Acid Identification from pH Titration Curves

Acid Identification from pH Titration Curves
Purpose and Introduction
Most acids are capable of donating one proton while others can donate as many as 2 or 3 protons.
These acids are referred to as monoprotic, diprotic and triprotic acids respectively. In a reaction
with a strong base such as NaOH, only one equivalent of hydrogen ion reacts at a time. When all
of the acid molecules in solution have lost one proton, the second proton will then be removed
with a second equivalent of base. This can be shown using a titration curve that plots the volume
of base added vs. the pH of the acid solution. The equivalence point is found when there is a
significant increase in pH of the acid solution under investigation with the addition of a small
amount of base. This can be located on a titration curve by plotting the first derivative of the
curve plot. This will have a sharp peak at the steepest part of the titration curve thus indicating
the exact equivalence point.
Many polyprotic acids will show several equivalence points on a titration curve. For weak acids,
prior to each equivalence point is a region in which the pH changes very little even when
relatively large amounts of base have been added. These are called buffer regions. A buffer
solution consists of a weak acid and its conjugate base in equilibrium and resists changes in pH.
For some polyprotic acids, only some of the equivalence points are easily observed in a titration
curve and in some cases are not detectable at all. This usually occurs when the pH at the
equivalence point is not significantly different from the pH of the titrant solution. The change in
pH upon reaching the equivalence point is not any greater than the change observed before and
after the equivalence point and so goes unnoticed.
The purpose of this laboratory is to determine the identity of a set of unknown acids from the
resulting titration curve. will be performed using a . The student will
measure and record the pH of an acid solution as a base solution is added. The pH will be
measured after every 1 mL of base is added to the acid solution. The data will be plotted using a
Safety/Waste Disposal
Safety goggles MUST be worn at all times when chemicals are in use.
Be cautious with all solutions. Some indicators may stain skin/clothing.
Any skin contact with solutions should be rinsed off as soon as possible.
All waste can be rinsed down the sink with water.
Chemicals and Equipment
100 mL volumetric flask
Plastic dropper
10 mL graduated cylinder
50 mL buret and clamp
250 mL Beaker
pH meter
0.10 M unknown acids (2 per station)
1. Calibrate the pH meter.
2. Fill a with 0.10 M NaOH. Open the stopcock to fill the tip of the buret and
remove any air bubbles. Fill the buret to the 0.00 mL line.
3. Place 10.00 mL of one of the 0. in the 250 mL beaker. Dilute with
deionized water so that the depth is about inch.
4. Insert the into the acid solution and record the pH of the Unknown
acid solution for 0.00mL of titrant.
5. Gently open the stopcock on the buret to add 1.00 mL of titrant. Record the pH of the
solution in Table 1 for 1.00 mL of titrant.
6. Continue to add titrant as shown in #5 until 25 mL of NaOH has been added to the
unknown acid. [Note: the pH reading should be around 10-12 at this point. If it is not,
add several more 1.00 mL aliquots of NaOH until the reading is in the 10-12 range]
7. Repeat the procedure for the second Unknown acid and record the results in Table 2.
8. Rinse the buret with deionized water before putting it away.
Table 1 Unknown Acid_______ Titration Data
Added pH Volume
Added pH Volume
Added pH
0.00 9.00 18.00
1.00 10.00 19.00
2.00 11.00 20.00
3.00 12.00 21.00
4.00 13.00 22.00
5.00 14.00 23.00
6.00 15.00 24.00
7.00 16.00 25.00
8.00 17.00 26.00
Table 2 Unknown Acid_______ Titration Data
Added pH Volume
Added pH Volume
Added pH
0.00 9.00 18.00
1.00 10.00 19.00
2.00 11.00 20.00
3.00 12.00 21.00
4.00 13.00 22.00
5.00 14.00 23.00
6.00 15.00 24.00
7.00 16.00 25.00
8.00 17.00 26.00
2 93 3 05 3.51
2 89 3.09 3 58
2 92 3 it 5.20
2 90 3.16 3 SO
Z 91 3.20 4 OO
Z 95 3.25 U 25
Z 95 3.29 S UJ
2 97 3 35 62
3 01 3.47 12.48
27.00 12.72
28 OO 12.88
3 28 12 96 13.3g
3 33 13.09 13 40
3 42 13.12 13 43
3 62 13.18 13 44
3 78 dB 22 13.46
5 38 13.26 13 47
12.14 13 30 13 51
12 61 3.32 13 SO
12 76 13.35 13 52

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