BS 7755-3 12-1996 Soil quality Chemical methods Determination of the potential cation exchange capacity and exchangeable cations using barium chloride solution buffered at pH = 8 1.pdf

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1、BRITISH STANDARD BS 7755-3.12: 1996 ISO 13536: 1995 Soil quality Part 3: Chemical methods Section 3.12. Determination of the potential cation exchange capacity and exchangeable cations using barium chloride solution buffered at pH = 8.1 ICS 13.080BS 7755-3.12:1996 This British Standard, having been

2、prepared under the directionof the Health and Environment Sector Board, was published under the authority of the Standards Board and comes into effect on 15 February 1996 BSI 07-1999 The following BSI references relate to the work on this standard: Committee reference EH/4/3 Draft for comment 94/501

3、662 DC ISBN 0 580 25334 1 Committees responsible for this British Standard The preparation of this British Standard was entrusted by Technical Committee EH/4, Soil quality, to Subcommittee EH/4/3, Chemical methods, upon which the following bodies were represented: Association of Consulting Scientist

4、s Association of Public Analysts British Ceramic Research Ltd. British Gas plc British Society of Soil Science Department of the Environment (Drinking Water Inspectorate) Department of Trade and Industry (Laboratory of the Government Chemist) National Rivers Authority Royal Society of Chemistry Soci

5、ety of Chemical Industry Soil Survey and Land Research Centre Water Research Centre Water Services Association of England and Wales Amendments issued since publication Amd. No. Date CommentsBS 7755-3.12:1996 BSI 07-1999 i Contents Page Committees responsible Inside front cover National foreword ii 1

6、 Scope 1 2 Normative references 1 3 Principle 1 4 Procedures 1 5 Test report 5 Annex A (informative) Bibliography 6 List of references Inside back coverBS 7755-3.12:1996 ii BSI 07-1999 National foreword This Section of BS7755 has been prepared by Subcommittee EH/4/3 and is identical with ISO13536:19

7、95 Soil quality Determination of the potential cation exchange capacity and exchangeable cations using barium chloride solution buffered atpH=8,1, published by the International Organization for Standardization (ISO). ISO 13536 was prepared by Subcommittee3, Chemical methods, of Technical Committee

8、ISO/TC190, Soil quality, with the active participation and approval of the UK. BS 7755 is being published in a series of Parts subdivided into Sections and Subsections that will generally correspond to particular international standards. Other Parts of BS7755 are, or will be, as follows. Part 1: Ter

9、minology and classification; Part 2: Sampling; Part 3: Chemical methods; Part 4: Biological methods; Part 5: Physical methods; Textual error. When adopting the text of the international standard, the following textual error was discovered. It has been marked in the text and has been reported to ISO

10、in a proposal to amend the text of the international standard. In4.2.5, the correct presentation of the second formula is: Additional information Although the use of ICP-AES is mentioned inclause3, no instructions are given in the procedural clauses for this method of analysis. In4.1.2, typical stor

11、age conditions are approximately 4 C for a maximum of24h. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from le

12、gal obligations. Cross-references International standard Corresponding British Standard ISO 3696:1987 BS EN ISO 3696:1995 Water for analytical laboratory use Specification and test methods (Identical) ISO 11464:1994 BS 7755 Soil quality Part: 3 Chemical methods Section 3.5:1995 Pretreatment of sampl

13、es for physico-chemical analyses (Identical) CEC 3 000c 2 c b1 () m - = Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments i

14、ncorporated. This will be indicated in the amendment table on the inside front cover.BS 7755-3.12:1996 BSI 07-1999 1 WARNING Barium is a very toxic element if ingested. The barium ion has health risks for laboratory personnel working with this chemical; it is also harmful for the environment. 1 Scop

15、e This International Standard specifies a method for the determination of the potential cation exchange capacity (CEC) of soil buffered at pH=8,1 and of the determination of the content of exchangeable sodium, potassium, calcium and magnesium in soil. This International Standard is applicable to all

16、 types of air-dried soil samples. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and p

17、arties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 3696:1987, Water for analytical lab

18、oratory use Specification and test methods. ISO 11464:1994, Soil quality Pretreatment of samples for physico-chemical analyses. 3 Principle The determination of CEC as specified in this International Standard is a modification of the method proposed by Mehlich1, 2 and modified by Bascomb3. The CEC o

19、f soil samples is determined in barium chloride solution buffered at pH=8,1 using triethanolamine. The soil is first saturated with respect to barium by treating the soil three times with buffered barium chloride solution. Subsequently, a known excess of0,02mol/l magnesium sulfate solution is added.

