Can some one find the pKa1 pKa2 and pKa3 for the curve A, and identify it? thank
ID: 475384 • Letter: C
Question
Can some one find the pKa1 pKa2 and pKa3 for the curve A, and identify it? thank you!
In the case of an amino acid with a dissociable R-group, you will not observe a steep equivalence point in the center of the titration curve, like that for glycine, rather there are two equivalence points. The analysis is further complicated because one of the equivalence points will probably be obscured since the pK_a values flanking it are close in value. The most noticeable equivalence point will lie at one third (or two thirds) of the total span of the plot. The obscured equivalence point will be at the other end of the plot, at two thirds (or one third) of the span of the data. Identification of the three pK_a values is otherwise similar to what you did in the lab for glycine. Referring to Figure 2 below, analyze the data for either A or B, but not both. What are the pK_a values for this amino acid? Speculate on the identity of the amino acid, A or B.Explanation / Answer
From the given figure 2 for curve A
At first equivalence point:
pH = 7
Volume of acid required = 4 units
pH = (pKa1 + pKa2) / 2
7 = (pKa1 + pKa2) / 2 …(1)
At half equivalence point:
pH = 10
Volume of acid required = 2 units
pH = pKa1
pKa1 = 10
Using equation (1) we get,
7 = (10 + pKa2) / 2
pKa2 = 14 -10 = 4
At second equivalence point (two-third part of curve):
pH = 3
Volume of acid required = 8 units
pH = (pKa2 + pKa3) / 2
3 = (4 + pKa3) / 2
pKa3 = 6 – 4 = 2