MHS AP Chemistry
Titration of a Polyprotic Acid

In this lab, you will make a 0.1 M solution of a polyprotic acid.  Possible acids are phosphoric, maleic, tartaric, or another suggested by your teacher.  You will then titrate the acid to determine Ka1, Ka2, and possibly Ka3.

Part 1 - Preparation

  1. Determine the amount of solute necessary to make a reasonable volume of 0.10 M solution.  Make sure there is as little excess as possible.  You may wish to share your solution with another group for convenience.
  2. Use the molecular weight of the acid given on the bottle.  Concentrated phosphoric acid is 14.8 M.
  3. Make a 0.10 M solution of your acid.
  4. Calibrate your pH meter using fresh buffer solutions.
Part 2 - Titration
  1. Using a 0.1 M solution of NaOH, titrate a sample of your acid until the pH is about 13.  Keep a record of pH vs. total mL of base added.
  2. Repeat for three trials.  Make sure you have as many points as possible in the ranges where the pH changes fastest.
  3. Rinse each solution down the drain with excess water.
  4. Rinse your buret with clean water several times, and leave it upside down in the clamp with the valve open.
Part 3 - Analysis
  1. Graph the data from each trial on separate sheets.  Put mL of base on the x-axis and pH on the y-axis.  The graph should be landscape and fill the page.
  2. Use a ruler to draw a vertical line at the steepest parts of each curve.
  3. Use this informatoin to determine Ka1, Ka2, and possibly Ka3 for each sample.
  4. Calculate your average Ka's and percent errors from the accepted Ka's.

[Syllabus 17][MHS AP Chem page]