CEC of the Clay and Fine Silt Fractions by Excess Salt Method

Background:  Read Chapter 5 in Jackson (2005) and this procedure.

Jackson, M. L. 2005. Soil chemical analysis: advanced course. Parallel Press, UW-Madison Libraries.

Objectives:  Measure CEC of the clay and fine silt fractions by the excess salt method.  Assess effects of pH on CEC.

Materials

  • Air-dry Na-saturated clay
  • Air-dry Na-saturated fine silt (2-20 µm)
  • 100 ml centrifuge tubes
  • 100 ml volumetric flasks with funnels
  • Plastic bottles with screw top lids for displaced solutions
  • 1.0 M CaCl2
  • 1.0 M MgCl2
  • 0.005 M CaCl2
  • 1 M NaOAc, pH 4
  • 1 M NaOAc, pH 7
  • Ethanol

Excess Salt Method

 

  1. Preweigh centrifuge tubes (to 0.0001 g) labeled with a Sharpie directly on the tube, no tape. Weigh about 0.2000 g of clay or fine silt into tubes.  Prepare four samples of each clay and fine silt (total eight tubes).  Weigh and record the weights of empty tubes and tubes plus clay or silt.

 

  1. To two tubes of each clay or silt, add 20 ml pH 4, 1 M NaOAc and to the other two tubes of clay or silt, add 20 ml pH 7, 1 M NaOAc. Sonicate in ultrasonic bath for 2 minutes to mix.  Centrifuge at 2000 RPM for 10 minutes.  Decant supernatant into clear beakers to insure that no sample is lost.  Discard supernatant.

 

  1. Repeat washing with 20 ml 1M NaOAc solutions, disperse (ultrasonic bath for 2 min +/- stirring with glass rod, but rinse stirring rod completely to avoid losing any sample) centrifuge, decant into beaker and measure pH with a pH meter, until supernatant pH is that of the added buffer (minimum of two washings, but this may be all that is needed).

 

  1. Add 20 ml 1.0 M CaCl2 to each tube, disperse with ultrasonic bath for 3 min (+/- stirring with glass rod).  Centrifuge at 2000 RPM for 10 min.  Discard supernatant.

 

  1. Wash, disperse, and centrifuge three times with 20 ml 0.005 M CaCl2 . Record the pH of the third wash.  We assume this brings the sample into equilibrium with the 0.005 M CaCl2.  Be careful that no sample is lost.

 

  1. Blot any large drops of solution in the tubes with Kim-Wipes and immediately weigh tubes plus wet samples (0.0001 g). Record weights.

 

  1. Add 20 ml 1.0 M MgCl2 to each tube. Disperse carefully and thoroughly (ultrasonic bath, 3 min.).  Centrifuge at 2000 RPM for 10 min.

 

  1. Decant supernatant into 100 ml volumetric flasks. Repeat three more times (total of four washes with 1.0 M MgCl2), each time collecting the supernatant in the 100 ml volumetric flask.  Bring flask to volume with 1.0 M MgCl2

 

  1. Mix flask contents thoroughly and transfer to plastic bottles. Ca to be determined by AA or ICP.

 

  1. Disperse samples with 20 ml ethanol and sonicate for 3 min. Centrifuge at 2000 RPM for 10 min.  Decant into clear beaker to ensure that no dispersion has occurred.  Discard clear supernatant.  Blot any large drops of solution in the tubes and weigh tubes immediately (0.0001 g).  Record weights.  Compare to weight in step 6.

 

  1. Oven-dry tubes overnight at 105 °C, cool in a desiccator, and weigh (0.0001g). Record weights. 

 

  1. Rinse and acid wash the volumetric flasks.

 

  1. Calculate CEC of clay and fine silt fractions. Total Ca is exchangeable Ca plus Ca in the 0.005 M CaCl(i.e., the excess salt)­­.  Ca in the liquid is calculated from the mass (hence, volume) and Ca concentration in the solution.  Solution mass is calculated from difference in tube weights:  step 6 minus step 11.  Sample mass on an oven-dry basis is calculated from step 11 minus original tube mass.

 

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