Determination Of An Unknown Amino Acid From Titration

Determination of An Unknown Amino Acid From Titration

Abstract

Experiment 11 used a titration curve to determine the identity of an unknown amino acid. The initial pH of the solution was 1.96, and the pKa’s found experimentally were 2.0, 4.0, and 9.85. The accepted pKa values were found to be 2.10, 4.07, and 9.47. The molecular weight was calculated to be

176.3 while the accepted value was found to be 183.5. The identity of the unknown amino acid was established to be glutamic acid, hydrochloride.

Introduction

Amino acids are simple monomers which are strung together to form polymers

(also called proteins). These monomers are characterized by the general structure shown in figure 1.

Fig. 1

Although the general structure of all amino acids follows figure 1, the presence of a zwitterion is made possible due to the basic properties of the NH2 group and the acidic properties of the COOH group. The amine group (NH2) is Lewis base because it has a lone electron pair which makes it susceptible to a coordinate covalent bond with a hydrogen ion. Also, the carboxylic group is a

Lewis acidic because it is able to donate a hydrogen ion (Kotz et al., 1996).

Other forms of amino acids also exist. Amino acids may exists as acidic or basic salts. For example, if the glycine reacted with HCl, the resulting amino acid would be glycine hydrochloride (see fig. 2). Glycine hydrochloride is an example of an acidic salt form of the amino acid. Likewise, if NaOH were added, the resulting amino acid would be sodium glycinate (see fig. 3), an example of a basic salt form.

Fig. 2

Fig. 3

Due to the nature of amino acids, a titration curve can be employed to identify an unknown amino acid. A titration curve is the plot of the pH versus the volume of titrant used. In the case of amino acids, the titrant will be both an acid and a base. The acid is a useful tool because it is able to add a proton to the amine group (see fig. 1). Likewise the base allows for removal of the proton from the carboxyl group by the addition of hydroxide. The addition of the strong acid or base does not necessarily yield a drastic jump in pH. The acid or base added is unable to contribute to the pH of the solution because the protons and hydroxide ions donated in solution are busy adding protons to the amine gr...

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...unded series of crude estimates which were required. Likewise, the deviance of the calculated molecular weight can be attributed to these crude vehicles, because the change in volume (between equivalence points) were used in calculation.

Conclusion

The identity of an unknown amino acid was determined by establishing a titration curve. The equivalence and half-equivalence point, the pKa values, and the molecular weight were directly or indirectly found through the titration curve. The equivalence points were found through a crude method known as the trapezoidal method. The establishment of the equivalence points gave rise to the half equivalence points and the D volume (used in calculating the molecular weight). The half-equivalence points were directly used to find the pKa values of the unknown. The molecular weight could also be calculated. This data led to the determination of the identity of the unknown amino acid--glutamic acid, hydrochloride. References

Jicha, D.; Hasset, K. Experiments in General Chemistry; Hunt: Dubuque, 1991:37-

53.

Kotz, J.C.: Treichel , P. Jr. Chemistry and Chemical Reactivity; Harcourt-Brace:

Fort Worth, 1996; 816- 837.