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Procedure

Safety You will be handling strong acids and bases in this lab.  Wash your hands immediately if you get some acid or base on your skin. Notify your instructor immediately regarding any spills that occur. You must wear goggles at all times.

Part I: Preparing the Buret

Collect a buret, pH meter, stir plate and several clean dry 100 ml beakers.

Obtain about 65-75 mL of 0.10 M NaOH

Clean a 50 mL buret and rinse it with the NaOH solution as follows:

Add a few mL of the titrant and tip the buret to allow the solution to run almost all the way to the open end.

Then rotate the buret slowly, so that all surfaces contact the solution.

Drain this solution into a waste beaker by opening the stopcock.

Repeat this procedure two more times, using no more than 15-20 mL total.

Fill the buret with the NaOH, and open and close the stopcock to force air bubbles out of the tip.

Use the buret clamp to place your buret on the ring stand.

Part II: The Titrations

Using a graduated cylinder, obtain 3 separate samples of 13 mL of the unknown acid in three 125 mL Erlenmeyer flasks.

Table of Possible Unknowns:

Name
Formula
Ka1
pKa1
Ka2
pKa2
Ka3
pKa3
Phosphoric acid
H3PO4
7.5 x 10-3
2.12
6.2 x 10-8
7.21
4.8 x 10-13
12.32
Citric acid
H3C6H5O7
8.4 x 10-4
3.08
1.8 x 10-5
4.74
4.0 x 10-6
5.40
Carbonic acid
H2CO3
4.3 x 10-7
6.37
5.6 x 10-11
10.25
Sulfuric acid
H2SO4
Large
1.2 x 10-2
1.92
Sulfurous acid
H2SO3
1.5 x 10-5
4.82
1.0 x 10-7
7.00
Hydrosulfuric acid
H2S
1.0 x 10-7
7.00
1.3 x 10-13
12.89
Ascorbic acid (Vitamin C)
H2C6H6O6
7.9 x 10-5
4.10
1.6 x 10-12
11.80

 

Add a few drops of phenolphthalein to each flask.

Use a utility clamp to suspend a pH electrode on the ring stand.  Obtain a short stir bar, stir plate, and set up your first flask for titration.

Calibrate the pH meter for data collection based on your instructor’s directions.

Turn on the stir plate.  The purpose is to gently and continuously mix the acid and base as it is added. Stir the solution carefully. Avoid striking the pH probe.  The probe is delicate and expensive.

You are now ready to begin the titration.

Read your buret to two decimal places, being sure that your eye is level with the meniscus.

Add 2 mL of 0.10 M NaOH with constant stirring.

Record the pH after each addition.  Continue adding NaOH until you reach a point where the pH no longer changes (> pH 11).

Buret Initial Reading

Observations

Titration 1: Data Collection

Buret Reading (mL)
pH
Buret Reading (mL)
pH
Buret Reading (mL)
pH

 

From the data collected above, you should be able to identify the region of the titration where the pH changes very slowly and also the region of the equivalence points.  You will now repeat the titration.  The goal of this second titration is to generate enough data points to create an accurate and smooth curve.
Refill the buret if necessary, note the volume and begin the second titration as previously instructed.

Buret Initial Reading

Observations

Titration 2: Data Collection

Buret Reading (mL)
pH
Buret Reading (mL)
pH
Buret Reading (mL)
pH

Repeat the titration a third time.

Buret Initial Reading

Observations

Titration 3: Data Collection

Buret Reading (mL)
pH
Buret Reading (mL)
pH
Buret Reading (mL)
pH

Clean up Any titrated solutions may go down the drain with LOTS of water. Wash your hands thoroughly before leaving the lab.