6-6
then entering the buret reading). Continue until the volume added and pH approach
the values at the endpoint you determined with the phenolphthalein. When the pH
nears this value, add the NaOH in 0.5 and finally 0.2 mL increments. When the pH
passes the equivalence point, add the NaOH again in 1-2 mL increments until you
have added 35 mL.
6. You should have taken approximately 30 data points. Press STO> when you have
finished collecting data.
7. Transfer your data to the Logger Pro program and transfer the pH/volume data to an
Excel file for later analysis. Store the volumes in the A column and the pH values in
the B column of the spreadsheet, beginning in cells A1 and B1.
Part B: Titration of an Unknown Polyprotic Acid with NaOH
1. Transfer 15.00 mL of an unknown 0.1 M acid solution to a 250 mL beaker with a
15.00 mL volumetric pipet you have pre-rinsed with a small amount of the solution.
2. Fill a buret to the 0.00 mL mark with standardized NaOH.
3. Rinse the pH meter with DI water. Set up a titration as you did for Part A using the
Vernier equipment to measure the pH in the EVENTS WITH ENTRY mode
4. Titrate the unknown acid with NaOH using 1 mL increments of the base except near
the first endpoint (as indicated by the more rapid rise in pH) where the increments
should be 0.2 mL. After the first endpoint, increase the increments to 1 mL until the
second end point is approached. Continue the titration until the second end point has
been passed by about 5 mL of added NaOH
5. Transfer this data into Logger Pro as before and save it to an Excel file (volume in
column A and pH in column B) under a different name for later analysis.
For Next Lab: Prepare both the saturated Ca(OH)
2
and the saturated Ca(OH)
2
with
CaCl
2
⋅H
2
O solutions according to the directions given in Experiment #7. Cover with Parafilm
for use during next lab.
Treatment of Data:
Part A: Titration of Acetic Acid with NaOH
1. Using the saved data in your Excel file prepare a plot of pH vs. Volume of NaOH
added to observe the equivalence point and half equivalence point.
2. To verify the equivalence point, determine the inflection point by calculating the
change in pH per change in volume, ΔpH/ΔV, for each recorded volume, as shown in
Figure 2 for the NaOH/HCl titration. Plot ΔpH/ΔV vs. the volume of titrant added. To
perform this, check that your volume and pH values are in columns A and B on your
Excel spreadsheet. In cell C1, type: =(A1+A2)/2. Click on cell C1 and drag down to
the C cell that is on the same row as the last filled cell in columns A and B. Column C
now contains the average volume of titrant between two readings.
3. In cell D1, type: =(B2-B1)/(A2-A1) which will calculate the ΔpH/ΔV values.
Highlight cell D1 and drag to fill the remaining cells as you did with column C.
Column D now contains the values of ΔpH/ΔV for each volume reading. The
inflection point is found by plotting ΔpH/ΔV versus the volumes in column C.
4. Once the volume at the equivalence point is known, the volume of the half-
UCCS Chem 106 Laboratory Manual Experiment 6