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1、INTERNATIONAL STANDARD IS0 13536 First edition 1995-06-I 5 Soil quality - Determination of the potential cation exchange capacity and exchangeable cations using barium chloride solution buffered at pH = 8,l Qua/it6 du sol - D it is also harmful for the environment. 1 Scope This International Standar
2、d specifies a method for the determination of the potential cation exchange ca- pacity KEC) of soil buffered at pH = 8,l and of the determination of the content of exchangeable sodium, potassium, calcium and magnesium in soil. This International Standard is applicable to all types of air-dried soil
3、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 publi- cation, the editions indicated were valid. All standards are subject to revision, and parties to agreements ba
4、sed on this International Standard are encouraged to investigate the possibility of applying the most re- cent editions of the standards indicated below. Members of IEC and IS0 maintain registers of cur- rently valid International Standards. IS0 3696:1987, Water for analytical laboratory use - Speci
5、fication and test methods. IS0 11464: 1994, Soil quality - Pretreatment of sam- ples for physico-chemical analyses. 3 Principle The determination of CEC as specified in this Inter- national Standard is a modification of the method proposed by Mehlichll,21 and modified by BascombW The CEC of soil sam
6、ples is determined in barium chloride solution buffered at pH = 8,l 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 of 0,02 mol/l magnesium sulfate solution is added. All t
7、he 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 ab- sorption spectrometry (FAAS) or inductively co
8、upled 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. NOTES 1 Besides sodium, potassium, calcium and magnesium, it is also possible to determine other cations, fo
9、r example manganese, in the same extract. 2 A 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. 3 The sum of exchangeable cations
10、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 be- cause it could change the relative proportions of cations in the CEC. -,-,- IS0 13536:1995(E) 63 IS0 4 Proce
11、dures 4.1 Leaching 4.1.1 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 than 02 mS/m at 25 “C (water conforming to grade 2 of IS0 3696). 4.1.1.2 Barium chloride solution, c(BaC
12、IJ = 1 mol/l. Dissolve 244 g of barium chloride dihydrate (BaCl,.2H,O) in water and dilute to 1 000 ml with water. 4.1.1.3 Hydrochloric acid, c(HCI) = 2mol/l. Dilute 166 ml of concentrated hydrochloric acid (p = I,19 g/ml) with water to 1 000 ml. 4.1.1.4 Triethanolamine solution, pH = 8,l. Dilute 90
13、 ml of triethanolamine (HOCH,CH,),N with water to about 1 litre and adjust the pH to 8,l + 0,02 with about 140 ml to 150 ml of hydrochloric acid (4.1.1.3). Dilute with water to 2 litres. 6 2 mm), pretreated according to IS0 11464, into a tightly stoppered polyethylene centrifuge tube of about 50 ml.
14、 Note the mass of the tube with cover and soil I,). Add 30 ml of extraction solution (4.1.1.5) to the soil and shake for 1 h. Balance the tubes and centrifuge at 3 OOOg for 10 min. Transfer the supernatant liquid to a 100 ml volumetric flask. Repeat the addition of extraction solution, shaking and c
15、entrifuging twice and adding the supernatant liquid to the 100 ml volumetric flask on each occasion. Dilute to the volume of the volumetric flask with extraction solution (4.1.1.5). Mix, filter through a coarse filter paper (diameter 7 cm) and store the filtrate A for the determination of sodium, po
16、tassium, calcium and magnesium following the procedure described in 4.3 and 4.4. Add approxi- mately 40 ml of water (4.1.1.1) to the soil cake and shake manually for 1 min to 2 min to break up the soil cake. Balance the tubes and centrifuge at about 3 OOOg for 10 min. Decant the supernatant to waste
17、. Weigh the tube with its contents and cover (q). Add 30 ml of magnesium sulfate solution (4.1 .I.61 to the soil cake and shake overnight. Balance the tubes and centrifuge at about 3 OOOg for 10 min. Decant the supernatant solution through a coarse fil- ter paper (diameter 7 cm) into a conical flask
18、 and store the filtrate B for the determination of magnesium fol- lowing the procedure described in 4.2.4. Prepare a blank by following the procedure described above completely, without the addition of soil. 4.1.1.5 Extraction solution. 4.2 Determination of the CEC Mix equal volumes of solutions 4.1
19、.1.2 and 4.1.1.4. Protect this solution from exposure to carbon dioxide during storage. 4.1.1.6 Magnesium sulfate solution, c(MgSOJ = 0,020 0 mol/l. Dissolve 4,930 g + 0.01 g of magnesium sulfate heptahydrate (MgS0,.7H,O, see note 4) in water and dilute to 1 000 ml. NOTE 4 Magnesium sulfate heptahya
20、rare may lose wa- ter of crystallization on standing. The reagent should be standardized by titration with ethylenediaminetetraacetic acid (EDTA) at pH = 10 using Eriochrome Black T as the in- dicator. Alternatively the reagent may be kept in a sealed polyethylene bag placed in a refrigerator. 4.1.2
21、 Procedure Transfer 2,50 g to 5,00 g of air-dried soil with a high content of clay and/or humus, or 10 g of sandy soil or soil with a low content of humus (particle size 4.2.1 Principle An acidified lanthanum solution is added to the filtrate B obtained according to the procedure de- scribed in 4.1.
