3-D Topo Surface Visualization of Acid–Base Species Distributions: Corner Buttes, Corner Pits, Curving Ridge Crests, and Dilution Plains

Species TOPOS is a free software package for generating three-dimensional (3-D) topographic surfaces ("topos") for acid base equilibrium studies. This upgrade adds. 3-D species distribution topos to earlier surfaces that showed pH and buffer capacity behavior during titration and dilution procedures. It constructs topos by plotting computed a values above a composition grid with volume of NaOH added as the x axis and overall system dilution 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 topos have been named corner buttes, corner pits, curving ridge crests, and dilution plains. Ramps connecting surface features are linear when tied to additions of NaOH and logarithmic when associated with the logarithmic dilution axis. The amphiprotic behavior of water is demonstrated by 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 pK(a) values of 4.000, 7.000, and 10.000. The Supporting Information includes the Species TOPOS software, macro-enabled spreadsheets that quickly generate pH, buffer capacity, and a surfaces for any mono-, di-, or triprotic acid desired. Only acid dissociation constant (K-a) values need be changed. Also included are a set of PowerPoint lecture slides and a document entitled "Suggestions on Using Species TOPOS for Teaching" with sections for lecture, worksheet applications, and laboratory activities in first-year college courses and third-year or graduate analytical chemistry courses.