CONTINUOUS TIME pH TITRATION PROCEDURE

1. Prepare the solution to be titrated as directed by your teacher.

2. Place the beaker onto a magnetic stirrer and add a small stirring bar.

3. Prepare the pH system for data collection.

• Plug the pH amplifier into the adapter cable in Channel 1 of the CBL. The pH electrode is already connected to the pH amplifier.
• Connect the CBL System to the TI-8X calculator with the link cable using the port on the bottom edge of each unit. Firmly press in the cable ends.

Prepare the buret by rinsing it first with distilled water and then with a sample of your titrant solution. Fill the buret above the zero with titrant solution being careful to limit the number of bubbles in the solution. Dispense some of the titrant solution out of the buret into a "spent chemical" beaker until the volume reads "0".

4. Turn on the CBL unit and the TI-8X calculator. Press PRGM and select CHEMBIO. Press ENTER, then press ENTER again to go to the MAIN MENU.

5. Set up the calculator and CBL for pH measurement.

• Select SET UP PROBES from the MAIN MENU.
• Enter "1" as the number of probes.
• Select PH from the SELECT PROBE menu.
• Enter "1" as the channel number.
• USE the STORED CALIBRATION, unless otherwise directed by your teacher.

6. Set up the calculator and CBL for data collection.

• Select COLLECT DATA from the MAIN MENU.
• Select TIME GRAPH from the DATA COLLECTION menu.
• Enter "3" as the time between samples, in seconds.
• Enter "80" as the number of samples. The CBL will collect data for a total of 4 minutes.
• IF ASKED
• Enter "0" as the minimum pH (Ymin).
• Enter "14" as the maximum pH (Ymax).
• Enter "1" as the pH increment (Yscl).

7. You are now ready to begin monitoring data. Follow the directions on the calculator screen to allow the CBL to warm up. Note: To determine the elapsed time when the indicator changes color in Step 8, monitor the time (in seconds) on a clock as soon as you press ENTER.

Press ENTER to begin data collection. Carefully open the buret stopcock to provide a dripping rate of about 1 drop per second. Do not worry if the rate is somewhat faster or slower when you first start; initial additions of titrant will have very little effect on the pH.

8. Watch to see if the indicator changes color before, at the same time, or after the rapid change in pH at the equivalence point. If phenolphthalein is a suitable indicator for this reaction, it should change from clear to red at about the same time as the jump in pH occurs. In your data table, record the elapsed time when the phenolphthalein color change occurs.

9. When you have finished collecting data after 3 minutes ("DONE" is displayed on the CBL), turn the buret stopcock to stop the flow of titrant. Press ENTER to display a graph of pH vs. time. Use RIGHT ARROW to examine the data points along the curve. As you move the cursor right or left, the time (X) and pH (Y) values of each data point are displayed below the graph. Determine the approximate time for the equivalence point; that is, for the biggest jump in pH in the steep vertical region of the curve. Record this time in the data table. Rinse the pH electrode and return it to the electrode storage solution. Dispose of the beaker contents as directed by your teacher. Clean and dry the 250-mL beaker for the next trial.

10. When you are finished, rinse the pH electrode with distilled water and return it to the electrode storage solution.