Generating a Titration Curve - Weak Acids or Bases

Generating a Titration Curve
Weak Acids or Bases

Unlike a strong acid-base titration, the titration curve for a weak acid or base is quite a bit more complex to calculate. For example, if we were to titrate a weak acid with a strong base:

the starting pH of the acid depends on the K_a value for the weak acid. The [H₃O⁺] for a weak acid is always less than the [Acid].
the salt formed during neutralization will hydrolyze and thus form a buffer solution with the acid.

These complications make it more difficult to generate a spreadsheet for a weak-acid or weak-base titration. An Excel spreadsheet has been prepared for you to use. You will not have to understand how it works in order to answer the following questions.

Questions:
1. Set up the spreadsheet with the following initial values (acetic acid is a monoprotic acid with a K_a = 1.8 x 10^-5). Compare the graph it produces to the spreadsheet graph you prepared for a strong acid and describe how it differs.

2. Compare your graph to the acid-base indicator dyes and determine which of the following indicators would be acceptable for this titration. For those which are not acceptable, explain why they will not work.

Thymol blue

Bromphenol blue

Bromthymol blue

Phenol red

Phenolphthalein

Alizarin yellow

Which would be the best choice(s) to use?
3. Change the curve to represent other acids. Locate the value for K_a in a table for the following acids, and enter it for K_a₁:

Phosphoric acid
Hydrogen sulfide
Boric acid

Select a suitable indicator for titrating these acids. Are there any in this list that would be almost impossible to titrate using an acid-base indicator? Explain your answer.
4. Does the volume of acid used or its concentration have any effect on the choice of an indicator? If so, explain what it does.