Soil solution part 3

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

[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 = 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 = M x γ

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+ + SO42- I = 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+ + 2Cl- I = 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”