Acid Base Titration Lab

Aim To determine the concentration of Acetic acid by titrating 10cm3 of acetic acid against a standard solution of NaOH(0.1 Molar) and hence calculate the Pka value.

ApparatusFunnel50 cm3 beaker10 cm3 pipette50 cm3 BuretteMagnetic StirrerClamp StandpH probe with LabproLaptop

Procedure1. Using the 10 cm3 Pipette measure out 10 ml of acetic acid (CH3COOH) and pour it into the 50

cm3 beaker.2. Attach the burette to the clamp stand.3. Using a funnel, pour the sodium hydroxide into the burette.4. Remove the Funnel.5. Add the magnetic stirrer into the 50 cm3 beaker and place it under the Burette.6. Open the Burette tap slowly to let the Base flow.7. Take pH readings every 0.5 or 1 cm3 of base that has flowed out by using the Labpro pH probe.8. Continue this process till the pH finally stabilizes9. Repeat this process 2 more time to minimize random error.

Concentration of NaOH = 0.1 MolarVolume of Acetic acid taken = 10 cm3

Data Processing and Collection

Raw DataThe table below gives the relation between the volume of base(Sodium Hydroxide) added and the pH value of the solution (Acetic acid – Sodium Hydroxide mixture) at that particular instant.

Volume(±0.1cm3)

(2 s.f.)

pH(reading 1)(±0.01 pH)

(3 s.f.)

pH(reading 2)(±0.01 pH)

(3 s.f.)

pH(reading 3)(±0.01 pH)

(3 s.f.)

0.5 2.53 2.53

1.0 2.68 2.65 2.19

1.5 3.11 2.71

2.0 3.16 2.67 2.78

2.5 3.64 3.02

3.0 3.42 3.11 3.46

3.5 3.78 3.04

4.0 3.79 3.23 3.71

4.5 3.83 3.39

5.0 3.88 3.51 3.77

5.5 3.89 3.46

6.0 4.06 3.68 3.83

6.5 4.25 3.83

7.0 4.30 3.83 3.61

7.5 4.26 3.71

8.0 4.38 4.14 3.97

8.5 4.12 4.17

9.0 4.32 4.37 3.80

9.5 4.63 4.52

10.0 4.69 4.59 4.09

10.5 4.83 4.34

11.0 4.89 4.71 4.27

11.5 4.91 4.82

12.0 5.20 4.94 4.51

Volume(±0.1cm3)

(2 s.f.)

pH(reading 1)(±0.01 pH)

(3 s.f.)

pH(reading 2)(±0.01 pH)

(3 s.f.)

pH(reading 3)(±0.01 pH)

(3 s.f.)

12.5 5.40 5.71

13.0 5.99 6.03 4.73

13.5 6.44 6.36

14.0 6.94 6.99 5.90

14.5 7.12 7.47

15.0 7.71 8.04 6.27

15.5 8.81 8.22

16.0 9.67 9.01 8.29

16.5 10.3 9.65

17.0 10.3 9.71 8.77

17.5 10.5 9.88

18.0 10.9 10.1 9.56

18.5 11.0 10.1

19.0 10.8 10.2 10.3

19.5 11.0 10.6

20.0 11.1 10.5 10.9

20.5 11.0 10.6

21.0 11.5 10.8 11.3

21.5 11.3 10.7

22.0 11.3 11.1 11.3

22.5 11.4 11.1

23.0 11.5 11.0 11.4

23.5 11.7 11.1

24.0 11.5 11.1 11.4

24.5 11.6 11.0

25.0 11.8 11.1 11.6

Qualitative Data

1) There was some amount of heat generated in the reaction since the beaker became warm. The reaction was exothermic.

2) The slope of the change in pH was not gentle and the pH rose suddenly at certain intervals of the experiment.

Processed DataBy looking at the data above the following graphs have been plotted. It also contains the Equivalence point and the half equivalence point of the graph.

The graph below shows the change in pH of 10 cm3 of acetic acid as 0.1M of NaOH is added to it for reading 1. It also contains the trend lines for certain parts of the graph along with the equation of the line.

The following graph has an uncertainty of ±0.01 pH on the y-axis and uncertainty of ±1cm3 on the x-axis.

The graph below shows the change in pH of 10 cm3 of acetic acid as 0.1M of NaOH is added to it for reading 1. It also contains the trend lines for certain parts of the graph along with the equation of the line.

