Experiment 17

1.	Safety
2.	Objectives/Overview
3.	Procedures
4.	Observations
5.	Equipment
6.	View Techniques
7.	Waste Disposal/Clean-up
8.	Conclusions
9.	Calculations/Set-Ups
10.	Grading Scale
11.	Review Prelab Questions
12.	Review Postlab Questions

Experiment 17
Titration of a Diprotic Acid: Identifying an Unknown

PROCEDURES

1. Obtain and wear goggles and do not forget to make observations.

2. Weigh out about 0.120 g of the unknown di-protic acid in a weighing container. Record the masses to the nearest 0.001 g. on the data page. Transfer the unknown acid to a 250-mL beaker and dissolve in 100 mL of distilled water. You may need to use a few milliliters of distilled water to rinse any powder sticking to the weighing boat into the beaker. The additional water will not change the quantity of acid present.

3. Place the beaker on a magnetic stirrer and add a stirring bar. At the end of lab, be sure to leave the stirring bar on the magnetic stirrer.

Figure 2

4. Obtain a 50-mL burette and rinse the burette three times with 5 mL of the standard 0.1000 M NaOH solution. The waste should be placed in a waste beaker at your station. Record the concentration of the NaOH solution on the data page. Use a utility clamp to attach the burette to the ring stand as shown in Figure 2. Fill the burette a little above the 0.00-mL level of the burette. Drain a small amount of NaOH solution into a waste beaker so it fills the burette tip and so the burette volume reads 0.00 mL. Be sure that there are no air bubbles in the tip of the burette. At the end of lab, pour the waste into the labeled container on the side bench. Your instructor will neutralize the waste and then flush it down the sink drain with an equal amount of water.

5. Open LoggerPro 3.4.6 on the desktop. From the “FILE” pull down menu, choose “Open” > ”Chemistry with Computers” > “Exp. 25a Titration Dip Acid”. Check the layout of the computer screen for this experiment. The vertical axis of the graph has pH scaled from 0 to 14 pH units. The horizontal axis has volume scaled from 0 to 25 mL. The live display below the data table shows pH. The value appearing in the live display is for the un-calibrated pH probe in a “keeper” solution. Please store the probe in the solution at all times except when it is being used.

6. Use a utility clamp to suspend a pH electrode on a ring stand as shown in Figure 2. Position the pH electrode in the acid solution and adjust its position toward the outside of the beaker so that it is not struck by the stirring bar.

CAUTION: the electrodes are fragile. Please handle them with great care!

7. Your first job is to calibrate the probe. Choose “Calibrate” from the “EXPERIMENT” pull-down menu. The window “LabPro:1CH1:pH” will appear. Click on the window. The “Sensor Settings” dialog box will pop up. Under the “Calibrate” tab click on “Calibrate Now.” Remove the probe from the keeper solution and rinse thoroughly with distilled water from a squeeze bottle. Very gently blot the probe dry with a “Kimwipe”. Immerse the probe in 20 mL of pH 4 buffer in a 50-mL beaker. Enter the number 4.00 in the “Reading 1” data entry box. Gently, use the probe to stir the solution continuously. When the voltage value displayed under the “Choose Sensor(s) to Calibrate” stabilizes, click “Keep”. Now rinse the probe again with distilled water, gently blot it dry, and immerse it in 20 mL of pH 10 buffer contained in another 50-mL beaker. Enter the number 10.00 in the “Reading 2”data entry box. Gently, use the probe to stir the solution continuously. When the voltage value displayed under the “Choose Sensor(s) to Calibrate” stabilizes, click “Keep”. Finally click on DONE to exit the “Sensor Settings” dialogue box. The probe is now calibrated, however, if you quit or close the program at any time, the calibration will be lost and it will have to be repeated. If you need to start over, Click on the “Collect” button at the top of the screen. Select “Erase and Continue” from the “Erase Data? dialog box . Alternatively, “Clear All Data” command or “FILE”, “New” and OK the erasure of data.

8. You are now ready to begin the titration. This process goes faster if one person manipulates and reads the burette while another person operates the computer and enters burette readings.

