Here's the the same diagram as in our last slide, but with some labels to talk about specific acids. From the right, we move through H2SO4 and H3PO4, which we envision from basic chemistry as losing the H+ ions listed first in their formulae. Next from the right, we get to silicic acid or H4SiO4, which we likewise envision dissociating at high pH as a much weaker acid. However, aqueous geochemists might encourage you to instead think of dissolved silicon as Si(OH)4, because it really is Si4+ surrounded by four OH groups.

        Those aqueous geochemist will also talk about aquoacidity of cations, and Al3+ is a favorite example. They point out that Al3+ either hydrated or in a hydroxocomplex like Al(OH)3 can act as an acid, giving off H+ ions. That usually perplexes those of us new to aqueous geochemistry. However, if you envision Al(OH)3H2O as H5AlO4, just as we see Si(OH)4 as H4SiO4, we can now conveniently see a progession of acids from right to left, all the way from the conventional acidity of sulfuric acid to the aquoacidity of Al3+: It's all the same thing.

        That's enough chemistry. Let's finish . . . .  

Image                                             Image


The first slide of this presentation
The last slide of this presentation
Railsback's main page about the Earth Scientist's Periodic Table of the Elements and Their Ions
e-mail to Bruce Railsback (
Railsback's main web page
UGA Geology Department web page

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.