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