Weak Diprotic/Dibasic Acid-Base Titration Curve

Experiment 6 Weak Dibasic Base-Strong Acid Titration Curves: Identification of an

Unknown Weak Base Purpose:

The purpose of this experiment is to identify a dibasic weak base by titrating with a strong acid. You will perform titrations of the type:

B + 2H3O+ BH22+ + 2H2O where B is your unknown dibasic base. The weak dibasic base is titrated with HCl (standardized with pure Na2CO3 in a previous experiment). During titration of the unknown base, the [H+] is measured by a pH sensor and the Vernier LabQuest data logger. The titration curve from plotting pH vs. mL of titrant added yields:

a) The two equivalence points (Ve and 2Ve) b) the base dissociation constants, Kb1 and Kb2 for the dibasic weak base c) the molecular weight of B

From this information and the list of possible unknown dibasic weak base, it should be possible to identify your unknown. Procedure:

1. Calibrate the pH electrode using pH 7.00 and 10.00 standard buffer solutions. 2. Weigh out ~0.1 g of the unknown sample into a 250 mL beaker. Add 50 mL of

deionized water. 3. Place the beaker on a magnetic stirrer and add a stirring bar. Turn on the stirrer to

dissolve the sample. 4. Connect the pH sensor to the Channel 1 of the Vernier Computer interface.

Connect the interface to the computer using the proper cable. 5. Set up a ring stand, buret clamp, and a 50 mL buret to conduct the titration. Rinse

and fill the buret with the previously standardized 0.1 M HCl. 6. Use a utility clamp and position the pH sensor in the HCl solution and adjust its

position so that it is not struck by the stirring bar. 7. Run the Logger Pro program on your computer. Open the file “07a Acid-Base”

from the Advanced Chemistry with Vernier folder. 8. You are now ready to begin the titration. 9. Titration:

- Record the initial pH by entering 0.00 mL volume of the titrant. - Continue adding HCl solutions in increments and record data points whenever

the pH drops by about 0.25 units or you add about 1 mL, whichever comes first. This process becomes faster if one person manipulates and reads the burette while another person reads the pH and enters the data on the Logger Pro program.

- During pH 9 – 7 and 5 – 3, change the additions to dropwise increments. - Continue through the initial drop in pH, through the buffer regions and

through the two equivalence points

- Stop the titration when the curve levels off around pH 2. Be careful close to and

- Repeat the titration, paying particular attention to the region around the equivalence points. If the total volume of titrant in the first titration was less than about 10 to 15 mL, use of a larger sample in the second titration will result in a smaller relative error.

Calculations & Results:

1. Use the molarity of the HCl solution from previous standardization titration experiment with pure Na2CO3 for further calculations.

2. Determine the pH at the two equivalence points from the second derivative titration curve.

3. On your titration curve, draw reference lines corresponding to first half- equivalence point volume and the second half-equivalence point volume.

4. Determine the pH values on the vertical axis that correspond to each of these volumes. These values are the pKa1 and pKa2 values, respectively.

5. From the pKa1 and pKa2 values in the previous step, calculate pKb1 and pKb2 values of the unknown dibasic base sample.

6. Calculate Kb1 and Kb2 for the dissociation of the unknown dibasic base sample and the standard deviation.

7. From the appropriate equivalence point and using stoichiometry, calculate the number of moles of the unknown dibasic base sample titrated.

8. Calculate the molar mass of the unknown dibasic base sample and the standard deviation.

9. Identify your unknown dibasic base sample by comparing your calculated values and the table below and write a brief explanation or justification.

Name & Formula of Selected Bases Kb1 Kb2 Molar Mass (g/mol)

Ethylenediamine, H2N(CH2)2NH2 8.47 x 10-5 7.04 x 10-8 60.12

1,2-Propylenediamine, H2N(CH2)3NH2 3.1 x 10-4 3.0 x 10-6 74.15

Sodium oxalate, Na2C2O4 1.85 x 10-10 1.8 x 10-13 134.00

Sodium carbonate, Na2CO3 2.13 x 10-4 2.24 x 10-8 105.99

Sodium malonate, NaO2CCH2CO2Na 4.98 x 10-9 7.04 x 10-12 148.03

Sodium succinate, NaO2C(CH2)2CO2Na 4.33 x 10-9 1.61 x 10-10 162.06

Potassium phthalate, KO2CC6H5CO2K 2.56 x 10-9 8.93 x 10-12 242.32

Piperazine, C4H10N2 5.38 x 10-5 2.15 x 10-9 86.14