E2: Ion Exchange Capacity

What is a soil’s ion exchange capacity?

How well the soil can hold on to these things that were formerly dissolved in water

What are permanent charges?

Isomorphic substitution

Where is CEC and AEC relevant?

Really small particles like colloids, clay and humus, essentially really tiny particles

What determines CEC?

The amount of negative charges a colloid has

What determines AEC?

The amount of positive charges a colloid has

Which colloids attract which cations?

Negative colloids attract positive cations

What is the smallest ion you can get?

H+ ion aka proton

What is protonation?

sticking protons onto stuff. Sticking usually involves forming a covalent bond between the H+ ion and the stickee

What is the ‘stuff’ that protons stick to during protonation?

Dissolved organic or inorganic molecules or ions (liquid) or chemically ‘suitable spots’ on surfaces like the negative end of a tetrahedral layer or octahedral layer (solid)

What is deprotonation?

Taking protons off of stuff

What is acidity caused by?

protons, H+ ions

What is acidity measured as?

A function of protons

Does lots of H+ mean lots of protonation or deprotonation? More or less acidity?

protonation; more acidity

More protonation causes more what charge on soil particles?

positive charge

Why does more H+ mean more potential for the system to be positively charged?

H+ will stick to stuff

What pH are acids?

<7

What pH are bases?

>7

What pH is water?

neutral, 7

What pH do soils typically range between?

3-8

Colloids get lots of positive charge when pH is what? Which EC is this?

<7; anion exchange capacity AEC

Colloids get lots of negative charge when pH is what?

>7; cation exchange capacity CEC

The more weathered a colloid is, the more what it has?

exchange capacity

Positively charged cations do what to the negative charge on colloids?

balance it

The closer the cation to the negative charge, the more strongly or weakly its held?

strongly

Which charge ions are repelled and easily flushed out by water?

Anions (-)

What determines nutrient availability?

mineralogy (and humus)

Is cation exchange reversible?

yes

What is the ratio law for cation exchange?

the ratio of ions of similar charge adsorbed will be equal to the ratio of the 2 ions in the soil solution, holding true for the overall system.

What is an example of exchangeable cations?

Ca2+ and Mg2+

When lime (CaCO3) is added to a soil and Ca2+ replaces H+ leading to H2O and CO2 as a byproduct in the soil solution, what direction does the reaction proceed more to? Why?

To the right because the CO2 precipitates and often leaves the system making the reaction irreversible.

What is the overall rule for when an ion displaced from the colloid is precipitated?

the reaction will be pulled more strongly to the right

What are other examples for when the reaction will be pulled more strongly to the right?

One of the ions displaced forms a strong association with an anion or is lost as gas.

The greater the charge of the cation, the tighter or looser it is held?

tighter the hold on the cation

Is the hold on the cation stronger if the hydrated radius of the cation is smaller or bigger?

smaller

What is the strongest held cation? Why?

Al3+; has the highest charge and smallest size

What is the name of Al3+ charge?

high density positive charge

What is the most weakly held ion?

Li+

What colloid has the highest cation exchange capacity?

humus

What is cation exchange capacity?

The number of centimoles of positive charge (cmolc) that can be adsorbed per unit mass (/kg) (amount of positive stuff you can hold)

Which soils/colloid have little to no CEC?

Kaolinite and sandy soils

Which soil/colloid has more CEC than even the highest 2:1 clays (vermiculites)?

humus

Does CEC increase or decrease as soil pH decreases? Why?

decrease because positive protonated minerals repel cations.

When pH is low and H+ is plentiful in solution, what else is plentiful?

protonated OH groups (neutral to + charge)

When pH is high and H+ ions are rare in solution, what is plentiful?

Deprotonated O- groups (these negative charges are cation exchange sites)

At pH 7, humic colloids have tons of CEC, what about at pH 3?

90% of it will disappear

Which clays offer places for H+ to covalently bond? (more AEC)

Older clays like kaolinite and hydrous oxides Fe and Al

Where are H+ binding spots located?

Mostly on edges of clay layers, some extra can bind to the octahedral sheet at the top or bottom of a stack of 1:1 layers

(more or fewer) protons are attached to the edges when protons are abundant in solution (low pH) and (more or fewer) protons are attached near neutral pH?

more; fewer

If the tetrahedral edge is protonated (OH), what does this mean for the acidity of the soil?

The soil is acidic

How do the protonation sites differ between gibbsite and kaolinite?

Gibbsite: OH in octahedral sheets, edges Kaolinite: OH on octahedral layer, edges (*remember Kaolinite is 1:1, one octahedral and one tetrahedral sheet and Gibbsite is 1:1, 2 octahedral sheets).

How do anions bond differently than cations during exchange?

Anions form a covalent bond, requiring a chemical reaction to break it apart, instead of an electrostatic “outer sphere” complex

Does anion exchange capacity increase or decrease as pH increases? Why?

decrease because there are fewer protons so the soil is more negatively charged meaning it can’t pick up as many anions, can pick up lots of cations.

Which can be undone easily, Hydrogen bond or covalent bond?

H bond

At higher pH, the soil has more (negative or positive charges) meaning it has (more or less) ability to exchange cations, (high or low) cation exchange capacity?

Negative charges; more; high

At pH of 4.5, what does the soil have as net charge?

Appears to have none but there is negative and positive spots that serve as exchange sites but cancel out

Which colloid is mostly pH dependent? What is its CEC and AEC?

Kaolinite; less CEC than others; some AEC

What does Smectite get its CEC from?

Permanent charge (*from interlayers)

Is humus pH dependent charge or permanent charge?

All pH-dependent charge

As pH goes up, what happens to CEC?

CEC also goes up

Why does humus have only pH dependent charge?

It does not undergo morphous substitution