Standardizing a Solution of Sodium Hydroxide
Using a pH Sensor
Materials
MaterialsLabQuest interface / wash bottle containing distilled water
ring stand
Vernier pH Sensor / utility clamp
50 mL buret / 250 mL beaker
buret clamp / buret clamp
Figure 1
1. Set up a ring stand, buret clamp, and buret to conduct a titration (see Figure 1). Rinse and fill the buret with the titrant. Make sure there are no bubbles in the buret. Allow titrant to flow into a waste beaker.
2. Rinse the pH sensor with distilled water using water bottle, allowing the waste water to go into the waste beaker. Use a nickel as a cap to protect the buffer solution from spilling.
3. Use an electrode support to suspend the pH Sensor on a ring stand as shown in Figure 1. Position the pH Sensor in the KHP solution and adjust its position so that it is not struck by the stirring bar. Add sufficient water to make sure the pH sensor is immersed in the solution. Gently stir the beaker of solution.
4. Connect a pH Sensor to Channel 1 of the LabQuest interface. Turn on the LabQuest. The screen should show CH 1: pH.
5. Set up the data-collection mode.
a. To select MODE, tap the box on the right side of the screen
b. Select EVENTS WITH ENTRY from the SELECT MODE menu.
c. Set NAME to Volume and UNITS to mL
d. Select OK to return to the Main screen.
6. Start the graph by pressing the play button
a. Before you have added any titrant, press “keep” and type in “0” as the buret volume in mL. Save this as the first data pair for this experiment.
b. Add the next increment of titrant (enough to raise the pH about 0.15 units). When the pH stabilizes, press “keep” and enter the current buret reading as precisely as possible. Save this as the second data pair for this experiment.
c. Continue adding the titrant in increments that raise the pH by about 0.15 units and enter the buret reading after each increment.
d. When the pH values begin increasing more rapidly (e.g. pH of about 3 for strong acid/strong base or about 6 for weak acid/strong base), change to 1-3 drop increments. Enter a new buret reading after each increment. As you reach the equivalence point, 1 drop increments will provide you with the most accurate data.
e. After a pH value of approximately 10 is reached, again add larger increments that raise the pH by about 0.15 pH units, and enter the buret level after each increment.
f. Continue adding titrant until the pH value remains constant.
7. Press stop when you have finished collecting data.
8. Use your titration data to find the equivalence point, which is the largest increase in pH upon the addition of a very small amount of NaOH solution. A good method of determining the precise equivalence point of the titration is to take the second derivative of the pH-volume data, a plot of D2pH/Dvol2.
(Note: An alternate method is to transfer your data to Logger Pro first, then use Logger Pro functions to derive the second derivative of the pH-volume data.)
a. Select the data table from the tabs at the top of the screen.
b. Tap the word table at the top of the screen and create a calculated column
c. Set the equation to Second Derivative. It should be all the way at the bottom of the list
d. Analyze the data table. The point where it goes from positive to negative is the equivalent point.
9. Attach your USB stick to the LabQuest and save your data to the USB. To do this press file, then save and tap the USB icon to save your data there.
10. Transfer the data to a computer using Logger Pro software.
Print two (or three) copies of your graph of pH vs. volume of NaOH (1 for you, 1 for your
partner, 1 for your instructor.
Advanced Chemistry with Vernier 6-5