22、2 and magnesium is determined by FAAS using an oxidizing (blue) air/acetylene flame. 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 de
23、ionized water for all solutions. 4.2.2.1 Water, with a specific conductivity not higher than 0,2 mS/m at 25 “C (water conforming to grade 2 of IS0 3696). 4.2.2.2 Hydrochloric acid, c(HCI) = 12 mol/l (p = 1 ,I9 g/ml). 2 Q IS0 IS0 13536:1995(E) 4.2.2.3 Magnesium standard soktion, c(Mg) = ml is the mas
24、s of the centrifuge tube with 0,001 0 mol/l. air-dried soil, in grams, Pipette 50,O ml of the 0,020 0 mol/l magnesium sul- fate solution (4.1 .I .6) into a 1 000 ml volumetric flask and dilute to the mark with water. 4.2.2.4 Acidified lanthanum solution, p(La) = 10 g/l . Dissolve 15.6 g of lanthanum
25、 nitrate hexahydrate La(NO,),.GH,O in water in a 500 ml volumetric flask, add 42 ml of hydrochloric acid (4.2.2.2) and dilute to the mark with water. 4.2.3 Calibration series Pipette 0 ml, 1 ml, 2 ml, 3 ml, 4 ml and 5 ml of magnesium standard solution (4.2.2.3) into a series of 100 ml volumetric fla
26、sks. Add 10 ml of acidified lanthanum solution (4.2.2.4), to each flask, dilute to the mark with water and mix. These calibration sol- utions have magnesium concentrations of 0 mmol/l, 0,Ol mmol/l, 0,02 mmol/l, 0.03 mmol/l, 0,04 mmol/l and 0.05 mmol/l, respectively. 4.2.4 Procedure Pipette 0.20 ml o
27、f filtrate B of the soil samples (see 4.1.2) and of the blank (see 4.1.2) into individual 100 ml volumetric flasks. Add 10 ml of acidified lanthanum solution (4.224) to each flask, dilute to the mark with water and mix. Determine the magnesium concentration in the di- luted filtrate B (c,) and the d
28、iluted blank (q,) using the calibration solutions (4.2.3) and FAAS at wave- length 285,2 nm, with the instrument set according to the manufacturer s instructions for optimum per- formance with an oxidizing (blue) air/acetylene flame. 4.2.5 Calculation Correct the concentration of magnesium in filtra
29、te B for dilution by the liquid retained in the centrifuged soil after being treated with water using the formula: c 2 = c1P+-?z - 4 30 where Cl is the magnesium concentration in the di- luted filtrate B, in millimoles per litre; c2 is the corrected magnesium concentration in the diluted filtrate B,
30、 in millimoles per litre; m2 is the mass of the centrifuge tube with wet soil, in grams. Calculate the cation exchange capacity (CEC) of the soil using the formula: CEC = 3 OOO(Cbl - c2) m where CEC is the cation exchange capacity of the soil, in centimoles positive charge per kilogram; c2 is the co
31、rrected magnesium concentration in the diluted filtrate B solution, in milli- moles per litre; cbl is the magnesium concentration in the di- luted blank solution, in millimoles per litre; m is the mass of the air-dried sample, in grams. If the CEC exceeds 40 cmol+/kg, repeat the determi- nation usin
32、g less soil, amending the calculation ac- cordingly. NOTE 5 The unit “centimoles positive charge per kilo- gram”, written in abbreviated form as cmol+/kg, is numeri- cally equal to the formerly used unit of milli-equivalents per hundred grams. 4.3 Determination of exchangeable sodium and potassium 4
33、.3.1 Principle Sodium and potassium are measured in an acidified barium chloride-triethanolamine extract of soil sam- ples by flame emission spectrometry. NOTE 6 Because of the high barium concentration in the medium, ionization interferences are eliminated. 4.3.2 Reagents Use only reagents of recog
34、nized analytical grade and distilled water or deionized water for all solutions. 4.3.2.1 Water, with a specific conductivity not higher than 0,2 mS/m at 25 “C (water conforming to grade 2 of IS0 3696). 3 -,-,- IS0 13536: 1995(E) 0 IS0 4.3.2.2 Potassium and sodium stock solution, p(K) = 1 000 mg/l an
35、d p(Na) = 400 mg/l. Dissolve 1,906 8 g of potassium chloride and 1,016 8 g of sodium chloride in water. Transfer to a 1 000 ml volumetric flask and dilute to the mark with water. NOTE 7 Pulverize the potassium chloride and sodium chloride, heat the powders obtained either at 400 “C to 500 “C for at
36、least 8 h or at about 200 “C for 24 h, and cool the powders in a desiccator before use. 4.3.2.3 Diluted stock solution, p(K) = 100 mg/l and p(Na) = 40 mgjl. Pipette 25,0 ml of the stock solution (4.3.2.2) into a 250 ml volumetric flask and dilute to the mark with water. 4.3.2.4 Hydrochloric acid, c(
37、HCI) = 1 mol/l. Dilute 83 ml of concentrated hydrochloric acid (p = 1 ,I9 g/ml) to 1 000 ml with water. 4.3.3 Calibration series Pipette 0 ml, 5 ml, 10 ml, 15 ml 20 ml and 25 ml of the diluted stock solution (4.3.2.3) into individual 50 ml volumetric flasks. Add 10,O ml of extraction solution (4.1 .
