gt extract exchangeable cations gt measure basic cations by AA spectroscopy gt measure acid cations by titration gt calculation CEC BS for your soil gt convert from ID: 707798
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Slide1
Soils and Hydrology - Lab 6
Cation Exchange Capacity (CEC) and Base Saturation (BS)Slide2
CEC and BS
CEC
(cation exchange capacity) is important for knowing the ability of a soil to store cations
BS
(base saturation) is important for knowing how many “good” cations are in a soil
Method
Extract exchangeable cations
Measure base cations by AA spectroscopy
Measure acid cations by titration
Calculate CEC and BS
Convert from meq/100 g to
lbs
/acSlide3
PROCEDURE
Add 3 g soil to 50 mL centrifuge tube
Add 35 mL
NaCl solutionShake for 20 minLabel your tubeCentrifuge Filter into Erlenmeyer flaskSlide4
Ca
+2
Mg
+2
K
+
Al
+3
Na
+
Na
+
Na
+
Na
+
Ion Exchange by
Mass Action
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Added Na soln …
Displaced cations
Na+
Na
+Slide5
Exchangeable Cations
Once the centrifuge tube has been removed from the centrifuge, filter your sample into an Erlenmeyer flask
Fill small vial with filtrate
Transfer remaining filtrate to a graduated cylinder, record volume, and then return filtrate to Erlenmeyer flaskyoutube.com/watch?v=HmEyymGXOfISlide6
CEC and BS procedure
40 mL
Centrifuge
Tube
3 g
Soil
35 mL
1M NaCl
Shake 20 Minutes
CENTRIFUGE
Funnel w
Filter Paper
Small Flask
50 mL Graduated Cylinder
Small Vial
Filtrate
Base Cations by AA
Exchangeable ions by titrationSlide7
Atomic Adsorption SpectrometerSlide8Slide9Slide10
Titration
of
Acidic Cations
with BuretteSlide11
Exchangeable acids by titration
Record volume of filtrate,
V
ARead initial value of NaOH (scrimmage line)Add stir bar and a packet of phenolphthalein indicator to Erlenmeyer flask
Turn on stirrer to
slow
speed.
Slowly add NaOH (drop by drop), NBStop when solution stays pink
for 20-30 s Read final value of NaOH (position at end of down)Record volume base used, VB (yards advanced!)Remember: amount of added base equals original amount of acid, so that
NB
x VB = NA x VASlide12
Titration Calculations:
N
A
= NB
x V
B
/ V
A
NA is acid concentration (meq
/mL)
NB
is base concentration (0.0044 meq
/mL) V
B is volume of base (mL) from burette
V
A is volume of acid (mL) from graduated cylinderSlide13
Solution Concentration
vs Soil Concentration
Our cations were extracted from 3 g of soil
These cations are now in 35 mL of solutionWe have to adjust our solution concentration back to the original soil concentration meq
acid
x
35 mL soln = meq acid
mL soln 3 g soil g soil meq Ca x 0.035
L soln
= meq Ca L
soln 3 g soil g soilWe then multiply by 100 to get meq/100
gSlide14
CEC
=
Cations (
meq/100 g) = Bases (Ca+Mg+K+Na) + Acids (H+Al) => Mg, K, Na provided belowBS = Bases (Ca+Mg+K+Na
) / CEC
Soil Type
Ca
Mg
K
Na
Cecil
Ap
Measured
0.257
0.086
0.031Cecil Bt
Measured
1.296
0.171
0.056Cecil C
Measured0.171
0.0860.049Floodplain A
Measured3.080
0.1710.121Floodplain C
Measured1.460
0.0100.022Slide15
Cations
Monovalent (H, K, Na): 1 mole = 1 equivalent
Divalent (Ca, Mg): 1 mole = 2 equivalents
Trivalent (Al, Fe): 1 mole = 3 equivalents
And remember…
1 mg/kg = 1 ppm = 2
lbs
/acSlide16
Converting
meq
/100 g to lbs/ac
Assume soil has 2 meq K+/100 g: How many lbs/ac? (“acre” = afs = 2x106
lbs)
Atomic mass of K = 39 g/mole, 1 mole = 1
eq
1 mg/kg = 1 ppm = 2
lbs/ac
780 ppm-K x 2 = 1560 lbs-K/ac Slide17
Show All Steps of Calculations Including Units
Hand In Lab Report Next Week
Please Write Legibly