This is another example of an acid-base titration laboratory experiment. However, this titration involves a weak acid and a strong base. Since the calculations pertaining to this type of titration are much more lengthy and involved than those for a strong acid-strong base titration, this experiment is recommended particularly for AP/Honors Chemistry instructors.
Objectives:
- Understand why the pH changes in the direction it does as an acid or base is added. In particular, why does the pH decrease as a solution gets more acidic, and why does the pH increase as a solution gets more basic?
- Understand how and why the concentrations of hydrogen and hydroxide ions change during a titration.
- Explain why the pH decreases as the hydrogen ion concentration increases and why the pH increases as the hydroxide ion concentration increases.
- Analyze the pH change over time with help from titration curves.
- Understand why the \(pK_{a}\) value is important when finding the pH of a weak acid solution.
Materials:
- Burette
- Flask
- 500 milliliters 1-molar aqueous hydrofluoric acid
- 50 milliliters .3-molar aqueous sodium hydroxide
- Universal indicator
- Safety goggles
- Funnel
- pH meter
Notes to instructor:
Have both solutions prepared in separate containers for students to use. Rinse out the glassware before use. Be sure to provide the acid dissociativity constant for hydrofluoric acid, which is \(3.5 \times 10^{-4}\). The corresponding \(pK_a\) value is \(3.45\) (Atkins, Jones 530).
Procedure:
- Equip goggles.
- Create a data table with columns reading as follows: volume of base added (milliliters), color of solution, pH of solution. Create eleven rows below this one.
- Make sure the burette valve is closed. With the funnel placed at the top of the burette, fill the burette with the basic solution.
- Pour the acidic solution into the flask.
- Put six drops of universal indicator into the solution. Fill out the first empty row in the table, where zero milliliters of base have been added.
- Position the burette above the flask.
- Titrate 5 milliliters of the base into the acidic solution. Record data for total base added (milliliters), the color of the solution, and the pH of the solution.
- Repeat Step 7 nine times, using up all of the basic solution.
- Rinse out all glassware. Dump the solution down a sink.
Sources:
Atkins, Peter and Loretta Jones. "Chemical Principles: The Quest for Insight" (2nd edition), W.H. Freeman and Company, New York, 2002.