Procedure for Cation Exchange Capacity (CEC) Determination in Soil Samples
1. Place 3 grams (weigh to 4 digits) of 1 mm air-dried soil sample in a 250 ml erlenmeyer flask, and add 100 ml of 1 N NH4OAC (pH = 7.0) solution. Shake the flask thoroughly by hand and allow it to stand overnight (cover the flask mouth with parafilm).
2. Filter the soil with light suction using a Büchner funnel and No. 2 filter paper into a clean flask. Add a small (25 ml) portion at a time.
3. Filter the soil with an additional 100 ml of 1 N NH4OAC (pH = 7). Check for Ca2+ : close vacuum, lift funnel carefully out of flask, transfer 3 drops of filtrate from funnel end into a test tube, add 3 drops 1 N NH4Cl, 3 drops 1:1 NH4OH, and 3 drops 10% ammonium oxalate. No precipitate indicates the completion of filtering. When the Ca2+ test is negative (no precipitate) save the filtrate for the later determination of exchangeable K+, Na+, Ca2+, and Mg2+.
4. Filter the soil with light suction using 200 ml 1 N NH4Cl followed by 100 ml 0.25 N NH4Cl.
5. Wash the soil with 150-200 ml of isopropyl alcohol, add a small (25 ml) portion at a time. Test for Cl-: Remove funnel as in step 3; add 10 drops of filtrate and 10 drops of 0.1 N AgNO3 to a clean test tube. A precipitate (AgCl) indicates the presence of chloride. When the chloride is no longer present, empty and clean the collection flask for step 6 (discard filtrate).
6. Filter the soil with 300 ml of 10% NaCl (in 5-6 portions). Save the filtrate in a clean bottle for CEC determination.
7. Transfer 20 ml of the filtrate to a microkjeldahl flask, add a spoon (calibrated) of MgO powder, and distill 40 ml of the solution into 5 ml of 2% H3BO3. Titrate the boric acid solution with standard H2SO4 (0.01 N). Refer to the following instructions for using the microkjeldahl apparatus.
1. Turn on the rheostat to the heater and allow the water to boil.
2. Prepare the steam distillation apparatus for use by opening the lower stopcock on the steam-bypass assembly and closing the upper stopcock, which connected to the distillation head. These should be the positions of these stopcocks while the water heats.
3. Add 5 ml H3BO3 indicator solution to 100 ml beaker marked to indicate a volume of 40 ml, and position the beaker under the condenser of the distillation apparatus so that the tip of the condenser is in contact with the side of the beaker.
4. Attach the kjedahl flask containing 20 ml NaCl filtrate and MgO powder to the distillation apparatus. Then seal the funnel with the peg stopper, and immediately commence distillation by opening the upper stopcock on the steam-bypass assembly and closing the lower stopcock.
5. When the volume of distillation reaches 40 ml, rinse the tip of the condenser, and stop the distillation by opening the lower stopcock on the steam-bypass assembly.
6. Determine NH4+-N in the distillate by titration with 0.01 N H2SO4. At the end-point, the color changes from green to a faint pink.
The CEC of the soil is calculated as follows:
CEC (in meq./100 g. soil) = V x 0.001 N x 300 ml/20ml x 100 g./Ms
Where,
V = Volume of 0.001
300 ml = Total volume of 10 % NaCl, used to substitute the NH4+.
20 ml = Volume of filtrate used for distillation.
Ms = Weight of the soil sample used.