Acid-Base Titrations Using pH Measurements
Prelab
1.
What is the purpose of this experiment?
2.
The following data were collected in the titration of 10.0 mL of 0.10 M weak acid, HA,
with 0.10 M NaOH solution. Tabulate and plot three graphs: titration curve, first
derivative and second derivative, and find the equivalence point.
mL 0.10 M NaOH
pH
0.00
1.00
2.50
5.00
7.50
8.75
9.50
9.75
9.90
10.00
10.10
10.25
10.50
12.75
15.00
3.00
4.05
4.52
5.00
5.48
5.85
6.28
6.59
7.00
8.85
10.70
11.09
11.39
12.08
12.30
An example of the procedure is given below
v (mL NaOH) pH
v ' (mL) f ' (ΔpH/v)
20.50
21.00
21.50
22.00
22.50
20.75
21.25
21.75
22.25
11.59
11.60
11.62
11.68
11.72
0.02
0.04
0.12
0.08
v ' is the average of two consecutive volumes:
v'' (mL)
21.00
21.50
22.00
f '' (Δ(ΔpH) /v’)
0.04
0.16
–0.08
21.00 + 20.50
= 20.75
2
and f ' is calculated by taking the difference in pH and dividing by the difference in
volume of NaOH.
11.60  11.59
0.01
=
= 0.02
21.00  20.50
0.50
1
21.25 + 20.75
= 21.00
2
v '' is the average of consecutive average volumes:
and f '' is calculated by taking the difference in f ' and dividing it by the difference in v'.
0.04  0.02
0.02
=
= 0.04
21.25  20.75
0.50
A spreadsheet to do this calculation for all your tabulated data looks like:
A
B
C
D
E
1 Volume
pH
2
3
4
5
6
7
B2
B3
B4
B5
B6
B7
A2
A3
A4
A5
A6
A7
f' (pH/v)
v'' (mL)
f'' (pH2/2v)
=(B3-B2)/(A3-A2)
=(B4-B3)/(A4-A3)
=(B5-B4)/(A5-A4)
=(B6-B5)/(A6-A5)
=(B7-B6)/(A7-A6)
–
=(C2+C3)/2
=(C3+C4)/2
=(C4+C5)/2
=(C5+C6)/2
–
–
=(D3-D2)/(C3-C2)
=(D4-D3)/(C4-C3)
=(D5-D4)/(C5-C4)
=(D6-D5)/(C6-C5)
–
–
v' (mL)
=(A2+A3)/2
=(A3+A4)/2
=(A4+A5)/2
=(A5+A6)/2
=(A6+A7)/2
–
F
Note: The last cell in C and D and the last two cells of E and F will contain no data.
Your graphs, which will be used as a template for your lab data, should look like:
Titration Curve of 0.1 M HA with 0.1 M NaOH
First Derivative of Titration Curve
25
13
12
11
20
10
9
ΔpH/ΔmL
7
6
5
15
10
4
3
5
2
1
0
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
1
2
3
4
5
volume NaOH (mL)
6
7
8
9
average volume (mL)
Second Derivative of Titration Curve
150
100
50
Δ2pH/ΔmL2
pH
8
0
0
1
2
3
4
5
6
7
8
9
10
-50
-100
-150
average average volume (mL)
2
11
12
13
14
15
10
11
12
13
14
Acid-Base Titrations Using pH Measurements
Introduction
When you titrate a weak acid with a strong base, such as NaOH, the reaction will go essentially
to completion, giving an acidic solution until just before the equivalence point
C2H3O2– + H2O
OH– + HC2H3O2
(1)
At the equivalence point, the pH will not be 7. This is because the conjugate base, acetate ion, is
the major species in solution.
In this experiment, you will be titrating acetic acid. A pH electrode will be used to determine the
equivalence point. A plot of pH vs. mL NaOH added will show an inflection point at the
equivalence point. The size of the inflection point depends on the Ka of the acid. If the acid is
too weak, this change is so small that it may not be seen.
In order to clearly find the equivalence point, you will plot the first and second derivatives of the
titration curve. This will make the equivalence point easier to locate.
Rough Titration of Acetic Acid
1.
Start LoggerPro. Plug in a pH electrode to a lab interface port on the right side of the
netbook (white Labquest Mini interface) and check to see that the electrode is recognized.
