pH-dependent Charge

P “Proton” = H+ ion (quicker to say).

P “Protonation” = to stick protons on “stuff”. The“sticking” usually involves forming a covalent bondbetween the H+ ion and the stickee, or (in the caseof clays) at least a bond with a good deal of“covalent character”.

P “Stuff”:< Dissolved organic or inorganic molecules or ions.< Chemically suitable “spots” on surfaces. On clays, edges

of layers have lots of “suitable spots”. On kaolinite, andthe sesquioxides of Fe and Al, octahedral sheets at layersurfaces have lots of “suitable spots”, as will be shown,shortly.

PDeprotonation = yanking protons off stuff.

With many protons, many protonations.Definitions:

You can try all day long, but you will have a hard time yankingthe proton off an alcohol molecule.

Some spots like protons; somehate them.

The protons in a glass of booze are so reluctant to come off thatyou would have to increase the pH of your drink to about 18 beforehalf of the ethanol molecules in your glass look like the (ethanolate)anion on the right

Alas, nobody has ever created an aqueous solution with a pH muchabove 16, so the ethanolate anion must be considered to be a “rarespecies”.

Conversely, it’s easier than sin in New Orleans to protonate anethanolate anion.

Well humified, colloidal organic matter particles have lots of attractivespots suitable for protonation. One common and fetching example isthe “carboxylate anion”, which is often written in text as R-CO2

!, butwhich is better explained in pictures.

The proton on a carboxylate group comes off nice and easy.

R-COOH X R-COO! + H+.

“Stuff” in soil

Depending on the local chemical environment, most carboxylategroups are half-protonated at about pH 4.5.

Thus, you need a lot more protons in solution to protonate carboxylateanions than to protonate ethanolate anions.

Back when you were eight, a nice third-grade teacher told youthat “like charges repel”. This powerful truth applies even to asingle charge, e.g., of !1. It hates itself and wants to get awayfrom its own stink by spreading out over many atoms.

Self-image Problems

Trapped like a rat! Concentrated around a single O atom

A much happier negativecharge schmeared overthree atoms. Whew! Room to breathe.

Fair play. When no obvious reason explains why one of two identicalatoms gets the double bond and the other gets stuck with the negativecharge, the two atoms “split the difference”. Both get a bond and ahalf and a partial negative charge. Chemists call this “resonance”.

The rules:

Organic chemical shorthand

P Carbon makes four bonds.

P Nitrogen makes three bonds, unless noted otherwise.

P Oxygen makes two.

P Hydrogen makes one.

The conventions:

P Straight lines represent covalent bonds.

P At an unlabeled angle and at the end of a line, a C atom isimplied.

P If you want a different atom, label it.

P Mentally add implied H atoms ‘til everybody is happy.

Positive charge on these colloids (last column) is probably measured ata pH of 3.5, since that pH nearly maximizes positive charge withoutbeing so acidic that the clays begin to disintegrate.

One more ugly fact about old clays

(i.e., about kaolinite and hydrous oxides of Fe and Al)

They bind phosphate tighter than derivatives traderscling to their tax breaks.

Save your breath and don’t try to tell a phosphate ionthat it has a negative charge and should repelnegatively charged clay particles. It won’t listen andwill stick anyway.

Phosphate anions are illiterate and never went to third grade.