Lab 6: Measuring CEC
Use soils from last week to –
> extract exchangeable cations
> measure basic cations by AA
spectroscopy
> measure acid cations by titration
> calculation CEC, % BS for your soil
> convert from meq/100g to lbs/acre
Procedure:
1. Weigh 3.0 g soil into 50 mL centrifuge tube
2. Pump in 35 mL NaCl extracting solution
3. Shake for 20 minutes to displace
exchangeable cations from colloid surface
into solution…
Ion Exchange by
Added Na soln …
Mass Action
Mg+2
K+
Ca+2
Al+3
Displaced cations
Measurement of exchangeable cations:
1. Centrifuge, filter to separate clear solution.
2. Fill glass vial about ½ full of solution for AA
analysis.
3. Measure remaining solution using grad
cylinder—record this number, this will be your
“volume titrated”. Pour back into Erlenmeyer
flask.
Titration of exchangeable acids:
1. To Erlenmeyer flask: add stir bar and 3-4
drops phenolpthalein indicator , turn on
stirrer to slow speed.
2. Add NaOH from burette SLOWLY (drop by
drop) until solution stays pink for 20-30
seconds. Record volume base used from
burette.
At endpoint: pH is near neutral, and
AMT BASE ADDED = AMT ACID PRESENT
NB x VB = NA x VA
Titration calculations:
NB x VB = NA x VA
NB: conc of base: 0.005 N (eq/L = meq/mL)
VB: volume of base (mL, burette)
VA: volume of acid (“mLs titrated”)
NA: conc of acid: UNKNOWN… meq acid/mL
Conc (solution)  Conc (soil)
We used 35 mL of NaCl solution to extract 3 g
soil to get cations into solution….
Meq acid x 35 mL soln x 100 = meq acid
mL soln
3 g soil
100
100 g soil
Same for basic cations by AA:
Meq Ca x 0.035 L soln x 100 = meq Ca
L soln
3 g soil
100
100 g soil
CEC of soil =  cations (in meq/100 g):
Ca + Mg + K + acidity (H+Al)
Mg and K: written on board (meq/L)…..
% base saturation (BS) =  bases (Ca+Mg+K)/CEC
Convert meq/100 g into lbs/acre-furrow slice….
CEC calculations: meq/100 g to lbs/acre
Assume a soil has 2 meq K+/100 g: how many lbs/acre?
(“acre” = acre-furrow slice= 2x106 lbs)
Note: Atomic mass of K=39 g/mole, and 1mol=1equiv (K+1)
39 g K X 1 mol X 1000 mg
1 equiv
X
1g
mol 1 equiv
1000 meq
39 mg
=
meq
2 meq K x 39 mg K x 1000 g =
1 kg
1 meq
100 g soil
780 mg K
kg soil
So--
Now: “mg/kg” is PPM = parts per million… and LBs/AFS =
lbs/2x106 lbs, which is “parts per 2 million”…. SO:
780 ppm K x 2 = 1560 lbs K/acre