Lab 6: Measuring CEC
Use soils from last week to –> extract exchangeablecations> measure basiccationsby AA spectroscopy> measure acidcationsby titration> calculation CEC, % BS for your soil> convert frommeq/100g to lbs/acre
Weigh 3.0 g soil into 50mLcentrifuge tube2. Pump in 35mLNaClextracting solution3. Shake for 20 minutes to displace exchangeablecationsfrom colloid surface into solution…
Ion Exchange byMass Action
Added Na soln …
Measurement of exchangeablecations:
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:
To Erlenmeyer flask: add stir bar and 3-4 dropsphenolpthaleinindicator , turn on stirrer to slow speed.AddNaOHfrom 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, andAMT BASE ADDED = AMT ACID PRESENTNBx VB= NAx VA
NBx VB= NAx VANB:concof base: 0.005 N (eq/L =meq/mL)VB:volume of base (mL, burette)VA:volume of acid (“mLstitrated”)NA:concof acid: UNKNOWN…meqacid/mL
We used 35mLofNaClsolution to extract 3 g soil to getcationsinto solution….Meqacidx35mLsolnx100=meqacidmLsoln3 g soil 100 100 g soilSame for basiccationsby AA:MeqCax0.035Lsolnx100=meqCaLsoln3 g soil 100 100 g soil
CEC of soil =cations(inmeq/100 g):Ca + Mg + K + acidity (H+Al)Mg and K: written on board (meq/L)…..% base saturation (BS) = bases (Ca+Mg+K)/CECConvertmeq/100 g intolbs/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= 2x106lbs)Note: Atomic mass of K=39 g/mole, and 1mol=1equiv (K+1)
Now: “mg/kg” is PPM = parts per million… and LBs/AFS = lbs/2x106lbs, which is “parts per 2 million”…. SO:780ppmK x 2 = 1560 lbs K/acre