Cation exchange capacity

Cation exchange capacity is determined by (1) replacing all of the exchangeable cations in a soil sample with K+ ions, (2) removing the excess K+ from solution, leaving behind only exchangeable K+, then (3) eluting and measuring the K+.

Reagents:

  • DI water (use the Milli-Q)
  • 1N Potassium acetate, KC2H3O2, at least enough for 40 mL per soil sample
  • Methyl alcohol, CH3OH, at least enough for 60 mL per soil sample
  • Ammonium acetate, CH3COONH4, at least enough for 40 mL per soil sample

Equipment:

  • Small soil corer
  • Ziplock bags
  • Sharpie
  • Cooler
  • Knife
  • Ruler
  • Analytical balance
  • Pipet and clean tips set for 20 mL samples
  • DI water in squirt bottle
  • Centrifuge
  • 100 mL centrifuge tubes and stoppers
  • 100 mL beakers

Procedure:

  1. COLLECT SOIL SAMPLES:
    1. Choose the first sample location. With a sharpie, label one of the bags with the sample site, treatment (e.g. soil type) and the initials of the investigator.
    2. Use the corer to sample the soil. Use the knife to remove and discard the litter layer from the top of the core. Measure down 10 cm from the top; cut off and discard the remaining soil. Put the 0-10 cm portion into the ziplock bag and put in the cooler.
    3. Repeat for your remaining samples.
  2. IN THE GEOCHEMISTRY LAB:
    1. Sieve and air-dry the soil sample, using the appropriate protocols.
    2. Make up your reagents and check that you have all of your equipment.
    3. Weigh out 10 g of soil to measure gravimetric water content (see the separate protocol for GWC).
    4. Weigh out about 5 g of soil, recording the exact mass.
    5. Using a sharpie, label a 100 mL centrifuge tube with the sample ID (which should also be in your lab notebook), then transfer the soil sample to the tube.
    6. Repeat the last three steps for your remaining soil samples.
    7. NOTE: For the remaining steps you may work with several centrifuge tubes simultaneously, up to the maximum number of tubes the centrifuge will hold. Remember that the centrifuge MUST BE BALANCED! Add additional tubes to balance the centrifuge even if you have no samples in them. If you are unsure about this step, ask someone.
    8. Swamp with K+ ions:
      1. With a pipet, add 20 mL of potassium acetate, stopper, and shake for 1 minute.
      2. Unstopper and rinse soil from the stopper and sides of tube with DI water, using as little water as possible.
      3. Stopper and centrifuge for 5 minutes. If the solution is still cloudy, centrifuge for another 5 minutes. When the solution is clear, pour off and discard the supernatant.
      4. Repeat the previous three steps one more time. Your sample should now have K+ ions on all of the exchangeable sites.
    9. Wash excess K+ ions out of the soil
      1. With a pipet and a fresh pipet tip, add 20 mL methyl alcohol to each sample tube.
      2. Stopper and shake until the soil is resuspended in the solution.
      3. Unstopper and rinse soil from the stopper and sides of tube with DI water, using as little water as possible.
      4. Stopper and centrifuge for 5 minutes. If the solution is still cloudy, centrifuge for another 5 minutes. When the solution is clear, pour off and discard the supernatant.
      5. Repeat the previous four steps two more times. The only extractable K+ ions remaining in the soil should be occupying the cation exchange sites.
    10. Elute the K+ ions from exchangeable sites into solution
      1. With a pipet and a fresh pipet tip, add exactly 20 mL of ammonium acetate to each sample tube.
      2. Stopper and shake until the soil is resuspended in the solution.
      3. Unstopper and rinse soil from the stopper and sides of tube with DI water, using as little water as possible.
      4. Centrifuge for 5 minutes.
      5. Label a 100 mL beaker with the sample ID, then carefully pour off the supernatant from the appropriate centrifuge tube into the beaker.
      6. Repeat the above five steps one more time, pouring the supernatant into the same beaker.
      7. Rinse and discard the soil samples from the tubes.
    11. Measure the K+ concentration of the solution using the Chemistry department's flame atomic absorption spectrometer (Fig. 2), using the Chem dept. protocols (see me for help)
Figure 1: Preparing samples for analysis



 
Whitman's Atomic Absorption Spectrometer
Figure 2.  The atomic absorption spectrometer, ready to analyze
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