Biochemistry I Fall Term
Calculations & Graphing
Topic #1: pH Titration
Recommended problems in Campbell, Chapt. 2: #10-13.
HA <=> A- + H+
Dissociation constant: Ka = [A-][H+]/[HA]
The initial pH of 100 mL of a monoprotic acid is shown below. We want to find 1) the pKa and 2) the concentration of the acid by titrating it with 1.0 M NaOH.
- The Henderson-Hasselbalch equation is:
pH = pKa + log(A-/ HA)
- pH = the measured pH, (-log[H+]);
- pKa = -logKa;
- HA = the concentration of the acid, and
- A- = the concentration of the conjugate base.
This is done in three steps.
(Each calculated pH value has a small "experimental error" added to it.)
3. Record the volumes of NaOH and the pH's you obtain; then graph them as pH vs. equivalents of NaOH. You should get enough data so as to have 3 or 4 values of pH, both above and below the pKa value and within ±1.0 of the pKa.
Note: In contrast to a "wet lab" titration where the pH would increase stepwise with each addition of NaOH, this titration uses a new solution of acid for each volume of NaOH added.
(Hint: Graph the values you obtain as they are calculated; then as the shape of the curve becomes apparent, choose values for NaOH volume that fall into the appropriate range.)
Answers to this problem.
A sample Answer Sheet for a similar problem shows the format of the results and the graph required.
This page is for practice, only; do not submit the results for Problem Set #1.
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