Chem 152 Lab 1

After rinsing the buret thoroughly with tap and DI water, rinse the buret twice with ~3 mL portions of ~0.1 M NaOH. Then fill the buret with this NaOH solution. Be sure that the stopcock is in the closed position and that it is not leaking before you fill the entire buret.

Make sure there are no bubbles in the tip of the buret. With the buret partially filled, you can open the stopcock and gently, but firmly, give the buret one vertical shake to dislodge the bubble, allowing some NaOH to drain into the beaker that contains the rest of your NaOH. Then close the buret and continue filling it to a level between 0 and 1 mL.

The graduated markings on a buret increase as you move down the buret so that you can easily read the volume of solution that has been delivered from the buret. The initial level does not have to be exactly zero, but you must know exactly what the level is. In this example, you should be able to read the volume to two decimal places : 2.65 mL.

For Part I, you will weigh out ~0.40 solid KHP, recording the exact mass of your sample in your notebook. Transfer the KHP to a clean 100 mL beaker, using a wash bottle with DI water to ensure all of the crystals are transferred to the beaker.

Add ~20 mL DI water, a stir bar, and 2 drops of phenolphthalein pH indicator. This indicator will change from clear (for an acid) to pink (for a base) as the titration reaction proceeds. (An Erlenmeyer flask is shown here and in the pictures below, but you will use a beaker in this lab.)

Place the beaker containing dissolved KHP, the indicator, and the stir bar on the magnetic stirrer below the buret filled with NaOH.

Begin the titration by turning the stopcock and allowing NaOH to drain from the buret into the beaker. Follow the procedure, making sure not to add the NaOH too quickly near the endpoint.

The endpoint of the titration has been reached when the solution turns a faint pink color that does not disappear.

If you add too much NaOH (called over-titrating) the pink color will intensify. Over-titrating will introduce error in your data and ultimately in the calculations you will perform to determine the concentration of the NaOH or of the unknown acid solution.

In part 2 of this lab, you will follow the same titration procedure from part 1, but will be using 10 mL of acetic acid delivered to a clean beaker using a 10 mL volumetric pipet.  A pH probe will be used to monitor the pH of the solution in the beaker after every addition of NaOH.  Instead of adding phenolphthalein, you will add a couple drops of Thymol Blue, a different acid-base indicator, which will help warn you that your are approaching the equivalence point and need to proceed carefully.