CATION EXCHANGE CAPACITY (CEC)
Ammonium Replacement Method(Buchner funnels vacuum flasks)
Tarleton State University Environmental Soils and Biogeochemistry Characterization Laboratory
Dr Donald G. McGahan
The following is adapted from Soil Survey Laboratory Methods Manual – Soil Survey Investigations Report No. 42 Ver. 4.0. USDA-NRCS and much text is lifted directly from their and then adapted where necessary for our local laboratory conditions.
Scope and Application
Cation exchange capacity(CEC) is the measure of a soil to retain readily exchangeable cations which neutralize the negative charge of soils. This method involves saturation of the cation exchange sites with ammonium, equilibration, removal of the excess ammonium with ethanol, replacement and leaching of exchangeable ammonium with protons from HCl acid (Horneck, et al. 1989). This method may be poorly suited to soils containing carbonates, vermiculite, gypsum and zeolite minerals. The procedure is time consuming and labor intensive. The speed at which samples filter depends on the strength of the vacuum applied and sample makeup. If using a water aspirated generated vacuum, some samples may never filter because of clogged filter paper. The method detection limit is approximately 1.0 cmol kg-1 (or meq/100 gm on a dry soil basis) and is generally reproducible within ± 10%.
- Analytical balance: 250 g capacity, resolution 0.01 g (greater resolution is acceptable and often desirable).
- Reciprocating horizontal mechanical shaker, capable of 180 oscillations per minute.
- Repipette dispensers (or pipets), calibrated to 20 ± 0.1, 8.0 ± 0.1 mL and 5.0 ± 0.1 mL.
- Whatman No. 1, No. 2 or equivalent filter paper.
- Buchner funnels vacuum flasks and source of vacuum
- Auto analyzer or Kjeldahl distillation equipment.
- Deionized water ASTM Type I Grade.
- Ammonium acetate (1.0 N) extraction solution neutral: Add 570 mL of glacial acetic acid CH3COOH (99%) to 8000 mL of deionized water. Add 680 mL of concentrated ammonium hydroxide adjust pH to 7.0 with 3.0 N glacial acetic acid or 3.0 N ammonium hydroxide and dilute to 10 L final volume.
- Ethanol, 95%
- Hydrochloric acid, 0.1 N HCl – Dilute 8.3 mL of concentrated HCl reagent to 1000 mL with deionized water.
- Standard calibration solutions of NH4-N. Prepare six calibration standard solutions of 1.0 – 20 mg L-1 of NH4-N mg L-1 from 1000 mg L-1 stock solutions. Dilute calibration solutions with 0.1 N HCl solution.
- Weigh 10.0 ± 0.1 g of air dry soil pulverized to pass 10 mesh sieve (< 2.0 mm) soil into a 125 mL Erlenmeyer flask. Add 50 mL of ammonium acetate solution (See Comment #1) and place the flask reciprocating shaker for thirty (30) minutes. Include a method blank.
- Connect a 1L vacuum extraction flask to a Buchner funnel fitted with a Whatman No. 5 or equivalent filter paper. Moisten the filter paper with 2 mL deionized water (See Comment #2).
- Transfer the soil suspension into the Buchner funnel and leach the sample with 175 mL of 1 N ammonium acetate. The soil extract may be analyzed for extractable K, Ca, Mg, and Na.
- Rinse the excess ammonium acetate from the soil sample in the Buchner funnel by leaching with a total volume of ethanol and discard the leachate. Note: Be sure to gently fill funnel to remove all excess ammonium and allow it to drain until only damp soil remains. Continue adding ethanol in this manner until 200 mL of solution has been used.
- Change to a clean 500-mL suction flask and leach the soil sample with 225 mL of 0.1 NHCl to replace the exchangeable ammonium. Bring leachate to a final volume of 250 mL volumetric flask using deionized water.
- The concentration of ammonium-N in the final leachate can be determined. The determination can be made using the Kjeldahl distillation method (See Comment #3 and #4). See Berthelot Reaction below for non-automated standard laboratory method which relies on ammonium to complex with salicylate to form indophenol blue. This color is intensified with sodium nitroprusside and measured at 660 nm.]
CEC in meq per 100 g of soil = (mg L-1 of NH4-N in leachate) x (0.25 / 14) x (100 / sample size (g)) mg L-1 NH4-N in leachate is determined using a standard curve.
- Check repipette dispensing volume calibration using an analytical balance.
- Check filter paper supply for possible contamination of and NH4-N. If contamination is greater than 0.2 mg L -1on a soil extract basis, rinse filter paper with 0.1 N HCl solution.
- Samples having ammonium concentrations exceeding the highest standard will require dilution and reanalysis.
- This procedure used is essentially the same as that of Schollenberger (1945) except that determination of NH4-N is done spectrophotometrically rather than Kjeldahl distillation and titration. To determine the NH4-N content using the Kjeldahl distillation method, follow steps 1 through 5 above, then proceed to Kjeldahl distillation. Care must be taken not to allow soil to dry and crack between ethanol leaching, as this could result in incomplete removal of excess NH4-N. A similar procedure is described by Rhoades (1982).
Horneck, D.A., J.M. Hart, K. Topperand, B. Koespell. 1989. Methods of Soil Analysis used in the soil testing laboratory at Oregon State University. Ag. Expt. Station SM 89:4.
