Amino acids can be defined as a group of organic molecules that are made up of basic amino groups (-NH2), an acidic carboxylic acid (-COOH), and a side chain comprised of an organic R group. Except for proline which has the amine group attached to the secondary amine, all other amino acids have free carboxylic acid attached to the primary amine group.
Amino acids are the building blocks of proteins. In fact, the structure of protein consists of twenty different amino acids. To know the structure of the amino acids, pH of the solution is determined since they can thrive in both acidic and basic solutions. When in strong acidic solutions, the carboxylate ion (-COO-) in the amino acid has the capability of accepting a proton to form a carboxylic (-COOH). Alternatively, an amidogen (NH2) is formed from the amino group (-NH3+). Therefore, amino acids are termed as amphoteric since they have the capability to react in either acid or basic medium. Similarly, their solutions can neutralize both an acid and a base and are therefore known as buffers solutions. The concept can be presented well through a titration reaction between an amino acid and a solution of sodium hydroxide. In our experiment, determining the degree of protonation (pKa) and isoelectric point (pI) of aniline and comparing the experimental values with the theoretical ones can assist in identifying the amphoteric properties the amino acid. The amino acids; glycine, alanine, and serine used in our experiment has the following pKa and pI values.
Amino acid
pKa1
pKa2
pI
Glycine
2.34
9.60
5.97
Alanine
2.34
9.69
6.00
Serine
2.21
9.15
5.68
Experimental Procedure
40ml of 0.01 M amino acid solution was measured and put into a 100ml beaker. The electrode of a calibrated pH meter was inserted into the solution, and the pH checked. Then, 6M HCl was added drop-wise to adjust the pH of the amino acid to less than 1.5 before titration started. A 50ml burette was filled with 0.25M NaOH solution, and titration began. Titration involved adding 1.00ml of NaOH into the amino acid solution, thoroughly mixing the solution after each addition, and recording the pH. Addition and recording were done cumulatively until the pH of 12 was reached.
Results
Burette reading
0
1
2
3
4
5
6
7
8
9
10
pH
1.50
1.47
1.47
1.50
1.56
1.58
1.61
1.64
1.69
1.74
1.79
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1.85
1.92
1.99
2.03
2.17
2.26
2.35
2.47
2.58
2.68
2.80
2.93
3.10
3.34
3.65
26
27
28
29
30
31
32
33
34
35
36
37
38
39
4.77
8.60
9.02
9.28
9.50
9.65
9.78
9.91
10.05
10.15
10.27
10.40
10.51
10.73
40
41
42
43
44
45
46
47
48
49
50
10.93
11.24
11.55
11.74
11.90
12.00
12.09
12.15
12.19
12.22
12.24
Graphical Presentation of the Results
Discussion
The pH of the amino acid is adjusted to 1.50 at the start of the experiment. The pH of the amino acid increases gradually with the addition of 0.25M sodium hydroxide. The pH at after addition of 0 ml and 26 ml of NaOH solution is 1.5 and 4.77 respectively. The pH then increases steeply and becomes 8.60 when another 1 ml of sodium hydroxide is added. Then pH rises gradually again with the addition of more sodium hydroxide solution until all 50 ml of NaOH is added. The final pH of the amino acid recorded is 12.24. There are two pKa obtained by getting the gradients of the curves separately. The low pKa is 2.03 while the high is 10.51. pI is 6.27 and is obtained by calculating the average between the low pKa and the high pKa.
Errors
The experimental values obtained varied from the theoretical values meaning that there were some errors during the experiment. The errors could have originated from using contaminated reactants, using faulty pH scale meter or due to variation in temperatures.
Conclusion
The data obtained allowed calculation of the pKa1 and pKa2 to show the amphoteric and buffer nature of the amino acid. Also, the aim of the experiment was achieved since the amphoteric nature of the amino acid was established.
