lab 6: measuring cec use soils from last week to – > extract exchangeable cations > measure...
TRANSCRIPT
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…
Ca+2Mg+2
K+ Al+3
Na+ Na+Na+ Na+
Ion Exchange by
Mass Action
Na+ Na+Na+
Added Na soln …
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 Kg m o l
X 1 m o l1 e q uiv
X 1000 m g1 g X 1 e q uiv
1000 m e q= 39 m g
m e q
So--
2 Km e q100 so ilg
x x39 m g K 1 m e q
1000 g1 kg
= 780 m g Kkg so il
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