Cation Exchange Capacity (CEC) pH 7 ammonium acetate (CEC7) by mechanical vacuum extractor
Much of this procedure is from the Soil Survey Laboratory Methods Manual – Soil Survey Investigations Report No. 42 Ver. 4.0. USDA-NRCS and then adapted for our local laboratory.
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: https://nrcspad.sc.egov.usda.gov/DistributionCenter/pdf.aspx?productID=886 [Accessed 10 March 2016].
Holmgren, G.G.S., R.L. Juve, and R.C. Geschwender. 1977. A mechanically controlled variable rate leaching device. Soil Sci. Am. J. 41:1207-1208.
Peech, M., L.T. Alexander, L.A. Dean, and J.F. Reed. 1947. Methods of soil analysis for soil fertility investigations. USDA Circ. 757, 25 pp.
Soil Survey Staff. 2004. Ion Exchange and Extractable Cations. p. 173-178. In R. Burt (ed.) Soil Survey Laboratory Methods Manual‚ Soil Survey Investigations Report 42 Version 4.0. USDA-NRCS, Washington D.C.
Sumner, M.E., and W.P. Miller. 1996. Cation exchange capacity and exchange coefficients. p. 1201-1229. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical methods. No. 5. ASA and SSSA, Madison, WI.
- Syringes, polypropylene, disposable, 60-mL, for extraction vessel, extractant reservoir and tared extraction syringe
- Rubber tubing, 3.2 ID x 6.4 OD x 25.4 mm (1/8 ID x 1/4 OD x 1 in) for connecting syringe barrels.
- Analytical filter pulp, ash-free, Schleicher and Schuell, No. 289.
- Plunger, modified. Remove rubber and cut plastic protrusion from plunger end.
- Electronic balance, ±1-mg sensitivity (note that increasing the electronic balance sensitivity can be valuable for research purposes)
(These may be prepared for you.)
- Distilled deionized (DDI) water
- Ammonium acetate solution (NH4OAc), 1 N, pH 0. Add 1026-mL of glacial acetic acid (CH3COOH) to 15 L DDI water. Add 1224-mL of concentrated ammonium hydroxide (NH4OH). Mix and cool. Dilute with DDI water to 18 L and adjust to pH 7.0 with CH3COOH or NH4OH.
- Ethanol (CH3CH2OH), 95%, U.S.P. (EtOH)
- Sodium chloride (KCl), reagent, crystal. Make a 2N solution.
Extraction of Bases
- Prepare extraction vessel by tightly compressing a 1-g ball of filter pulp into the bottom of a syringe barrel with a modified plunger.
- Weigh 2.50-g of
- Place 25.4-mm (1 in) length rubber tubing on the extraction vessel and clamp.
- Use a squeeze bottle to add NH4OAc solution to 5-7.5-mm above the soil. Let stand for at least 20 min. Add more NH4OAc as necessary to maintain 5-7.5-mm solution above soil.
- To the extractor and connect a tared extraction syringe to the 25.4-mm tubing that is attached to the extraction vessel. Recheck and add NH4OAc as necessary to maintain 5-7.5-mm solution above soil.
- Put reservoir tube on top of the extraction vessel.
- Add 45-mL of NH4OAc solution to the reservoir tube.
- Pull the plunger of the tared extraction syringe down at a modest rate such that the solution in the reservoir tube is emptied in no less than 10-min (longer is better). Do not pull plunger from the barrel of the tared extraction syringe.
- Carefully remove the tared extraction syringe containing the extract. Leave the rubber tubing on the extraction vessel and retain for Removal of Excess Ammonium Acetate and replacement of NH4.
- Note: It is easiest to number the extraction vessels and record the tare wt on a separate sheet.
- Pull the plunger to add 1-2.5-cm air to the tared extraction syringe.
- Weigh the extraction syringe containing the NH4OAc extract to [at least] the nearest 0.01 g.
