Visualization of Buffer Capacity with 3-D “Topo” Surfaces: Buffer Ridges, Equivalence Point Canyons and Dilution Ramps

Abstract

BufCap TOPOS is free software that generates 3-D topographical surfaces (“topos”) for acid–base equilibrium studies. It portrays pH and buffer capacity behavior during titration and dilution procedures. Topo surfaces are created by plotting computed pH and buffer capacity values above a composition grid with volume of NaOH as the x axis and overall system dilution as the y axis. What emerge are surface features that correspond to pH and buffer behaviors in aqueous solutions. Topo surfaces are created for pH, log buffer capacity, and linear buffer capacity. Equivalence point breaks become pH cliffs and logarithmic buffer capacity canyons that grow shallower with dilution. Areas of high buffer capacity become rounded ridges. Dilution alone generates 45° ramps. Example systems include hydrochloric acid, HCl (a strong acid); acetic acid, CH3COOH (a weak monoprotic acid); oxalic acid, HOOCCOOH (a weak diprotic acid); and l-glutamic acid hydrochloride, C5H9NO4·HCl (a weak triprotic acid). The Supporting Information includes a copy of the interactive BufCap TOPOS program, macro-enabled spreadsheets that quickly generate surfaces for any mono-, di-, or triprotic acid. Only acid dissociation constants, Ka values, need be changed. Other materials include teaching slides for lectures, plus worksheets and laboratory activities for first-year college courses and third-year or graduate analytical chemistry courses.