38、l .5) and 5,0 ml of hydrochloric acid (4.3.2.4). Dilute to the mark with water. These cali- bration solutions have potassium concentrations of 0 mg/l, 10 mg/l, 20 mg/l, 30 mg/l, 40 mg/l and 50 mg/l and sodium concentrations of 0 mg/l, 4 mg/l, 8 mg/l, 12 mg/l, 16 mg/l and 20 mg/l, respectively. 4.3.4
39、 Procedure Pipette 2.0 ml of filtrate A (see 4.1.2) and of the blank extract (see 4.1.2) into individual test tubes. Add 1.0 ml of hydrochloric acid (4.3.2.4) followed by 7.0 ml of water to each test tube and mix. Determine the concentrations of sodium and potassium in the diluted filtrate A, blank
40、and calibration solutions by flame emission spectrometry at wavelengths of 589 nm and 766 nm, respectively, using an air/propane flame. NOTES 8 To prevent contamination with sodium, clean the glass- ware by soaking it overnight in 4 mol/l nitric acid, technical grade. 9 Recheck the standard with the
41、 highest concentration frequently (e.g. after every five measurements). 4.3.5 Calculations Calculate the exchangeable sodium and potassium contents in the soil samples using the equations: b(Na, exch) = 2,174 g(P3 - Pb2) m b(K, exch) = 1,278 f3(3 - Pb2) m where b(Na, exch) is the content of exchange
42、able sodium in the soil, in centimoles positive charge per kilogram; b(K, exch) is the content of exchangeable pot- assium in the soil, in centimoles positive charge per kilogram; P3 is the concentration of sodium or potass- ium in the diluted filtrate A, in milligrams per litre; Pb2 is the concentr
43、ation of sodium or potass- ium in the diluted blank solutions, in milli- grams per litre; m is the mass of air-dried soil, in grams. 4.4 Determination of exchangeable calcium and magnesium 4.4.1 Principle Magnesium and calcium are determined in the acidi- fied barium chloride triethanolamine extract
44、 by FAX. NOTE 10 Due to the high barium concentration in this medium, there is no formation of refractory compounds of magnesium or calcium with phosphate, aluminium, etc. in the flame. 4.4.2 Reagents Use only reagents of recognized analytical grade and distilled water or deionized water for all sol
45、utions. 4.4.2.1 Water, with a specific conductivity not higher than 0,2 mS/m at 25 “C (water conforming to grade 2 of IS0 3696). 4.4.2.2 Hydrochloric acid, c(HCI) = 4 mol/l. Dilute 330 ml of concentrated hydrochloric acid (p = 1 ,19 g/ml) to 1 000 ml with water. -,-,- 8 IS0 IS0 13536:1995(E) 4.4.2.3
46、 Magnesium stock solution, p(Mg) = 100 mg/l. Dissolve 0,837 g of magnesium chloride hexahydrate (MgCI,.GH,O) in water. Transfer to a 1 000 ml volumetric flask and dilute to the mark with water. NOTES 11 Magnesium chloride hexahydrate may lose water of crystallization on standing. The reagent should
47、be standard- ized by titration with EDTA buffered at pH = 10 using Eriochrome Black T as the indicator. 12 The use of commercial standard solutions and the dis- solution of a weighed amount of metal is also possible. 4.4.2.4 Calcium stock solution, p(Ca = 1 000 mg/l. Weigh 2,497 g of calcium carbona
48、te (CaCO,) into a 100 ml beaker. Dissolve the calcium carbonate in 12,5 ml of 4 mol/l hydrochloric acid (4.4.2.2). Boil the solution to expel carbon dioxide, cool to room tem- perature, transfer to a 1 000 ml volumetric flask and dilute to the mark with water. NOTE 13 Calcium carbonate should be hea
49、ted for 2 h at 400 “C, then cooled and stored in a desiccator, before it can be used as a standard. 4.4.2.5 Mixed stock solution, p(Mg) = 5 mg/l and p(Ca) = 50 mg/l. Pipette 5,0 ml of the magnesium stock solution (4.4.2.3) and 5,0 ml of the calcium stock solution (4.424) into a 100 ml volumetrjc flask and dilute to the mark with water. blank and calibration solutions by FAAS at wave- lengths of 285,2 nm for magnesium and 422,7 nm for calcium, using an oxidizing (blue) ai
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