20、 All the barium present, in solution as well as adsorbed, is precipitated in the form of highly insoluble barium sulfate and the sites with exchangeable ions are then readily occupied by magnesium. The excess magnesium is determined by either flame atomic absorption spectrometry (FAAS) or inductivel

21、y coupled plasma atomic emission spectrometry (ICP-AES). It is also possible to determine sodium, potassium, calcium and magnesium (and other elements) in the barium chloride extract of the soil. NOTE 1Besides sodium, potassium, calcium and magnesium, it is also possible to determine other cations,

22、for example manganese, in the same extract. NOTE 2A yellowish-brown colour of the barium chloride extract indicates that some organic matter has been dissolved. Since organic matter contributes to the CEC, the measured value will be an underestimate of the potential CEC. NOTE 3The sum of exchangeabl

23、e cations may give a result that is greater than the CEC due to the dissolution of salts present in the soil. Preliminary washing of the soil with water to remove these salts should not be employed because it could change the relative proportions of cations in the CEC. 4 Procedures 4.1 Leaching 4.1.

24、1 Reagents Use only reagents of recognized analytical grade and distilled water or deionized water for all solutions. 4.1.1.1 Water, with a specific conductivity not higher than0,2mS/m at25 C (water conforming to grade2of ISO3696). 4.1.1.2 Barium chloride solution, c(BaCl 2 =1 mol/l. Dissolve 244 g

25、of barium chloride dihydrate (BaCl 2 .2H 2 O) in water and dilute to1000ml with water. 4.1.1.3 Hydrochloric acid, c(HCl) = 2mol/l. Dilute166ml of concentrated hydrochloric acid ( =1,19g/ml) with water to1000ml. 4.1.1.4 Triethanolamine solution, pH = 8,1. Dilute90ml of triethanolamine (HOCH 2 CH 2 )

26、3 N with water to about1litre and adjust thepHto8,1 0,02 with about140ml to150ml of hydrochloric acid(4.1.1.3). Dilute with water to2litres. 4.1.1.5 Extraction solution Mix equal volumes of solutions4.1.1.2 and4.1.1.4. Protect this solution from exposure to carbon dioxide during storage. 4.1.1.6 Mag

27、nesium sulfate solution, c(MgSO 4 )=0,0200mol/l. Dissolve4,930g 0,01g of magnesium sulfate heptahydrate (MgSO 4 .7H 2 O, see note4) in water and dilute to1000ml. NOTE 4Magnesium sulfate heptahydrate may lose water of crystallization on standing. The reagent should be standardized by titration with e

28、thylenediaminetetraacetic acid (EDTA) atpH=10 using Eriochrome Black T as the indicator. Alternatively the reagent may be kept in a sealed polyethylene bag placed in a refrigerator.BS 7755-3.12:1996 2 BSI 07-1999 4.1.2 Procedure Transfer 2,50g to5,00g of air-dried soil with a high content of clay an

29、d/or humus, or10g of sandy soil or soil with a low content of humus (particlesizek2mm), pretreated according to ISO11464, into a tightly stoppered polyethylene centrifuge tube of about50ml. Note the mass of the tube with cover and soil (m 1 ). Add30ml of extraction solution(4.1.1.5) to the soil and

30、shake for1h. Balance the tubes and centrifuge at3000g for10min. Transfer the supernatant liquid to a100ml volumetric flask. Repeat the addition of extraction solution, shaking and centrifuging twice and adding the supernatant liquid to the100ml volumetric flask on each occasion. Dilute to the volume

31、 of the volumetric flask with extraction solution (4.1.1.5). Mix, filter through a coarse filter paper (diameter7cm) and store the filtrate A for the determination of sodium, potassium, calcium and magnesium following the procedure described in4.3 and4.4. Add approximately40ml of water(4.1.1.1) to t

32、he soil cake and shake manually for1min to2min to break up the soil cake. Balance the tubes and centrifuge at about3000 g for10min. Decant the supernatant to waste. Weigh the tube with its contents and cover (m 2 ). Add30ml of magnesium sulfate solution (4.1.1.6) to the soil cake and shake overnight

33、. Balance the tubes and centrifuge at about3000 g for10min. Decant the supernatant solution through a coarse filter paper (diameter7 cm) into a conical flask and store the filtrateB for the determination of magnesium following the procedure described in4.2.4. Prepare a blank by following the procedu

34、re described above completely, without the addition of soil. 4.2 Determination of the CEC 4.2.1 Principle An acidified lanthanum solution is added to the filtrateB obtained according to the procedure described in4.1.2 and magnesium is determined by FAAS using an oxidizing (blue) air/acetylene flame.

