise and knowledge of the necessary safety precautions. Standard chemical procedures should be followed throughout. A British Standard does not purport to include all the necessary provisions of a contract. Users of British S
9、tandards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 4, an inside back cover and a back cover. Th
10、is standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Amendments issued since publication Amd.No Date of issue CommentsBS6068-2.34:1988 BSI 02-1999 i Contents Page Foreword Inside front cov
11、er 0 Introduction 1 1 Scope 1 2 Definition 1 3 Principle 1 4 Reagents and materials 1 5 Apparatus 2 6 Sampling and samples 2 7 Procedure 2 8 Expression of results 3 9 Interferences 3 10 Test report 4 Publications referred to Inside back coverii blankBS6068-2.34:1988 BSI 02-1999 1 0 Introduction The
12、chemical oxygen demand, COD, of water as determined by this dichromate method can be considered as an approximate measure of the theoretical oxygen demand, i.e. the amount of oxygen consumed in total chemical oxidation of the organic constituents to inorganic end products (seealso clause 9). The deg
13、ree to which the test results approach the theoretical value depends primarily on how complete the oxidation is. A great number of organic compounds are oxidized to an extent of between 90 % and 100 %, and for waters where these compounds predominate, such as municipal effluents, the COD value is a
14、realistic measure of the theoretical oxygen demand. For other waters which contain large quantities of certain substances that are difficult to oxidize under the conditions of the test (see clause 9), the COD value is a poor measure of the theoretical oxygen demand. This may be the case for some ind
15、ustrial effluents. The significance of a COD value thus depends on the composition of the water studied. This should be borne in mind when judging results obtained by the method described in this Section of BS 6068. 1 Scope This Section of BS 6068 describes a method for the determination of the chem
16、ical oxygen demand, COD, of water. It is applicable to most kinds of waters having a COD value higher than 30 mg/L (see also the note in clause 9). The maximum COD value that can be determined on an undiluted sample is 700 mg/L. It is not applicable to highly saline waters containing (after dilution
17、) more than 1000 mg/L of chloride, such as sea and oilfield waters. NOTE 1For interferences, see clause 9. NOTE 2The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definition For the purposes of this Section of BS 6068 the following definition applies.
18、 chemical oxygen demand (COD) the mass concentration of oxygen equivalent to the amount of dichromate consumed by dissolved and suspended matter when a water sample is treated with that oxidant under defined conditions 3 Principle A test portion is refluxed in the presence of mercury(II) sulphate wi
19、th a known amount of potassium dichromate and silver catalyst in strong sulphuric acid for a fixed period of time, during which part of the dichromate is reduced by the oxidizable material present. The remainder of the dichromate is titrated with ammonium iron (II) sulphate. The COD value is calcula
20、ted from the amount of dichromate reduced. 1 mol of dichromate (Cr 2O 7 2 ) is equivalent to1.5 mol of oxygen (O 2 ). 4 Reagents and materials WARNING. This method involves the handling and boiling of strong solutions of sulphuric acid and dichromate. Protective clothing, gloves and full face protec
21、tion are necessary. In the event of spillage immediate washing with a copious volume of clean water is the simplest and most effective remedy. It is essential that addition of concentrated sulphuric acid to water should always be carried out with care and with gentle swirling of the contents of the
22、flask. Care is required when preparing and handling solutions containing silver sulphate and mercury(II) sulphate as these substances are toxic. Used reagents contain mercury, silver and chromium salts. Mercury salts discharged into receiving streams may be converted to very toxic methyl mercury com
23、pounds by bacterial action. Attention is drawn to national and local regulations for the handling and treating of reagents before disposal. (See also BS 6070-2.) 4.1 General During the analysis, use only reagents of recognized analytical grade and only distilled water or water of equivalent purity.
