Biochemistry I Fall Term
Calculations & Graphing

Topic #1: pH Titration

Recommended problems in Campbell, Chapt. 2: #10-13.
Model reaction:     HA <=> A- + H+
Dissociation constant: Ka = [A-][H+]/[HA]
The Henderson-Hasselbalch equation is:
pH = pKa + log(A-/ HA) ,  where
pH = the measured pH,   (-log[H+]);
pKa = -logKa;
HA = the concentration of the acid, and
A- = the concentration of the conjugate base.
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.
This is done in three steps.
1. Enter a volume of NaOH in this box:
1.0 M NaOH = mL
2. For the above volume of NaOH, Calculate the pH
(Tabbing out of the volume entry slot or clicking anywhere on the page will also calculate the pH.)
pH =
    (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.

Back to the Calculations & Graphing Index