Determination Of An Unknown Amino Acid From A Titration Curve

1691 words - 7 pages

AbstractExperiment 11 used a titration curve to determine the identity of an unknown amino acid. The initialpH of the solution was 1.96, and the pKa's found experimentally were 2.0, 4.0, and 9.85. The acceptedpKa values were found to be 2.10, 4.07, and 9.47. The molecular weight was calculated to be 176.3 whilethe accepted value was found to be 183.5. The identity of the unknown amino acid was established to beglutamic acid, hydrochloride.IntroductionAmino 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. 1Although the general structure of all amino acids follows figure 1, the presence of a zwitterion is madepossible 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. 2Fig. 3Due 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, thetitrant will be both an acid and a base. The acid is a useful tool because it is able to add a proton to theamine group (see fig. 1). Likewise the base allows for removal of the proton from the carboxyl group bythe addition of hydroxide. The addition of the strong acid or base does not necessarily yield a drasticjump in pH. The acid or base added is unable to contribute to the pH of the solution because the protonsand hydroxide ions donated in solution are busy adding protons to the amine group and removing protonsfrom the carboxyl group, respectively. However, near the equivalence point the pH of the solution may increase or decrease drastically with the addition of only a fraction of a mL of titrant. This is due to the fact that at the equivalence point the number of moles of titrant equals the number of moles of acid or base originally present (dependent on if the amino acid is in an acidic or basic salt form). Another point of interest on a titration curve is the half-equivalence point. The half-equivalence point corresponds to the point in which the concentration of weak acid is equal to the concentration of its conjugate base. The region near the half-equivalence point also establishes a buffer region (Jicha, et al., 1991). (see figure 4).Fig....

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