24、The quality of the water is of great importance for the precision of the results. Check the quality of the water by running blanks (described in 7.2 and 7.4) and similar parallel tests without any boiling, but otherwise exactly as stated. Note the consumption of iron (II) ammonium sulphate solution
25、(4.4) in both cases. A difference of more than 0.5 mL indicates poor water quality. For determination of COD values below 100 mg/L the difference shall not exceed 0.2 mL. The quality of distilled water can often be improved by redistilling it from an acidified solution of potassium dichromate or pot
26、assium permanganate, using all-glass distillation equipment.BS 6068-2.34:1988 2 BSI 02-1999 4.2 Sulphuric acid, c (H 2 SO 4 ) = 4 mol/L. Add to about 500 mL of water, 220 mL of sulphuric acid (r = 1.84 g/mL) in portions and with caution. Allow to cool and dilute to 1000mL. 4.3 Silver sulphate-sulphu
27、ric acid. Add 10 g of silver sulphate (Ag 2 SO 4 ) to 1 L of sulphuric acid (r = 1.84 g/mL). Allow 1 or 2 days for dissolution. The dissolution is enhanced by stirring. 4.4 Iron (II) ammonium sulphate, standard volumetric solution, c (NH 4 ) 2 Fe(SO 4 ) 2 .6H 2 O 0.12 mol/L. Prepared as follows. a)
28、Preparation. Dissolve 47.0 g of iron (II) ammonium sulphate hexahydrate in water. Add20 mL of sulphuric acid (r = 1.84 g/mL). Cool and dilute with water to 1000 mL. b) Standardization. Standardize the solution daily before use as follows. Dilute 10.0 mL of potassium dichromate solution(4.5) to about
29、 100 mL with 4 mol/L sulphuric acid (4.2). Titrate this solution with the iron (II) ammonium sulphate solution 4.4 a) to be standardized, using 2 or 3 drops of ferroin (4.7) as indicator. c) Calculation of concentration. The concentration, c, expressed in moles per litre, of the iron (II) ammonium s
30、ulphate solution is given by the formula where V is the volume, in millilitres, of iron (II) ammonium sulphate solution consumed. 4.5 Potassium dichromate, standard reference solution, c (K 2 Cr 2 O 7 ) = 0.040 mol/L. Dissolve 11.768 g of potassium dichromate, dried at105 C for 2 h, in water and dil
31、ute to 1000mL. The solution is stable for at least 1 month. 4.6 Potassium hydrogen phthalate, standard reference solution, c (KC 8 H 5 O 4 ) = 2.082 4 mmol/L. Dissolve 0.425 1 g of potassium hydrogen phthalate, dried at 105 C, in water and dilute to 1000mL. The solution has a theoretical COD value o
32、f 500 mg/L. This solution is stable for at least 1 week if stored at4 C. 4.7 Ferroin, indicator solution. Dissolve 0.7 g of iron (II) sulphate heptahydrate (FeSO 4 .7H 2 O) in water. Add 1.50 g of 1,10-phenanthroline monohydrate, and shake until dissolved. Dilute to 100 mL. This solution is commerci
33、ally available. 4.8 Anti-bumping granules. Clean the granules as described in 5.2. 4.9 Mercury (II) sulphate (HgSO 4 ), crystals. 5 Apparatus 5.1 Ordinary laboratory equipment. The glassware used shall be scrupulously clean and shall be protected from dust. NOTEThis is particularly important in the
34、case of low COD values. 5.2 Reflux apparatus, consisting of a 250mL flask or tube with ground glass neck connected to a condenser so that there may be no significant loss of volatile material. Clean the apparatus and anti-bumping granules(4.8) by repeatedly refluxing with fresh mixtures of5 m L of p
35、otassium dichromate solution(4.5), 15 mL of silver sulphate-sulphuric acid (4.3) and 10 mL of water until constant blank values (7.2) are obtained. Reserve this apparatus solely for use in the COD test. 5.3 Heating mantle, hotplate or other heating device, capable of bringing the sample to boiling w
36、ithin10min. Ensure that the device works without causing local over-heating to solutions being heated. 5.4 Burette, of capacity 10 mL, graduated in divisions of 0.02 mL, and complying with class A of BS 846. 6 Sampling and samples Laboratory samples shall be collected in glass or polyethylene bottle