Before adding any NaOH titrant to the unknown, click on the "Start" button and monitor pH for 5-10 seconds. After the pH has stabilized, click on the "Collect" button and then the “Keep” button. Enter the current burette level, “0.00,” in the edit box. Press the “Enter” key. You have now stored the first data pair, which should appear in the data table.
Add enough NaOH to raise the pH by about 0.20 units. After the NaOH has been added, wait for the pH to stabilize and then click the "Keep" button. Enter the new burette reading in the edit box, to the nearest 0.01 mL.

ALWAYS read the burette to 2 decimal places!
Continue adding NaOH solution in increments that raise the pH about 0.20 unit. Enter the burette reading after each addition. Proceed in this manner until the pH is 3.5.
When pH 3.5 is reached, change to 2-drop increments. Enter the burette reading after each increment.
After pH 4.5 is reached, again add larger increments that raise the pH by about 0.20 units and enter the burette reading after each addition. Continue in this manner until a pH of 7.5 is reached.
When pH 7.5 is reached, change to 2-drop increments again. Enter the burette reading after each increment.
When pH 10 is reached, again add larger increments that raise the pH by 0.20 units. Enter the burette reading after each increment. Continue in this manner until you reach a pH of 11.4. Click the "Stop" button.

DO NOT "CLOSE" OR "QUIT" THE PROGRAM! Your instructor will clear the data from the screen at the end of class.

PROCESSING THE DATA
1. Select “Graph Options” from the "OPTIONS" pull-down menu. Check the “Connect Points” option found under the “Appearance” box of the “Graph Options” tab. Click on DONE. You can also adjust the scales of the x and y axes by selecting “Axis Options” tab. This will allow you to display all of the titration data with as much enlargement as possible. Ask your instructor if you have trouble.

2. You are almost ready to print your graph. Select "Print" from the "FILE pull-down menu. The “Printing Options” dialog box will pop up. Enter both your names. Under “Comments” enter the experiment number (Experiment 21) and your laboratory section number. Click OK. Since you are to print two copies of the graph, one for you and one for your partner, enter the number 2 in the “Number of Copies” cell. Then click OK. Have your instructor verify that your graph is acceptable. Then, close the program. Answer NO to the question “Do you want to save the changes you made to 25a. Titration Dip Acid.cmbl”?

3. On your graph, the second equivalence points is probably more clearly defined than the first; the two-drop increments near the equivalence points frequently result in larger increases in pH (a steeper slope) at one equivalence point than the other. Indicate the more clearly defined equivalence point (first or second) on line 5 of your data page.

4. Use your graph and the data table on the screen to determine the volume of NaOH titrant used for your most clearly defined equivalence point. To do so, find the largest value in the ΔpH/ Δvol column (d1) of the data table. Then find the NaOH volume before the pH jump and the volume after the pH jump. Record these two volumes on the data page in your report.

5. To determine the volume of NaOH added at the equivalence point, add the two NaOH volumes determined in Step 4 above, and divide by two. For example:

6. Calculate the number of moles of NaOH used at the equivalence point you selected.

7. Determine the number of moles of the di-protic acid, H₂X. Use Equation 3 or Equation 5 to obtain the ratio of moles of H₂X to moles of NaOH, depending on which equivalence point you selected in Line 5.

8. Using the mass of di-protic acid you measured out in Step 1 of the procedure, calculate the molar mass of the di-protic acid, in g/mol.

9. From the following list of five di-protic acids, identify your unknown.

Diprotic Acid	Formula	Molecular Weight
Oxalic Acid	H₂C₂O₄	90
Malonic Acid	H₂C₃H₂O₄	104
Maleic Acid	H₂C₄H₂O₄	116
Malic Acid	H₂C₄H₄O₅	134
Tartaric Acid	H₂C₄H₄O₆	150

10. Calculate the percent error between the molar mass value in Step 8 and the acid closest to it in Step 9.

11. Determine the volume of NaOH titrant used to reach the alternate equivalence point (the one you did not use in previous calculations). Use the same method as for the previous determination: examine the data table to find the largest increase in pH values during the 2-drop additions of NaOH. Find the NaOH volume just before and after this jump. Record these volumes on the data page. Note: Dividing or multiplying the other equivalence point volume by two may help you confirm that you have selected the correct two data pairs in this step.

12. On your graph, clearly specify the position of the equivalence point volumes you determined in Steps 5 and 11, using dotted reference lines like those in Figure 1. Specify the NaOH volume of each equivalence point on the horizontal axis of the graph. Also be sure to label the axes of your graph and the lines you just drew.