Cation Exchange Capacity by Barium Saturation – Calcium Replacement Method

CATION EXCHANGE CAPACITY

Barium Saturation – Calcium Replacement Method

Source: University of California Division of Agriculture and Natural Resources

Scope and Application

The method determines the cation exchange capacity (CEC) of soil as described by Rible and Quick (19601) and Janitzky (19862). The soil is quantitatively displaced of all exchangeable cations with Ba, followed by four deionized rinses to remove excess Ba. A known quantity of calcium is then exchanged for Ba and excess solution calcium is measured. CEC is determined by the difference in the quantity of the Ca added and the amount found in the resulting solution. The method has a detection limit of approximately 1.0meq l00g-1 (soil basis) and is generally reproducible within ±7%.

Equipment

  1. Analytical balance: l000g capacity, resolution ±.0.01g.
  2. Mechanical shaker.
  3. Repipette dispenser(s), calibrated to 10 and 15mL.
  4. 150mL glass beakers.
  5. Whatman No. 2 filter paper.
  6. Heating plate or steam table (90° C).
  7. Atomic Absorption Spectrometer (AAS), Perkin Elmer 2380.
  8. Peristaltic (Istamac) two channel pump diluter with variable speed, 0-100rpm.

Reagents

  1. Deionized water, ASTM Type I grade.
  2. Barium Acetate Reagent, 1.0 N pH 7.0. Dissolve 1280g of reagent grade barium acetate (Ba(C2H3O2)2 in 9000mL of distilled water. Adjust pH to 7.0 with acetic acid or barium hydroxide.
  3. Calcium Sulfate Reagent. Dissolve 10.0g of reagent grade calcium sulfate (CaSO4.2H2O) in 1 0L of deionized water and place on magnetic stir plate with stir bar for one hour. Settle for twelve hours.
  4. Standard calibration solution, calcium. Prepare six calibration standards, concentration range 0.5-400mg L-1 of calcium made up in 5% nitric acid (v/v) made from 1000mg L-1 standard solution.

Procedure

  1. Weigh out 4.00±0.05g of air dry soil into a 150mL beaker. Add 15.0mL of 1.0 N Barium acetate reagent using repipette dispenser (see comments #1 Р#3).
  2. Place extraction vessel(s) on a steam table for 30 minutes.
  3. Immediately filter, refilter if filtrate is cloudy.
  4. Leach with 10.0mL Barium acetate reagent and then wash with four 50mL increments of deionized water. Allow to drain between increments.
  5. Air dry filtrate and filter for twelve hours on lab bench or dryingoven for two hours at 50ºC.
  6. Transfer to second extraction rack vessel and add 100.0±0.1mL of Calcium sulfate reagent. Place on mechanical shaker for twenty minutes.
  7. Filter and collect extract. Analyze Ca in both soil filtrate and Calcium sulfate reagent by atomic absorption spectrometry and pump diluter in accordance with standard instrument operating protocol using the prepared standard working solutions and report results as mg L-1 calcium in solution.

Calculations

Cation Exchange Capacity can be calculated by:

CEC meq/l00g = [(Calcium Sulfate Ca – Soil Filtrate Ca) x 0.1 L x 100]/4

Record CEC results to three significant digits.

Comments

  1. Check calibration of repipette dispenser, recalibrate using analytical balance as necessary.
  2. For samples having CEC concentrations (>60meq 100g-1) decrease soil sample size to 2.0g. Cation exchange capacity can also be reported as cmol kg-1 which equal to meq 100g-1.
  3. Soils containing large quantities of gypsum, should be washed extensively with 0.01 N NaCl to remove calcium sulfate.

Literature

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