37、s, although glass bottles are preferred. Analyse the samples as soon as possible and not later than 5 days after sampling. If the samples have to be stored prior to analysis, add10mL of sulphuric acid (4.2) per litre of sample. Keep them at 0 C to 5 C. Shake the storage bottles and make sure that th
38、eir contents are well homogenized when withdrawing a test portion for analysis. 7 Procedure 7.1 Test portion Place 10.0 mL of the laboratory sample (clause 6) into the flask (5.2). If the COD value of the sample is expected to exceed 700 mg/L make a dilution of the original sample with water so that
39、 a COD value of between 350 mg/L and 700 mg/L is obtained. 7.2 Blank test Carry out a blank test in parallel with the determination, by the same procedure but replacing the test portion by 10 mL of water. 10.00.040 6 V - 2.4 V - =BS6068-2.34:1988 BSI 02-1999 3 7.3 Check test Check the technique and
40、the purity of the reagents by analysing 10.0 mL of the standard solution (4.6) by the same procedure as given for the test portion. The theoretical oxygen demand of this solution is500 mg/L; the experimental procedure is satisfactory if the result of the check test is at least96 % of this value. 7.4
41、 Determination Add 0.4 g of mercury (II) sulphate (4.9) to the test portion (7.1). Swirl thoroughly and add 5.0 mL of potassium dichromate solution (4.5) and a few anti-bumping granules (4.8). Mix well. Add slowly 15 mL of silver sulphate-sulphuric acid(4.3), while swirling the flask under cold runn
42、ing water or in an ice-bath to prevent loss of volatile organic matter. (Without adequate mixing, local heating may occur at the bottom of the flask, and the mixture may be blown out of the flask.) Attach the flask to the condenser, and reflux the mixture for 2 h. Allow to cool and wash down any mat
43、erial on the inside of the condenser into the flask with a small volume of water. Remove the flask and dilute the mixture to about 75 mL with water and cool to room temperature. Using the burette (5.4) titrate the excess dichromate with iron (II) ammonium sulphate solution (4.4) using one or two dro
44、ps of ferroin (4.7) as indicator. NOTE 1The reaction mixture has to boil gently without any bumping. Bumping indicates local overheating of the solution, which may lead to false results. Bumping may be caused by intense heating or by inefficient anti-bumping granules. NOTE 2The consumption of iron (
45、II) ammonium sulphate will be 10 mL or less, provided it has not deteriorated. NOTE 3Although the quantity of ferroin added is not critical, it should be kept as constant as possible. Take as the end-point the first sharp colour change from blue-green to reddish brown, even though the blue-green col
46、our may reappear after some minutes. 8 Expression of results 8.1 Calculation The chemical oxygen demand, COD, expressed in milligrams of oxygen per litre, is given by the formula where Give the result to the nearest milligram per litre and report values below 30 mg/L as “ 30 mg/L”. 8.2 Reproducibili
47、ty Data obtained, using a very similar procedure 1) , from 10 different laboratories analysing the same trade waste with a COD value of about 300 mg/L gave within-batch standard deviations for the individual laboratories from 3.5 mg/L to 9.8 mg/L. For synthetic potassium hydrogen phthalate solutions
48、 of COD value 64 mg/L and 400 mg/L, standard deviations from 1.7 mg/L to 6.6 mg/L and1.4 mg/L to 6.2 mg/L respectively were found for the same laboratories. Data obtained, using another very similar procedure 2) , from 74 laboratories analysing a synthetic potassium hydrogen phthalate solution of CO
49、D value 200 mg/L gave an inter-laboratory standard deviation of 13 mg/L. The method in this standard would be expected to exhibit results of a similar order of reproducibility to those quoted above. 9 Interferences The test is sensitive to some interferences, principally chlorides. Inorganic reducing agents, such as nitrites, sulphides, and iron (II), will increase the result. It is acceptable practice to include the oxygen demand from such agents as part of the overall COD value of the sample.