35、 Lanthanum is added to prevent the formation of refractory compounds of magnesium with phosphate, aluminium etc. in the flame. 4.2.2 Reagents Use only reagents of recognized analytical grade and distilled water or deionized water for all solutions. 4.2.2.1 Water, with a specific conductivity not hig

36、her than0,2mS/m at25 C (water conforming to grade2of ISO3696). 4.2.2.2 Hydrochloric acid, c(HCl)=12mol/l ( =1,19g/ml). 4.2.2.3 Magnesium standard solution, c(Mg)=0,0010mol/l. Pipette50,0ml of the0,0200mol/l magnesium sulfate solution (4.1.1.6) into a1000ml volumetric flask and dilute to the mark wit

37、h water. 4.2.2.4 Acidified lanthanum solution, (La)=10g/l. Dissolve15,6g of lanthanum nitrate hexahydrate La(NO 3 ) 3 .6H 2 O in water in a500ml volumetric flask, add42ml of hydrochloric acid (4.2.2.2) and dilute to the mark with water. 4.2.3 Calibration series Pipette 0 ml,1ml,2 ml,3 ml,4 mland5 ml

38、 of magnesium standard solution (4.2.2.3) into a series of100ml volumetric flasks. Add10ml of acidified lanthanum solution (4.2.2.4), to each flask, dilute to the mark with water and mix. These calibration solutions have magnesium concentrations of0mmol/l,0,01mmol/l,0,02mmol/l, 0,03mmol/l,0,04mmol/l

39、 and0,05mmol/l, respectively. 4.2.4 Procedure Pipette0,20ml of filtrateB of the soil samples (see4.1.2) and of the blank (see4.1.2) into individual100ml volumetric flasks. Add10ml of acidified lanthanum solution (4.2.2.4) to each flask, dilute to the mark with water and mix. Determine the magnesium

40、concentration in the diluted filtrate B (c 1 ) and the diluted blank (c b1 ) using the calibration solutions (4.2.3) and FAAS at wavelength285,2nm, with the instrument set according to the manufacturers instructions for optimum performance with an oxidizing (blue) air/acetylene flame. 4.2.5 Calculat

41、ion Correct the concentration of magnesium in filtrate B for dilution by the liquid retained in the centrifuged soil after being treated with water using the formula: where c 1 is the magnesium concentration in the diluted filtrate B, in millimoles per litre; c 2 is the corrected magnesium concentra

42、tion in the diluted filtrate B, in millimoles perlitre; m 1 is the mass of the centrifuge tube with air-dried soil, in grams, m 2 is the mass of the centrifuge tube with wet soil, in grams. c 2 c 1 30m 2 m 1 +() 30 - =BS 7755-3.12:1996 BSI 07-1999 3 Calculate the cation exchange capacity (CEC) of th

43、e soil using the formula: where If the CEC exceeds40cmol+/kg, repeat the determination using less soil, amending the calculation accordingly. NOTE 5The unit “centimoles positive charge per kilogram”, written in abbreviated form as cmol+/kg, is numerically equal to the formerly used unit of milli-equ

44、ivalents per hundred grams. 4.3 Determination of exchangeable sodium and potassium 4.3.1 Principle Sodium and potassium are measured in an acidified barium chloride-triethanolamine extract of soil samples by flame emission spectrometry. NOTE 6Because of the high barium concentration in the medium, i

45、onization interferences are eliminated. 4.3.2 Reagents Use only reagents of recognized analytical grade and distilled water or deionized water for all solutions. 4.3.2.1 Water, with a specific conductivity not higher than0,2mS/m at25 C (water conforming to grade2of ISO3696). 4.3.2.2 Potassium and so

46、dium stock solution, (K)=1000mg/l and (Na)=400mg/l. Dissolve1,9068g of potassium chloride and1,0168g of sodium chloride in water. Transfer to a1000ml volumetric flask and dilute to the mark with water. NOTE 7Pulverize the potassium chloride and sodium chloride, heat the powders obtained either at400

47、 C to500 C for at least8h or at about200 C for24h, and cool the powders in a desiccator before use. 4.3.2.3 Diluted stock solution, (K) =100mg/l and (Na) =40mg/l. Pipette25,0ml of the stock solution (4.3.2.2) into a250ml volumetric flask and dilute to the mark with water. 4.3.2.4 Hydrochloric acid,

48、c(HCl) =1mol/l. Dilute83ml of concentrated hydrochloric acid ( =1,19g/ml) to1000ml with water. 4.3.3 Calibration series Pipette0ml,5ml,10ml,15ml20ml and25ml of the diluted stock solution (4.3.2.3) into individual50ml volumetric flasks. Add10,0ml of extraction solution (4.1.1.5) and5,0ml of hydrochlo

49、ric acid (4.3.2.4). Dilute to the mark with water. These calibration solutions have potassium concentrations of0mg/l,10mg/l,20mg/l, 30mg/l,40mg/l and50mg/l and sodium concentrations of0mg/l,4mg/l,8mg/l, 12mg/l,16mg/l and20mg/l, respectively. 4.3.4 Procedure Pipette2,0ml of filtrate A (see4.1.2) and of the blank extract (see4.1.2) into individual test tubes. Add1,0ml of hydrochloric acid (4.3.2.4) followed by7,0ml of water to each test