Transcript Soil solution part 3

Soil solution part 3
Combined tensiometersoil solution sampler
hopmans.lawr.ucdavis.edu/images/research_2_5.jpg
Pore water
sampler
www.decagon.com
Measuring soil solution in laboratory
more common, but not as accurate
•
Displacement techniques
With or w/o non-polar displacing chemicals
•
•
Centrifugation (spinning the soil at a
high speed pulls the liquid out of the
pores)
Saturation paste extracts or any ratio of
soil to water mixture
Displacement by a non-polar
chemical (e.g., CCl4)
http://commons.wikimedia.org/wiki/Image:Carbon-tetrachloride-3D-balls.png
“Saturated Paste” or Soil:Water mixture
Extracts
Collecting soil solution
http://www.envisci.ucr.edu/faculty/graham/students/photos/blee05.jpg
Speciation -
the distribution of free ions and
complexes in their various forms (species)
Free, hydrated ions  complexed or hydrolyzed ions
http://www-ocean.tamu.edu/Quarterdeck/QD2.2/Sant-Gill/sant-gillfig2.html
Hydrolysis
• Metal cations in water accept electrons
“Lewis acid”
• Water donates unshared electrons
“Lewis base”
• Acidic water is produced
• M-OH forms an inner-sphere complex
• Water dissociation upon metal hydrolysis
 KH or hydrolysis constant
Larger KH means lower pH
Al3+ has KH >>> Ca2+ (10-5 >>>10-12)
Hydration and Hydrolysis of Metal Cations
(different forms of ions result from reaction in soln)
[M(H2O)n]z+ + H2O

[M(H2O)n-1(OH)](z-1)+ + H3O+
KH =
http://www.wou.edu/las/physci/ch412/hydrxn2.jpg
Solution pH affects degree of hydrolysis
[M(H2O)n]z+ + H2O

[M(H2O)n-1(OH)](z-1)+ + H3O+
More H+ in solution drives eqn to the left
Less H+ in solution drives eqn to the right
Complexation or ion pairs
• Hydrated metal + ligand  ion pair
• Outer-sphere complex
Water molecules between the metal and ligand
• Ligands can be inorganic or organic
• Analytical methods don’t differentiate
between free and complexed ions:
Mtotal = Mn+free + Mcomplexed
Activity vs concentration
• The effective concentration of a substance
• Measure of deviation from standard T,P and
ideal solutions
• Activity (α) is a correction factor to account
for non-ideality and is between 0 and 1
as solution concentration decreases, α  1
• Activity = molarity x activity coefficient
α=Mxγ
Ionic Strength
• Estimate of the interaction between ions in
solution
• Related to concentration (m) and valence
(z) of ions
Example of I calculation
(monovalent)
0.01 M NaCl
I = 0.5[(0.01 M x 12)+(0.01 M x -12)]
= 0.5[0.01 + 0.01]
= 0.5[0.02]
I = 0.01 M
Example of I calculation (Divalent)
0.01 M CaSO4 ↔ Ca2+ + SO42I = 0.5[(0.01 M x 22)+(0.01 M x -22)]
= 0.5[0.04 + 0.04]
= 0.5[0.08]
I = 0.04 M
(note effect of valence makes I
more than twice as much as the monovalent salt)
Example of I calculation (mixed
valence single salt)
0.01 M CaCl2 ↔ Ca2+ + 2ClI = 0.5[(0.01 M x 22) + 2(0.01 M x -12)]
= 0.5[0.04 + 2(0.01)]
= 0.5[0.04 + 0.02]
= 0.5[0.06]
I = 0.03 M
I calculation (mixed salt solution)
0.01 M CaCl2 and 0.02 M NaNO3
I = 0.5[(0.01 M x 22) + 2(0.01 M x -12) +
(0.02 M x 12) + (0.02 M x -12)]
= 0.5[0.04 + 2(0.01) + 0.02 + 0.02]
= 0.5[0.04 + 0.02 + 0.02 + 0.02]
= 0.5[0.10]
I = 0.05 M
Activity coefficients
Extended Debye-Hückel eqn for solutions with I < 0.2 M
log γ = -AZ2[I0.5 / (1 + BaiI0.5)]
(eqn 4.15)
I = ionic strength (moles/L)
A = ~0.5 at 298K
B = ~0.33 at 298K
ai = angstroms related to the size of the hydrated
ion (not the activity!). It is the “Distance of
Closest Approach”