The following graph has an uncertainty of ±0.01 pH on the y-axis and uncertainty of ±1cm3 on the x-axis.

The graph below shows the change in pH of 10 cm3 of acetic acid as 0.1M of NaOH is added to it for reading 1. It also contains the trend lines for certain parts of the graph along with the equation of the line.

The following graph has an uncertainty of ±0.01 pH on the y-axis and uncertainty of ±1cm3 on the x-axis.

The equivalence point was found by taking the group of points between which the curve increased and then found the mid point of its pH value. The value of the volume of base added could then be found out by using the pH value.

The half equivalence point can be found out by the following steps-

1. After finding the equivalence point (pH and volume of base added), divide the base by 2 and mark the point on the graph.

2. The point should then be extended up to the graph.3. The point at which it intersects gives the pH value at the half-equivalence point.4. Using the Hinderson-Hasselbach equation we know that the pH = pKa at half equivalence point. ∴ by finding out the pH value at half equivalence point we can find the pKa value for the particular acid.

Calculations shown below are done using the steps spoken about above.For Reading 1

Finding the part of the graph where the slope becomes steep i.e. Between pH 5.99 and pH10.5.The midpoint of this slope can be calculated by finding the average of the maximum value (pH10.5) and the minimum value (pH 5.99). Average = (10.5 + 5.99)/2

= 8.245= 8.25 pH (3 s.f.)

Now using this value we can find the x-value using the equation of the trend line (shown in the diagram) i.e. y = 1.022x - 6.522.Therefore substituting the value of pH in the equation we get:-(8.25) = 1.022x - 6.52214.772 = 1.022xx = 14.456 x = 14.46 (3 s.f.)

This is the equivalence point. On dividing this by 2 we get pH 7.23, which is the half equivalence volume.

Using the trend line as shown in the diagram to find the half equivalence pH. The equation is as follows:- y = 0.19x + 2.81

Substituting the half equivalence volume into the equation we get:y = 0.19 x (7.23) + 2.81y = 1.38 + 2.81y = 4.19 pH (3 s.f.) Using the Hinderson-Hasselbach equation we know that the half equivalence pH equals the pKa value. Hence the pKa value for the 1st reading equals 4.19.

Calculating uncertaintyFor Reading 1

Using the process and calculation as shown above we can find the pKa value for the other 2 readings as well:-1. 4.19 ± 0.0792. 3.93 ± 0.0883. 3.83 ± 0.071

Average pH (=pKa) value = 3.98 pH ± 0.0790pH (2 s.f.)The actual pKa value of acetic acid = 4.76 pH1, which can be found on page 13, table 15 (strengths of organic acids and bases) of the IB chemistry data booklet.

Percentage error= 16.38= 16.4% (3 s.f.)

Conclusion and Evaluation

ConclusionFrom the above we can see that without the use of any calculation we can easily find the pKa value of the acid only using a pH probe and other basic apparatus. Using the Hinderson-Hasselbach

1http://www.wiley.com/college/pratt/0471393878/student/review/acid_base/4_strong_and_weak.html

equation( ) we know that the pH value at the half equivalence point is the pKa value of the acid. Thus using graphs and the calculaitons, the pH values of the acid at the equivalence point as well as the half equivalence point are shown. Using the pH value at the half equivalence point we can determine the pKa value of the acid. The pKa value of acetic acid found experimentally equals 3.98pH ± 0.037pH or 3.98pH ± 0.930% while the literature value of the pKa value of acetic acid equals 4.76. Therefore the percentage error equals 16.4%.

Evaluation1. The readings may not have been taken at the correct times since the pH value kept fluctuating

even thought the base was not being added.2. The base may have fallen directly on the probe at time and hence the readings may have been

affected.3. There is a major role of parallax error to play in the variation in the actual answer since a burette

was being used to add the base.4. The base that was left exposed to air, and hence it could have absorbed moisture from the air

decreasing the concentration of the base.

Methods of Improvement1. The readings should have been take only after the pH had stabilized which would have reduced

the error.2. It should have been made sure that the base was not falling onto the probe but instead it was

falling directly into the beaker.3. The magnetic stirrer shouldn't have been on a very high speed, so that the base/acid doesn’t

splatter on the sides of the beaker.4. Several readings should have been taken to reduce the random error. Also acetic acid could have

been titrated with other bases so that the final answer is more accurate.