A simple application from the chapter 'Aquatic Chemistry'

in Chemed Manual, p.70
Titration curves

The titration curve of a mixture of three acids with NaOH 0.1 M is computed for the case with changing total volume as in a real titration. The points show the behavior of the buffer intensity. This script allows to change all input parameters in real time.
Klick anywhere on the figure to open or download the script "titration3.d" (size 8 kB). If you have George Harth's Dialect on your computer, you can run this script now. Download "Dialect Standard" or "Dialect Professional" for free!
Of course, such a program can also be used to determine pK-values, or even all analytical concentrations and pK-values from a measured titration curve in one go. See the output of such a fit routine:

Output of fit

If you consider to apply a fit program to extract analytical data and pK-values from the output of your electrometric titrator, free of first-order approximations, incl. error correction for pH-calibration and carbonate content of an NaOH-Titrator, consult with me. The thin black vertical lines show equivalence points as determined with sophisticated algorithms by most commercial titrators (which interpolate the extremum of the first derivative). They are wrong for the first two lines from the left. The true equivalence points are the thin red lines which do not coincide with the extrema! For the large pH-step both correctly coincide within the precision visible on the graph.-
The colored boxes show values determined automatically by the fit routine. The white boxes are input parameters. The program analyzes the raw data and finds reasonable starting values for the concentrations and pK values of the analytes without operator input. The whole fit procedure lasts less than 10 seconds.

Below is a blown-up view of the buffer region of the intermediate analyte CH3COOH = HAc with concomitant equivalence points (the sample is slightly different from the one in the analysis above): The magenta points pH(v) are from the experimental titration curve, the blue line pH(v) is the result of the fit procedure. The fat black points have been determined as equivalence points by the commercial titrator using an interpolation of the maximum value of the experimental first derivative, the small blue points. The fat red points on the titration curve are the equivalence points as determined by the fit. They are named "true" because they mark the correct beginning (after H3PO4) and end (before H2PO4(-))of the existence region of HAc. Here the titrator's evaluation finds only 90.6% of HAc present and H3PO4 is overestimated. No commercial titrator known to the author finds correct values for this analysis of a mixture of HNO3, H3PO4 and HAc! They apply first order approximations and a flawed theory instead of making use of all measured points of the titration curve to extract the correct equivalence points.

Last modified April 2, 2009