Chemistry 3251 Laboratory Report

Experiment #3 – Titration of Amino Acids

Ryan Moore: 960514735

Purpose: To determine the pKa value of 2 unkown amino acids by titration.

Method: follow the procedures as outlined in the Chemistry 3251 Lab Manual excepting that instead of doing the procedure twice over, 2 groups will each do the experiment for one each and we share the results!

Theory: We will titrate the unknown Amino acid with first a solution of 1 N HCl and then do another titration with the same unknown with then 1M KOH. When titrating the unknown with the acid the low pH causes it to protenate the amino group giving a pK value where the ratio of the concentration of the protenated vs the unprotenated (positive charge vs neutral) is 1. The same thing happens at when titrating with the base, except in this case the proton is extracted from the carboxyl group leaving a negative charge. Graphing the titration curve we can find the inflection points revealing these pK values. For reference in this report we will use pK1 to represent the protenated amino group / neutral point, and pK2 to represent the deprotenated / neutral point. Graphing both of these on one graph (as will be seen) we can see a third inflection point. This is the isoelectric point – the point where the concentration of both the protenated and the deprotenated are equal. At this point we have what is called a zwitterion. This is also roughly the mid point of both the pK values. From determining these 3 points we can roughly assume which Amino acid is our unknown from literature reference values. Now in doing these titrations it is well to note that we will have to correct for natural occurences. Since we are conducting the titration of the amino acid at around 0.1M so a blank must be titrated as well and those values subtracted. This is done by overlapping the corresponding titration curves and for each pH taking the pH of the unknown’s curve then subtracting the blanks’ curve pH value. It’s a long and tedious process, and unfortunately I didn’t know enough of Excel to get it to work properly so it was done in long hand. Now, of course, the R group from the Amino Acid may cause interference with the titration. Having an ionizable portion can cause ‘bumps and squiggles’ on the titration curve.

Observations:

1N HCl + AA(b)     1N HCl + H20  

mL

dmL

pH

  

mL

dmL

pH

0.36

0

5.15

  

0.57

0

4.1

0.42

0.06

3.4

  

0.64

0.07

3

0.475

0.115

3.2

  

0.68

0.11

2.8

0.55

0.19

3.09

  

0.8

0.23

2.5

0.6

0.24

3.05

  

0.88

0.31

2.47

0.66

0.3

3

  

0.98

0.41

2.4

0.79

0.43

2.9

  

1.05

0.48

2.35

0.925

0.565

2.85

  

1.15

0.58

2.3

1.05

0.69

2.85

  

1.26

0.69

2.23

1.08

0.72

2.8

  

1.36

0.79

2.19

1.24

0.88

2.79

  

1.46

0.89

2.15

1.42

1.06

2.75

  

1.57

1

2.1

1.57

1.21

2.72

  

1.74

1.17

2.05

1.65

1.29

2.72

  

1.9

1.33

2.01

1.73

1.37

2.7

  

2.08

1.51

1.95

1.79

1.43

2.7

  

2.29

1.72

1.9

1.87

1.51

2.69

  

2.48

1.91

1.87

2.06

1.7

2.65

  

2.73

2.16

1.85

2.13

1.77

2.62

  

2.95

2.38

1.8

2.22

1.86

2.6

  

3.18

2.61

1.75

2.34

1.98

2.59

  

3.33

2.76

1.7

2.69

2.33

2.5

  

3.56

2.99

1.6

3.2

2.84

2.4

        

3.41

3.05

2.37

        

3.65

3.29

2.32

        

4.03

3.67

2.2

        

4.41

4.05

2.15

        

4.94

4.58

2.11

        

5.34

4.98

2.05

        

5.76

5.4

1.99

        

5.97

5.61

1.95

        

6.08

5.72

1.93

        

6.35

5.99

1.9

        

6.66

6.3

1.89

        

6.94

6.58

1.85

        

7.25

6.89

1.85

        

7.525

7.165

1.8

        

7.78

7.42

1.8

        

8.27

7.91

1.7

        

8.75

8.39

1.65

        

9.1

8.74

1.65

        

9.48

9.12

1.65

        

10

9.64

1.6

        
             
             
1M KOH + AA(b)     1M KOH + H20  

mL

dmL

pH

  

mL

dmL

pH

1.31

0

5.4

  

0.74

0

4.6

1.34

0.03

6.51

  

0.79

0.05

9.6

1.41

0.1

7.1

  

0.85

0.11

10.1

1.46

0.15

7.25

  

0.92

0.18

10.3

1.5

0.19

7.38

  

0.96

0.22

10.5

1.61

0.3

7.7

  

1.04

0.3

10.7

1.72

0.41

7.8

  

1.1

0.36

10.8

1.77

0.46

7.85

  

1.31

0.57

11

1.88

0.57

7.9

  

1.38

0.64

11.15

2.05

0.74

8.05

  

1.55

0.81

11.25

2.11

0.8

8.1

  

1.74

1

11.4

2.2

0.89

8.12

  

1.82

1.08

11.5

2.52

1.21

8.32

  

1.92

1.18

11.6

2.71

1.4

8.4

  

2.04

1.3

11.7

2.9

1.59

8.5

  

2.19

1.45

11.8

3.07

1.76

8.6

  

2.32

1.58

11.8

3.49

2.18

8.75

  

2.57

1.83

11.85

3.81

2.5

8.9

  

2.76

2.02

11.9

4.07

2.76

9

  

2.99

2.25

12

4.42

3.11

9.1

        

4.77

3.46

9.3

        

5.03

3.72

9.4

        

5.3

3.99

9.55

        

5.75

4.44

9.85

        

6.17

4.86

10.475

        

6.51

5.2

11.2

        

6.73

5.42

11.4

        

6.9

5.59

11.6

        

7.21

5.9

11.7

        

7.35

6.04

11.8

        

7.5

6.19

11.8

        

7.75

6.44

11.9

        

8

6.69

11.95

        

8.25

6.94

12

        

(results for unknown A are attached)

  • Discussion: A graph of the titration curves are attached as well as the corrected titration curves for unknown A and B. Here are the rough estimates of the corresponding variables:
    •

    • Unknown

    pK1

    pI

    pK2

    Assumed Amino Acid

    A

    2.6

    5.7

    9.5

    		 Tryptophan

    B

    2.25

    5.3

    8.7

    		 Asparagine

    Now, of this is done in a very crude fashion, done exactly we might have come up with something a little more ‘actual.’ But, just ‘eyeballing’ the points on a graph causes a huge degree of error. We will have a better shot at guessing the unknown in the following lab.

    References: