Biochemistry 03-232 - Dry Lab

pH Titration

You are given 100 ml of an unknown monoprotic acid with an unknown concentration.
You have a 1.0 M NaOH solution to use in a titration experiment.
a) Plot the pH versus added NaOH (in ml) using Excel. You probably want to increase the amount of NaOH by 1 ml at a time.
Note:In contrast to a "wet lab" titration where the pH would increase step-wise with each addition of NaOH, this titration uses a new solution of acid for each volume of NaOH added, e.g. when you enter 2 ml, a total of 2ml would have been added to the starting solution.
b) Determine the pKa from the titration curve.
c) Identify the acid based on the table below.
d) Determine the concentration of the weak acid.

Notes on titrations:

  • When 1/2 of the ionizable group on the weak acid is deprotonated, the pH=pKa. This is also an inflection point in the curve.
  • When all of the ionizable group has been deprotonated an equal number, or equivalent number, of moles of NaOH have been added per mole of the weak acid. In a monoprotic buffer there is one equivalence point, a diprotic two, etc.
  • The x-axis can be plotted in units of ml of NaOH or equivalents of NaOH. In the case of equivalents, the scale is from 0 to 1 for a monoprotic buffer, where an equivalence of 1 indicates that one mole of NaOH has been added/mol of weak acid. In the case of diprotic buffers the x-axis scale would range from 0 to 2 equivalents.
Enter a volume of NaOH:
1.0 M NaOH = ml
For the above volume of NaOH, Calculate the pH
pH =
AcidpKa Values
Acetate4.76
pyridine5.23
BIS-Tris6.50
HEPES7.48
TRIS8.06
Bicine8.26
AMPSO9.00
CAPSO9.60