- Mix the extract in each syringe by manually shaking. Fill a labeled storage bottle with extract solution and discard any excess. The solution in the storage bottle is reserved for analysis of extracted cations Na+, K+, Ca2+, Mg+ by ICP-spectroscopy.
- Remove any excess NH4OAc solution at this time using the now evacuated extraction syringe to draw out the solution and discard the excess NH4OAc solution.
- NOTE: If it is necessary to pause at this point, don’t allow the sample to dry, as this could cause NH4+ fixation. Keep the soil in the extraction reservoir covered with about 5 mm of NH4OAc solution until you are ready to proceed with the following steps.
Removal of Excess Ammonium Acetate
- NOTE: Ethanol is flammable. Keep away from sparks and sources of fire. Proceed immediately from the ethanol washing procedure to the replacement of NH4+ with K+. Don’t allow the sample to dry, as this could cause NH4+ fixation.
- Remove the reservoir tube on top of the extraction vessel. Clamp the 25.4-mm tubing that is attached to the extraction vessel. Rinse the sides of the extraction vessel with ethanol from a wash bottle. Fill the extraction vessel to the 20-mL mark (again ensuring that at least 5-7.5-mm of ethanol cover the soil) with ethanol and let stand for 15 to 20 min.
- Attach the evacuated extraction syringe to the extraction vessel, un-clamp tubing, and place reservoir tube on the extraction vessel. Rapidly extract the ethanol level to a 0.5- to 1.0-cm height above the sample.
- Add 55 to 60-mL of ethanol to the reservoir. Extract at a modest rate such that the solution in the reservoir tube is emptied in no less than 10-min.
- Pull the plunger of the extraction syringe down. Do not pull the plunger from the syringe barrel. Remove the extraction syringe and discard the ethanol. You may need to clamp the connection tubing to avoid ethanol from dripping.
- Repeat the ethanol wash. Evacuate any remaining ethanol from the extraction vessel by attaching the extraction syringe. Rinse the extraction vessel well with DI water.
- Note: Proceed immediately from the ethanol washing procedure to the replacement of NH4+ with K+. Don’t allow the sample to dry, as this could cause NH4+ fixation. If it is necessary to pause at this point, don’t allow the sample to dry, as this could cause NH4+ fixation. Keep the soil in the extraction reservoir covered with about 5 mm of EtOH solution until you are ready to proceed with the following steps.
Replacement of Ammonium (NH4+)
- Clamp the connector. Use a squeeze bottle to fill extraction vessel to the 20-mL mark with KCl solution (~10-mL). Thoroughly wet the sample to a level 0.5- to 1.0-cm height above the sample. Let stand for at least 20 min.
- Put reservoir tube on top of the extraction vessel. Rapidly extract the NaOAc solution to a 0.5- to 1.0-cm height above sample. Add 45-mL of KCl solution to the reservoir tube.
- Attach the extraction syringe, unclamp tubing, and, pull the plunger of the extraction syringe down at a modest rate such that the solution in the reservoir tube is emptied in no less than 10-min. Do not pull plunger from the barrel of the syringe.
- Carefully remove the extraction syringe containing the extract. Leave the rubber tubing on the extraction vessel.
- Pull the plunger to add 1-2.5-cm air to the tared extraction syringe.
- Weigh each syringe containing the KCl extract to the nearest 0.01-g.
- Mix the extract in each syringe by manually shaking. Fill a storage bottle with extract solution and discard the excess. The solution in the storage bottle is reserved for analysis of extracted NH4+.
Measurement of Ammonium (NH4+)
- Measure NH4+ on Carlson, Lachat, by Berthelot reaction, or Kjeldahl digest.
The above procedure can be repeated substituting NH4Cl for NH4OAc in the initial cation displacement. By doing so the CEC is unbuffered.
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
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 February 2008].Soil Survey Staff. (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.
Last updated on Thursday, August 1, 2013