This chapter adds 3-D species distribution topos to earlier surfaces that showed pH (Chapter 1.1) and buffer capacity behavior (Chapter 1.2) during titration and dilution procedures. It constructs trend surfaces by plotting computed alpha distribution coefficients above a composition grid with “mL of NaOH” as the x-axis and overall system dilution (log C) as the y-axis. The systematic shift from protonated to deprotonated forms is clearly visualized on a linear z-axis. Because pH and buffer capacity surfaces accompany the species topos, it is easy to see their interrelationships. On the basis of their graphical appearance, features on species topo surfaces have been named corner buttes, corner pits, curving ridge crests, curving canyons and dilution plains. Ramps connecting surface features are linear when tied to additions of NaOH and logarithmic when followed on the log C dilution axis. The amphiprotic behavior of water is demonstrated through dilution procedures. Systems examined include acetic acid, CH3COOH (a weak monoprotic acid); carbonic acid, H2CO3 (a weak diprotic acid), and phosphoric acid, H3PO4 (a weak triprotic acid). For comparative purposes, species topos are depicted for a set of three acids with hypothetical pKas of 4.0, 7.0, and 10.0. Supplementary files include the Species TOPOS software, a macro-enabled Excel workbook that quickly generates pH, buffer capacity and alpha surfaces for any mono-, di-, or triprotic acid desired. Only the acid dissociation constants, the Ka values, are needed as inputs. Also included are a set of PowerPoint lecture slides and a document “Teaching with Species TOPOS” with sections for lecture, practice exercises, and suggested laboratory activities for introductory college courses and upper-division or graduate courses in analytical chemistry, biochemistry and geochemistry.
© 2021 Garon C. Smith and Md Mainul Hossain
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Smith, Garon C. and Hossain, Md Mainul, "Chapter 1.3: 3-D Topo Surface Visualization of Acid-Base Species Distributions: Corner Buttes, Corner Pits, Curving Ridge Crests and Dilution Plains" (2021). Water Topos: A 3-D Trend Surface Approach to Viewing and Teaching Aqueous Equilibrium Chemistry. 4.