Click Experiment, Data Collection. On the menu, on the Collection tab, change Mode:
from Time Based to Events With Entry (on pull down menu). On the new menu, make
Column Name: Volume, Short Name: Volume, Units: mL. Click Done. This sets pH
data to be collected as a function of volume of standard NaOH added.
2.
Keeping the buret in its clamp, rinse a with a few milliliters of the provided standardized
sodium hydroxide solution (drain and discard), fill to the top and remove the air bubble
from the tip. Make sure you record the molarity of this sodium hydroxide solution
since you will need it to calculate the molarity of the acetic acid solution. Record the
initial volume to 0.01 mL. Do not start at 0.00 mL. Make a table in your notebook of
buret reading, total added volume and pH.
3.
Obtain an unknown acetic acid solution and record its letter. Pipet 25.00 mL of the
solution into a 250-mL beaker. Add 25 mL of distilled water (graduated cylinder) and
two drops of phenolphthalein to help visualize the equivalence point.
4.
Place a stir bar in the beaker and set it a little off to one side on a stir plate. Suspend the
pH electrode in the solution with an electrode clamp and position it to the same side as
the offset of the beaker (so the stir bar does not hit it). Adjust the stir rate to about 300
rpm (no splashing). Position the buret tip about 1 ½ inches from the solution in the
beaker, over the stir bar’s vortex. Keep the pH electrode in the solution throughout
the following steps.
3
5.
Click
. When the pH stabilizes (should not take long), click
dialog box will appear asking you to enter the volume. Enter 0, then click OK.
. A
6.
Add about 1 mL of NaOH solution from the buret. Record the final volume to 0.01 mL.
7.
When the pH has stabilized, click
. Enter the total volume added (final reading
minus initial reading), and then click OK. Both partners should record all data in your
notebooks.
8.
Repeat steps 6 and 7. Enter the total volume added up to that point each time. You will
see a sharp increase in the pH of the solution at the equivalence point. Go past the
equivalence point in the same way until the pH reaches about 11. Click
.
9.
Click Analyze, Interpolate. Find the approximate equivalence point (mL). Clear the
data (do not save): on the file menu click Data, Clear All Data.
The titration will be repeated, but with more data recorded in near the equivalence point.
Good Titration of Acetic Acid
10.
Refill the buret and record the initial volume of the buret to 0.01 mL. Make a table in
your notebook of buret reading, total added volume and pH.
11.
Pipet 25.00 mL of unknown solution into a clean 250-mL beaker. Add 25 mL of distilled
water (graduated cylinder).
Do not add any indicator. Set up the beaker, pH
electrode and buret as in step 4.
12.
Click
. When the pH stabilizes (should not take long), click
Enter 0 in the dialog box, then click OK.
13.
Add about 1 mL of NaOH solution from the buret. Record the final volume to 0.01 mL.
14.
When the pH has stabilized, click
. Enter the total volume added (final reading
minus initial reading), and then click OK. Record the data in your notebook.
15.
Repeat 13 and 14. When you are within about 5 mL of the equivalence point
(determined by interpolation of the first titration), add the NaOH in increments of about
0.5 mL for the next 3 mL, then in increments of about 0.2 mL or less for the next 2 mL.
Continue for 5 mL on the other side of the equivalence point in a mirror image (about
0.2, then 0.5 mL increments). Record all volumes to 0.01 mL accuracy. Save the data.
4
.
Data Treatment
Using the same spreadsheet from the prelab, tabulate just the second set of titration data.
Calculate the first and second derivatives. Plot pH vs. mL of NaOH solution added, the first
derivative plot and the second derivative plot.
The equivalence point is found at the volume corresponding to the x-intercept of the second
derivative curve. This point should be coincident with the inflection point (the point at which
the curve changes direction) of the original data plot and the peak of the first derivative plot.
Read the equivalence point volume, to 4 significant figures, off the graph. Expand the x axis
(and attach the plot) to read it accurately.
Calculate the molarity of the acetic acid solution using the equivalence point determined from
the graph. The volume of the solution was 25.00 mL. Do not include the 25 mL of water added
from the graduated cylinder; you are titrating moles.
Determine the Ka of acetic acid from the pH at the half-titration point.
Conclusion
Give the molarity of the acetic acid solution. Be sure to give the unknown letter. Give the Ka of
acetic acid.
Also address:
What is the advantage to using a pH electrode and meter to find the equivalence point versus a
visual indicator?
5