pH Titration

You are given 100 ml of an unknown diprotic acid at a concentration of 100mM.
You have a 1.0 M NaOH solution to use in a titration experiment.

a) Plot the pH versus added NaOH (in equivalents) 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 two pK_a from the titration curve.

c) Identify the acid based on the table below.

Notes on titrations:

• When 1/2 of the ionizable group on the weak acid is deprotonated, the pH=pK_a. 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.