1. Soil solution part 3

2. Combined tensiometer- soil solution sampler
Pore water sampler
hopmans.lawr.ucdavis.edu/images/research_2_5.jpg

3. 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

4. Displacement by a non-polar chemical (e.g., CCl4)

5. “Saturated Paste” or Soil:Water mixture Extracts

6. Collecting soil solution

7. Speciation - the distribution of free ions and complexes in their various forms (species)
Free, hydrated ions  complexed or hydrolyzed ions

8. 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)

9. [M(H2O)n]z+ + H2O  [M(H2O)n-1(OH)](z-1)+ + H3O+
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 =

10. 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

11. 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

12. 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 = M x γ

13. Ionic Strength Estimate of the interaction between ions in solution
Related to concentration (m) and valence (z) of ions

14. Example of I calculation (monovalent)
0.01 M NaCl
I = 0.5[(0.01 M x 12)+(0.01 M x -12)]
= 0.5[ ]
= 0.5[0.02]
I = 0.01 M

15. Example of I calculation (Divalent)
0.01 M CaSO4 ↔ Ca SO42-
I = 0.5[(0.01 M x 22)+(0.01 M x -22)]
= 0.5[ ]
= 0.5[0.08]
I = 0.04 M (note effect of valence makes I more than twice as much as the monovalent salt)

16. Example of I calculation (mixed valence single salt)
0.01 M CaCl2 ↔ Ca Cl-
I = 0.5[(0.01 M x 22) + 2(0.01 M x -12)]
= 0.5[ (0.01)]
= 0.5[ ]
= 0.5[0.06]
I = 0.03 M

17. 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[ (0.01) ]
= 0.5[ ]
= 0.5[0.10]
I = 0.05 M

18. 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”