Rhoades, J.D. 1982. Cation exchange capacity. In: A.L. Page, R.H. Miller, and D.R. Keeney (eds.) Methods of soil analysis. Part 2. Agron. Monogr. 9, Am. Soc. Agron., Madison, WI. p. 149-157.
Schollenberger, C.J. andR.H. Simon. 1945. Determination of exchange capacity and exchangeable bases in soils-ammonium acetate method. Soil Sci. 59:13-24.
Thomas, G.W. 1982. Exchangeable cations. In A.L. Page (ed.). Methods of soil analysis, Part 2, 2nd ed. Agronomy Monograph 9, American Society of Agronomy, Madison, WI.
Ammonium (NH4+) Determination
Berthelot reaction, salicylate analog of indophenol blue. Ammonia reacts with hypochlorite to form chloramine, which couples with two non-para-substituted phenols. Citrate and tartrate are included to avoid precipitation of Ca, Mg, and other hydroxides.
Forster JC. 1995. Soil nitrogen. In: Alef K and Nannipieri P (eds.) Methods in Applied Soil Microbiology and Biochemistry. Academic Press, San Diego. pp. 79-87.
Kempers AJ, Kok CJ. 1989. Re-examination of the determination of ammonium as the indophenol blue complex using salicylate. Anal. Chim. Acta. 221:147-155.
Verdouw H, van Echteld CJA, Dekkcrs EMJ. 1977. Ammonia determination based on indophenol blue formation with sodium salicylate. Water Res. 12:399-402.
-The procedure below most closely matches Forster 1995.
Reagent A: in 100 ml water
- 6.5 g sodium salicylate
- 5 g sodium citrate
- 5 g sodium tartrate
- 0.025 g sodium nitroferricyanide (sodium nitroprusside)
This reagent is somewhat sensitive to light. It may be stored cold. Discard if dark.
Reagent B: in 100 ml water
- 6 g sodium hydroxide
- 2 ml bleach (5.25% sodium hypochlorite) or equivalent
Quantities in reagent preparation need not be exact.
- In semimicro cuvets mix sample, then A, then B, in the appropriate amounts. Cap and invert to mix. Read absorbance at 650 nm against a reagent blank after 50-60 minutes.
- For 1-20 ppm NH use 40 µl sample, 500 µl A, 500 µl B.
- For 1-10 ppm use 80 µl sample, 500 µl A, 500 µl B.
- For 1-5 ppm, LISC 140 µl sample, 400 µl A, 400 µl B. (to ~0.1)
- For less than 1 ppm*. use 600 µl sample, 150 µ1 A, 150 µl B. Working lower limit is approximately 0.025 ppm N.
- For 5-60 ppm*, use macrocuvets, 45 µl sample, 1500 µl Ω x A, 1500 µl Ω x B. (lower limit ≈ 1 ppm)
Primary amines e.g. amino acids) and certain other N compounds may interfere slightly when present in large amounts (more than 10-20 times the ammonia concentration). It is usually not a problem with soil extracts and water samples. For reducing such interference substitute sodium 2-phenylphenolate for sodium salicylaic (Rhine et al Soil Sci. Soc. Am. J. 62:473-480).
*Note: When using <1 ppm range with high Ca/Mg samples (e.g. soil extracts), use 700 µl sample, 300 µl 2.5 x A*, 200 µl B. 2.5 x A: in 100 ml, 17 g sodium salicylate, 12.5 g each sodium citrate and tartrate, 62 mg sodium nitroprusside.
Direct Distillation of Adsorbed Ammonia, Kjeldahl
Soil Survey Staff. 2004. Soil Survey Laboratory Methods Manual – Soil Survey Investigations Report No. 42 Ver. 4.0. USDA-NRCS. Lincolin, Nebraska, USA. [Online WWW] URL: http://soils.usda.gov/technical/lmm/, [Accessed 8 March 2016]. (p 619-620)
Peech, M., L.T. Alexander, L.A. Dean, and J.F. Reed. 1947. Methods of soil analysis for soil fertility investigations. U.S. Dept. Agr. C. 757, 25 pp.
- Sodium chloride (NaCl).
- Antifoam mixture. Mix equal parts of mineral oil and n-octyl alcohol.
- Sodium hydroxide (NaOH), 1 N.
- Hydrochloric acid (HCl), 0.2 N, standardized.
- Boric acid (H3BO3), 4-percent.
- Mixed indicator. Mix 1.250 g methyl red and 0.825 g methylene blue in 1 liter 95-percent ethanol.
- Brom cresol green, 0.1-percent, aqueous solution.
- Transfer the soil plus filter paper from CEC7 method to a Kjeldahl flask. Add 400 ml water and about 10 g NaCl, 5 drops antifoam mixture, a gram or two of granular zinc, and 40 ml 1 N NaOH. Connect the flask with the condenser and distill 200 ml into 50 ml 4-percent H3BO3 solution. Titrate the distillate to the first tinge of purple with 0.2 N HCl, using 10 drops mixed indicator and 2 drops brom cresol green.
- CEC(meq/100 g) =(A/B) x N x 100
A = Volume HCl (mL)
B = Sample weight (g)
N = Normality of acid
Report on ovendry (OD) basis.
Thursday, August 1, 2013
Cation Exchange Capacity-Buchner funnels vacuum flasks.pages